A method for purifying crude lactide to produce polymer grade lactide
By using a aziridine compound auxiliary agent to react with lactide followed by vacuum distillation, the problems of high energy consumption and difficulty in reducing free acid content in lactide purification have been solved, achieving low-cost and high-efficiency preparation of polymer-grade lactide to meet the production needs of polylactic acid.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- PRICE BIOTECHNOLOGY CO LTD
- Filing Date
- 2026-03-30
- Publication Date
- 2026-06-05
AI Technical Summary
Existing technologies consume a lot of energy and have difficulty effectively reducing the free acid content during the purification of lactide, which increases the production cost of polylactic acid, especially for crude lactide with a high content of racemic lactide.
Aziridine compounds were used as aids for removing free acid from lactide. The reaction was carried out under inert gas protection after mixing with lactide, followed by vacuum distillation, with the reaction temperature and time controlled to remove the free acid.
This method effectively reduces the free acid content in lactide, meets polymerization requirements, reduces crystallization operations, lowers energy consumption, and increases lactide yield, resulting in high molecular weight polylactic acid products.
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Figure CN122145429A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of lactide purification, and more specifically to a purification method for preparing polymer-grade lactide from crude lactide. Background Technology
[0002] Polylactic acid (PLA), as a bio-based and biodegradable material, possesses excellent mechanical properties and can be widely used in straws, coatings, biaxially oriented films, 3D printing, and other fields, showing promising application prospects. Currently, commercially available PLA is generally prepared via the "lactic acid-lactide-PLA" route. Latex is the key monomer in PLA production, and its chemical purity, optical purity, and free acid content all significantly affect the synthesis and properties of the polymer. In particular, the free acid content in lactide significantly influences the molecular weight of PLA. High free acid content makes ring-opening polymerization difficult. The latest national standard for "Bio-manufacturing of L-lactide" also clearly requires a free acid content of ≤7 mmol / kg for high-grade L-lactide. Therefore, the purification process of lactide in production becomes crucial. Currently, it is generally prepared using distillation and crystallization. Distillation is carried out under reduced pressure and high temperature, which results in high energy consumption. While crystallization is energy-efficient, its effect on reducing acid value is limited for crude lactide with a high meso-lactide content, and it requires multiple crystallization operations, leading to high production costs. Therefore, there is an urgent need to develop a low-cost and efficient method for preparing lactide with low free acid content. Summary of the Invention
[0003] To address the shortcomings of existing technologies, this invention proposes a purification method for preparing polymer-grade lactide from crude lactide.
[0004] The objective of this invention can be achieved through the following technical solutions: A first aspect of the present invention relates to the use of aziridine compounds as adjuvants for removing free acid from lactide.
[0005] Optionally, the aziridine compound includes one or more of the following: trimethylolpropane tri[3-(2-methylaziridine)propionate], tri(1-aziridine)trimethylolpropane triacrylate, tri(1-aziridine)ethoxylated trimethylolpropane triacrylate, tri(1-aziridine)pentaerythritol triacrylate, pentaerythritol tri(3-aziridine)propionate, tetramethylolmethane-tri(3-aziridine)propionate, trimethylolpropane tri(3-isobutyleneimino)propionate, glycerol tri(3-aziridine)propionate, and 1,1'-(1,3-phenylenedicarbonyl)-bis(2-methylaziridine).
[0006] Optionally, the aziridine compound accounts for 0.5 to 15% of the weight of the lactide.
[0007] A second aspect of the present invention relates to a method for removing free acid from lactide, comprising the following steps: The reaction involves mixing lactide with a aziridine compound.
[0008] Optionally, the reaction conditions include inert gas protection, a reaction temperature of 80–160 degrees Celsius, and a reaction time of 10–40 minutes.
[0009] Optionally, lactide can be extracted by vacuum distillation after the reaction is complete.
[0010] Optionally, during the vacuum distillation process, the distillation temperature is 100~180℃, and the vacuum degree of vacuum distillation is less than 2000Pa.
[0011] Preferably, the crude lactide comprises, by mass percentage: 5% to 90% meso lactide, 10% to 95% L-lactide and 1% to 20% D-lactide.
[0012] Preferably, the crude lactide raw material includes one or more of L-lactide, D-lactide, and meso-lactide, and there are no particular limitations on the composition of the raw lactide. The free acid content of the crude lactide is 20~500 mmol / kg.
[0013] Preferably, the melting temperature of the crude lactide is 80-110°C.
[0014] Preferably, the reaction temperature between the additive and crude lactide is 80-160°C.
[0015] Preferably, the temperature for preparing polymer-grade lactide by vacuum distillation of crude lactide is 100-180℃.
[0016] Preferably, the pressure of the vacuum distillation is less than 500 Pa.
[0017] The beneficial effects of this invention are: The process of this invention reduces the free acid content in lactide by adding aziridine compounds, thereby meeting the requirements for polymer-grade lactide. At the same time, it reduces the need for crystallization, lowers process energy consumption, and ultimately achieves a lactide yield of 80% or higher. Attached Figure Description
[0018] The invention will now be further described with reference to the accompanying drawings.
[0019] Figure 1 The molecular weight of the product obtained by polymerization of lactide, a comparative example of this application; Figure 2This is a schematic diagram of the GPC test analysis results of polylactic acid prepared from the purified polymer-grade lactide in the embodiments of this application. Detailed Implementation
[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0021] Comparison Example This comparative example studies the effect of reducing acid value directly through vacuum distillation without the addition of any additives. 200g of crude lactide with an initial free acid content of 79.8 mmol / kg was added to a thoroughly dried round-bottom flask. The round-bottom flask was placed in a 120℃ oil bath and reacted for 0.5h. Then, an air condenser, a receiving flask, and other equipment were connected, along with a cold trap (liquid nitrogen) and a vacuum pump. The vacuum pump was turned on to perform reduced pressure distillation. A heating band was added to the air condenser to prevent the lactide from cooling too quickly and clogging the entire reaction system. The lactide obtained in the collecting flask was the distilled lactide, and the free acid content was determined by titration using a potentiometric titrator.
[0022] Example 1 This embodiment provides a method for removing free acid from crude lactide to prepare polymer-grade lactide, the steps of which are as follows: 200g of crude lactide with an initial free acid content of 79.8 mmol / kg was added to a thoroughly dried round-bottom flask. 1g (0.5%) of trimethylolpropane tris[3-(2-methylaziridinyl)propionate] was added to the flask. Nitrogen gas was purged three times to replace the air in the flask. The round-bottom flask was placed in a 120℃ oil bath and reacted for 0.5h. An air condenser, a receiving flask, and other equipment were then connected, along with a cold trap (liquid nitrogen) and a vacuum pump. The vacuum pump was turned on to perform reduced pressure distillation. A heating band was added to the air condenser to prevent the lactide from cooling too quickly and clogging the entire reaction system. The lactide obtained in the collecting flask was the lactide with free acid removed. The free acid content was determined by titration using a potentiometric titrator.
[0023] Example 2 This embodiment provides a method for removing free acid from crude lactide to prepare polymer-grade lactide, the steps of which are as follows: 200g of crude lactide with an initial free acid content of 79.8 mmol / kg was added to a thoroughly dried round-bottom flask. 2g (1%) of trimethylolpropane tris[3-(2-methylaziridinyl)propionate] was added to the flask. Nitrogen gas was purged three times to replace the air in the flask. The round-bottom flask was placed in a 120℃ oil bath and reacted for 0.5h. An air condenser, a receiving flask, and other equipment were then connected, along with a cold trap (liquid nitrogen) and a vacuum pump. The vacuum pump was turned on to perform reduced pressure distillation. A heating band was added to the air condenser to prevent the lactide from cooling too quickly and clogging the entire reaction system. The lactide obtained in the collecting flask was the lactide with free acid removed. The free acid content was determined by titration using a potentiometric titrator.
[0024] Example 3 This embodiment provides a method for removing free acid from crude lactide to prepare polymer-grade lactide, the steps of which are as follows: 200g of crude lactide with an initial free acid content of 79.8 mmol / kg was added to a thoroughly dried round-bottom flask. 4g (2%) of trimethylolpropane tris[3-(2-methylaziridinyl)propionate] was added to the flask. Nitrogen gas was purged three times to replace the air in the flask. The round-bottom flask was placed in a 120℃ oil bath and reacted for 0.5h. An air condenser, a receiving flask, and other equipment were then connected, along with a cold trap (liquid nitrogen) and a vacuum pump. The vacuum pump was turned on to perform reduced pressure distillation. A heating band was added to the air condenser to prevent the lactide from cooling too quickly and clogging the entire reaction system. The lactide obtained in the collecting flask was the lactide with free acid removed. The free acid content was determined by titration using a potentiometric titrator.
[0025] Example 4 This embodiment provides a method for removing free acid from crude lactide to prepare polymer-grade lactide, the steps of which are as follows: 200g of crude lactide with an initial free acid content of 79.8 mmol / kg was added to a thoroughly dried round-bottom flask. 8g (4%) of trimethylolpropane tris[3-(2-methylaziridinyl)propionate] was added to the flask. Nitrogen gas was purged three times to replace the air in the flask. The round-bottom flask was placed in a 120℃ oil bath for 0.5h. After that, an air condenser, a receiving flask, and other equipment were connected, along with a cold trap (liquid nitrogen) and a vacuum pump. The vacuum pump was turned on to perform reduced pressure distillation. A heating band was added to the air condenser to prevent the lactide from cooling too quickly and clogging the entire reaction system. The lactide obtained in the collecting flask was the lactide with free acid removed. The free acid content was determined by titration using a potentiometric titrator.
[0026] Example 5 This embodiment provides a method for removing free acid from crude lactide to prepare polymer-grade lactide, the steps of which are as follows: 200g of crude lactide with an initial free acid content of 405.4mmol / kg was added to a thoroughly dried round-bottom flask. 20g (10%) of trimethylolpropane tris[3-(2-methylaziridinyl)propionate] was added to the flask. Nitrogen gas was purged three times to replace the air in the flask. The round-bottom flask was placed in a 120℃ oil bath and reacted for 0.5h. An air condenser, a receiving flask, and other equipment were then connected, along with a cold trap (liquid nitrogen) and a vacuum pump. The vacuum pump was turned on to perform reduced pressure distillation. A heating band was added to the air condenser to prevent the lactide from cooling too quickly and clogging the entire reaction system. The lactide obtained in the collecting flask was the lactide with free acid removed. The free acid content was determined by titration using a potentiometric titrator.
[0027] Example 6 This embodiment provides a method for removing free acid from crude lactide to prepare polymer-grade lactide, the steps of which are as follows: 200g of crude lactide with an initial free acid content of 513.6mmol / kg was added to a thoroughly dried round-bottom flask. 24g (12%) of trimethylolpropane tris[3-(2-methylaziridinyl)propionate] was added to the flask. Nitrogen gas was purged three times to replace the air in the flask. The round-bottom flask was placed in a 120℃ oil bath and reacted for 0.5h. An air condenser, a receiving flask, and other equipment were connected, along with a cold trap (liquid nitrogen) and a vacuum pump. The vacuum pump was turned on to perform reduced pressure distillation. A heating band was added to the air condenser to prevent the lactide from cooling too quickly and clogging the entire reaction system. The lactide obtained in the collecting flask was the lactide with free acid removed. The free acid content was determined by titration using a potentiometric titrator.
[0028] Example 7 This embodiment provides a method for removing free acid from crude lactide to prepare polymer-grade lactide, the steps of which are as follows: 200g of crude lactide with an initial free acid content of 513.6mmol / kg was added to a thoroughly dried round-bottom flask. 30g (15%) of trimethylolpropane tris[3-(2-methylaziridinyl)propionate] was added to the flask. Nitrogen gas was purged three times to replace the air in the flask. The round-bottom flask was placed in a 120℃ oil bath and reacted for 0.5h. An air condenser, a receiving flask, and other equipment were connected, along with a cold trap (liquid nitrogen) and a vacuum pump. The vacuum pump was turned on to perform reduced pressure distillation. A heating band was added to the air condenser to prevent the lactide from cooling too quickly and clogging the entire reaction system. The lactide obtained in the collecting flask was the lactide with free acid removed. The free acid content was determined by titration using a potentiometric titrator.
[0029] It is evident from the comparative and exemplary cases that vacuum distillation alone has a very limited effect on reducing the free acid content in crude lactide. However, the free acid content in polymer-grade lactide can be adjusted by adding additives and adjusting their dosage. Furthermore, as the free acid content in crude lactide continues to increase, the dosage of additives can be increased to meet the requirements of polymer-grade lactide.
[0030] Using the lactide obtained in the control example as raw material, stannous octoate as catalyst, and diethylene glycol as initiator, after long-term prepolymerization (140℃ / 6h), the viscosity of the lactide was still relatively low. Figure 1 As shown, the molecular weight was too low after sampling and testing, which is difficult to meet the usage requirements. This indicates that the product is difficult to polymerize effectively when the lactide acid value is too high in the control example.
[0031] Using the polymer-grade lactide obtained in Example 2 as raw material, stannous octoate as catalyst, and diethylene glycol as initiator, the polylactic acid obtained by polymerization, after devolatilization to remove unreacted monomers, has the following GPC test results: the number average molecular weight can reach 105.9 kDa, and the polydispersity index (PDI) is 1.77, which can meet the market demand for low melt index products.
[0032] In summary, by comparing the lactide obtained from the control example and the selected Example 2 as raw materials, the lactide with acid value reduced by additives can meet the polymerization requirements and produce high molecular weight polylactic acid products.
[0033] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.
[0034] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.
Claims
1. Application of aziridine compounds as adjuvants for removing free acid from lactide.
2. The application of the aziridine compound according to claim 1 as an adjuvant for removing free acid from lactide, characterized in that, The aziridine compounds include one or more of the following: trimethylolpropane tri[3-(2-methylaziridine)propionate], tri(1-aziridine)trimethylolpropane triacrylate, tri(1-aziridine)ethoxylated trimethylolpropane triacrylate, tri(1-aziridine)pentaerythritol triacrylate, pentaerythritol tri(3-aziridine)propionate, tetramethylolmethane-tri(3-aziridine)propionate, trimethylolpropane tri(3-isobutyleneimino)propionate, glycerol tri(3-aziridine)propionate, and 1,1'-(1,3-phenylenedicarbonyl)-bis(2-methylaziridine).
3. The application of the aziridine compound according to claim 1 as an adjuvant for removing free acid from lactide, characterized in that, The aziridine compound accounts for 0.5% to 15% of the weight of the lactide.
4. A method for reducing the free acid content in lactide, comprising the following steps: The reaction involves mixing lactide with a aziridine compound.
5. The method for reducing the free acid content in lactide according to claim 4, characterized in that, The reaction conditions include inert gas protection, reaction temperature of 80–160 degrees Celsius, and reaction time of 10–40 minutes.
6. The method for reducing the free acid content in lactide according to claim 4, characterized in that, After the reaction was completed, lactide was extracted by vacuum distillation.
7. The method for reducing the free acid content in lactide according to claim 6, characterized in that, During the vacuum distillation process, the distillation temperature is 100~180℃, and the vacuum degree of vacuum distillation is less than 2000Pa.
8. The method for reducing the free acid content in lactide according to claim 4, characterized in that, The aziridine compounds include one or more of the following: trimethylolpropane tri[3-(2-methylaziridine)propionate], tri(1-aziridine)trimethylolpropane triacrylate, tri(1-aziridine)ethoxylated trimethylolpropane triacrylate, tri(1-aziridine)pentaerythritol triacrylate, pentaerythritol tri(3-aziridine)propionate, tetramethylolmethane-tri(3-aziridine)propionate, trimethylolpropane tri(3-isobutyleneimino)propionate, glycerol tri(3-aziridine)propionate, and 1,1'-(1,3-phenylenedicarbonyl)-bis(2-methylaziridine).
9. The method for reducing the free acid content in lactide according to claim 4, characterized in that, The aziridine compound accounts for 0.5% to 15% of the weight of the lactide.