Method for preparing lactide with different optical purities

Abstract

The invention relates to a method for preparing lactide with different optical purities, belongs to the technical field of lactide preparation, and discloses a method for preparing lactide with different optical purities through multistage depolymerization, lactic acid oligomer is depolymerized, segmentation is carried out at least twice in the depolymerization process to obtain crude lactide with different m-lactide contents, and optionally, the crude lactide is separated from the crude lactide to obtain the lactide with different optical purities. And respectively purifying crude lactide with different contents of m-lactide. According to the invention, mixing of m-lactide and lactide with optical purity is reduced from the source, and lactide with different optical purity can be obtained as required; the method provided by the invention can obtain gt; according to the present invention, the lactide with the optical purity of 96% does not need the separation of m-lactide and L-lactide, such that the amount of the lactide crude product requiring the separation of m-lactide and L-lactide is substantially reduced, and the separation energy consumption is reduced.

Description

Technical Field

[0001] This invention belongs to the field of lactide preparation technology, specifically relating to a method for preparing lactide with different optical purities. Background Technology

[0002] Biodegradable materials are polymeric materials that exhibit excellent performance during use and can be rapidly hydrolyzed and degraded by enzymes or microorganisms after use. Among various biodegradable materials, polylactic acid (PLA) is attracting widespread attention due to its good biocompatibility, degradability, and excellent processing properties.

[0003] There are two main methods for synthesizing polylactic acid (PLA). One method is direct polycondensation of lactic acid. This method has a simple production process, but due to the presence of impurities in the system and the fact that the polycondensation reaction of lactic acid is reversible, the resulting PLA has a relatively small molecular weight, poor strength, and no practical value. The other method for synthesizing PLA is to first obtain cyclic dimer lactide by oligomerization and depolymerization of lactide, and then use lactide as a monomer for ring-opening polymerization to obtain PLA. This method can yield products with a high relative molecular weight of millions and has become the main method for synthesizing PLA.

[0004] As a key intermediate in the synthesis of polylactic acid (PLA), the synthesis process and key equipment for lactide are a major research focus in the industry. Currently, the industrial synthesis of lactide generally involves the pyrolysis and cyclization reaction of PLA with a certain degree of polymerization in the presence of a catalyst to obtain lactide products, as disclosed in technologies such as US5247058A, US5338822A, US5521278A, and EP0893462B1.

[0005] Lactic acid is a chiral molecule, with L-lactic acid being the most commonly used. Lactide and polylactic acid (PLA) exhibit chirality. Applications involving L-lactide require high optical purity, and existing technologies for achieving this purity necessitate the separation of meso-lactide (m-lactide). Firstly, m-lactide arises from a racemic reaction during the oligomerization-depolymerization process in lactide synthesis. A racemic reaction refers to the inversion of either the L-configured chiral carbon atom to the D-configuration, or vice versa. Commercially available PLA primarily uses L-lactic acid as a raw material, corresponding to a racemic inversion of the L-configuration to the D-configuration. Each molecule of lactide contains two chiral carbon atoms. When neither chiral carbon atom races and retains its original L-configuration, the resulting lactide is L-lactide, which is the primarily desired product. When one of the two chiral carbon atoms races, the resulting lactide is meso-lactide (m-lactide). It is called meso-lactide because the chirality of the two chiral carbons is exactly opposite, making the entire molecule optically inactive. Even when using optically pure L-lactic acid (usually requiring an L content >99.5%) as a raw material, a large amount of m-lactide is inevitably generated during the lactide production process due to the racemic reaction, and it usually needs to be separated from L-lactide. Secondly, the formation of m-lactide is due to insufficient optical purity of the raw material lactic acid. Commercially available L-lactic acid usually contains a small amount of D-lactic acid, for example, 1% or 3% D-lactic acid. When lactic acid containing a small amount of D-lactic acid is used for lactide synthesis, it is inevitable that the resulting lactide will have one D and one L chiral center, which is the case of obtaining m-lactide.

[0006] Currently, various lactide purification processes are known, such as those described in US5521278A, US5357034A, and US5214159A, which include one or more integrated distillation, condensation, and melt crystallization steps. However, m-lactide and L-lactide have the same molecular weight, differing only in the chirality of one carbon atom, making their separation extremely difficult and energy-intensive.

[0007] Different optical purities of lactide can be used to prepare polylactic acid (PLA) with corresponding optical purities, resulting in variations in crystallinity and heat resistance. Commercially available PLA brands, such as TotalEnergies Corbion's LX175, have a melting point of 155°C, an optical purity of 96%, and contain 96% lactide (L) and 4% dilution (D). Accordingly, the content of m-lactide in the lactide needs to be approximately 8% or less. Since lactide generally also contains a small amount of D-lactide, the actual m-lactide content needs to be less than 8% to ensure a final optical purity of over 96%. Further, PLA with optical purity of 98% or >99% exhibits correspondingly higher melting points and crystallinity. On the other hand, there are also amorphous PLAs with optical purity below 96%, even lower melting points, or no melting point, possessing only a glass transition temperature. The market and applications of polylactic acid (PLA) with different optical purities vary, but generally speaking, high optical purity and high heat resistance PLA are more popular in the market. High optical purity PLA is essential in applications such as heat-resistant straws and heat-resistant lunch boxes.

[0008] Due to market usage habits, obtaining polylactic acid with 96% optical purity is highly commercially valuable. Accordingly, it is required to control the m-lactide content in lactide to within 8%. Furthermore, assuming the obtained lactide has an m-lactide content greater than 4% but less than 8%, it is desirable to further classify this lactide into two specifications: one with an m-lactide content of less than 4%, and the other with an m-lactide content of less than 8%.

[0009] Among known methods, distillation is typically used to separate a stream rich in m-lactide and a stream rich in L-lactide. For example, CN101696203A uses distillation for separation. The feed stream contains 6.06% m-lactide and 86.96% L-lactide. First, a stream with 66.69% m-lactide and 11.54% L-lactide is obtained at the top of the light-light ...

[0010] Among known methods, the approach to obtaining lactide with an optical purity of over 96% can be broadly described as follows: 1) Lactic acid is dehydrated to obtain lactic acid oligomers. 2) The lactic acid oligomers are depolymerized under high temperature and vacuum to obtain crude lactide. The crude lactide contains water, lactic acid, lactide, and a small amount of lactic acid oligomers. 3) The crude lactide is separated by distillation to remove water, lactic acid, and oligomers, yielding refined lactide. 4) The refined lactide is separated by distillation or melt crystallization to obtain a stream rich in m-lactide and a stream rich in L-lactide, respectively. The L-lactide-rich stream obtained in step 4 typically has an optical purity of over 96%, meaning the m-lactide content is below 8%. The m-lactide-rich stream obtained in step 4 can have an m-lactide content of over 60%. Summary of the Invention

[0011] As mentioned above, existing methods for preparing high-optical-purity lactide produce a mixture of optically pure lactide and m-lactide. Separating the two still has high commercial value, but separating the mixture is extremely difficult and energy-intensive. Therefore, it is necessary to develop a simpler and more efficient method to separate the two and obtain lactide with more than 96% optical purity. After extensive research, the inventors proposed that minimizing the mixing of m-lactide and optically pure lactide from the beginning is a more ideal solution, and lactide with different optical purities can be prepared for different needs.

[0012] Based on this, the present invention provides a method for preparing lactide with different optical purities. This method has the advantages of reducing the technical difficulty and energy consumption of enriching and separating m-lactide, and can prepare lactide with different optical purities according to different needs. The method includes: depolymerizing lactic acid oligomers, wherein at least two splits are performed during the depolymerization process to obtain crude lactide with different contents of m-lactide. Optionally, the crude lactide with different contents of m-lactide is purified separately.

[0013] Through the above technical solution, the present invention has at least the following beneficial effects:

[0014] 1. This invention reduces the mixing of m-lactide and optically pure lactide from the source. Because at least two splitting and crude product collection are used, the crude lactide obtained from the first or second depolymerization can be processed without separating and enriching m-lactide as needed. Only the crude lactide obtained from the last depolymerization needs to be enriched and separated for m-lactide, or m-lactide enrichment and separation can be omitted. Ultimately, lactide with different optical purities can be obtained as needed.

[0015] 2. In this invention, the lactic acid oligomer undergoes at least two splits during the depolymerization process. The crude lactide product obtained in the first depolymerization step has a high lactic acid and water content, while the crude lactide product obtained in the subsequent degradation step has a lower lactic acid and water content. At the same time, it greatly reduces the amount of crude lactide product that needs to be separated into m-lactide and L-lactide, thus reducing the separation energy consumption and the difficulty of separation. Detailed Implementation

[0016] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0017] Unless otherwise specified, all pressures in this invention are absolute pressures.

[0018] This invention provides a method for preparing lactide with different optical purities. The method includes: depolymerizing a lactic acid oligomer, wherein the depolymerization process involves at least two splitting steps to obtain crude lactide with different contents of m-lactide, and optionally, purifying each of the crude lactide with different contents of m-lactide.

[0019] In this invention, the conversion rate of lactic acid oligomers at each cutting refers to the total conversion rate of the original polylactic acid raw material.

[0020] The inventors of this invention discovered that during the production process of lactic acid oligomers depolymerizing into lactide, the proportion of m-lactide in the crude lactide obtained from depolymerization is not constant, but changes with the conversion rate. In this invention, at least two segments are performed during the depolymerization of lactic acid oligomers, that is, the depolymerization reaction of polylactic acid oligomers is set to multiple depolymerizations (i.e., divided into several segments), and the crude lactide obtained from each depolymerization (each segment) is collected separately. According to different conversion rates, crude lactide with different contents of m-lactide can be obtained, thereby meeting the requirements of different optical purity lactide in different scenarios.

[0021] According to one embodiment of the present invention, at least two splits are performed during the depolymerization process to obtain crude lactide with an increased m-lactide content.

[0022] The inventors of this invention further discovered that the proportion of m-lactide in crude lactide is not constant, but increases with the increase of conversion rate. That is, the crude lactide obtained in the early stage has a lower proportion of m-lactide, while the crude lactide obtained in the later stage has a higher proportion of m-lactide. Through appropriate cutting, crude product with an m-lactide content of no more than 8%, or even no more than 4%, can be obtained directly. This reduces the mixing of m-lactide and optically pure lactide from the source, greatly reduces the amount of crude lactide product that needs to be separated from m-lactide and optically pure lactide, and reduces separation energy consumption.

[0023] The inventors speculate that the technical principle of this method may be explained as follows:

[0024] A polylactic acid (PLA) oligomer has n PLA structural units. When the end of the molecular chain bites back two units into the chain in a transesterification reaction, one molecule of lactide is removed, and a smaller PLA oligomer with n-2 structural units is generated. This chain continues to undergo the above reaction, yielding more lactide and smaller oligomers such as n-4, n-6, etc., until the oligomer contains only 1-2 structural units and cannot be depolymerized. During the above depolymerization process, if chiral inversion (called racemization) occurs, m-lactide is produced. If no racemic side reaction occurs, the chirality of the product is consistent with that of the raw lactic acid. That is, L-lactide is obtained when L-lactic acid is used as a raw material, and D-lactide is obtained when D-lactic acid is used as a raw material. Existing processes, to varying degrees, involve racemic side reactions.

[0025] Furthermore, the researchers of this invention hypothesize that when a polylactic acid oligomer begins to depolymerize, the lactide obtained from the first depolymerization (such as the first and second lactides) has high optical purity and a very low probability of racemic side reactions. Along this chain, the probability of racemic side reactions increases continuously with the number of depolymerization reactions. It can be inferred that when a polylactic acid oligomer has a relatively low molecular weight and undergoes only 6-8 depolymerization reactions, the resulting product will have fewer racemic side reactions than when the polylactic acid oligomer has a higher molecular weight and undergoes 10-20 reactions. This corresponds to the phenomenon proposed in this application where the content of m-lactide in the depolymerized lactide increases with increasing conversion rate.

[0026] The reason why the probability of racemic side reactions increases with the number of depolymerization reactions when a polylactic acid oligomer begins to depolymerize is currently unclear. One possible hypothesis proposed by the researchers of this invention is that the end groups of the original lactic acid oligomer (hereinafter referred to as proto-end groups) may undergo a certain probability of some side reaction during depolymerization, generating another active end group (hereinafter referred to as new end groups). These new end groups are more likely to undergo racemic reactions to generate m-lactide in subsequent depolymerizations, and the new end groups cannot or rarely revert to proto-end groups. Therefore, when depolymerizing the first lactide, assuming there are 1000 polylactic acid oligomers in the system, the end groups of these oligomers are almost all proto-end groups, so racemic reactions rarely occur. As the number of depolymerization reactions increases, more and more proto-end groups are converted into new end groups. For example, when there are 940 proto-end groups and 60 new end groups in the system at a certain conversion rate, the proportion of m-lactide in the lactide obtained from depolymerization is higher than when there were only 1000 proto-end groups initially.

[0027] It should be noted that the researchers have not yet fully verified the above hypothesis. Whether the above hypothesis is valid or not does not affect the phenomenon observed by the researchers that the proportion of m-lactide increases with the increase of conversion rate.

[0028] According to the present invention, as long as the purpose of the present invention can be achieved, there are no special requirements for the segmentation points of each segmentation. The present invention provides the following preferred segmentation points as examples to illustrate the advantages of the present invention.

[0029] According to one embodiment of the present invention, the method performs a first split when the conversion rate of the lactic acid oligomer is not higher than 50 wt%, obtaining crude lactide with an m-lactide content not higher than 8 wt%. In the aforementioned embodiment, the first split is performed by depolymerizing the lactic acid oligomer, and the first split is controlled at a conversion rate of not higher than 50 wt%, resulting in crude lactide with an m-lactide content not higher than 8%. Furthermore, studies have shown that the crude lactide obtained from the first split has a high content of lactic acid and water, which can reduce the lactic acid and water content in the crude lactide product obtained in subsequent steps, thus reducing the difficulty of subsequent separation. Moreover, purification of the crude lactide obtained from the first split can yield lactide with an optical purity of over 96%.

[0030] In this invention, the final cutting point can be selected as needed. According to one embodiment of the invention, the final cutting is performed when the conversion rate of lactic acid oligomers is not higher than 100 wt%.

[0031] According to one embodiment of the present invention, when the conversion rate of lactic acid oligomers is 20-50 wt%, a first split is performed to obtain crude lactide with an m-lactide content of 4-8 wt%.

[0032] According to one embodiment of the present invention, when the conversion rate of the lactic acid oligomer is less than 20 wt%, a first split is performed to obtain crude lactide with an m-lactide content of no more than 4 wt%. The lower the depolymerization conversion rate of the lactic acid oligomer, the lower the proportion of m-lactide in the obtained crude lactide. In the aforementioned embodiment, by controlling the conversion rate of the lactic acid oligomer to be less than 20 wt%, the proportion of m-lactide in the obtained crude lactide is no more than 4 wt%. Subsequently, the crude lactide is purified to obtain lactide with an optical purity greater than 98%.

[0033] According to one embodiment of the present invention, the depolymerization process involves 2-5 splitting steps, such as 2 splitting steps, 3 splitting steps, 4 splitting steps, and 5 splitting steps, preferably 2-3 splitting steps. In the aforementioned embodiments, crude lactide with an increasing m-lactide content can be obtained sequentially. The proportion of m-lactide increases with the increase of conversion rate. The crude lactide obtained in the early stages has a lower proportion of m-lactide, while the crude lactide obtained in the later stages has a higher proportion of m-lactide.

[0034] According to one embodiment of the present invention, specifically, the depolymerization process involves two splitting steps. The splitting point for the second split is not particularly critical and can be selected as needed. Preferably, the first split is performed when the conversion rate of the lactic acid oligomer is not higher than 50 wt%, and the second split is performed when the conversion rate of the lactic acid oligomer is higher than 50 wt% but not higher than 100 wt%. The aforementioned embodiment employs two splitting steps and crude product collection, which can yield crude lactide with m-lactide content not higher than 8 wt% and not higher than 50 wt%, respectively. In the crude lactide obtained from the first depolymerization, the m-lactide content is less than 8%, therefore, m-lactide separation and enrichment are not required. Only the crude lactide obtained from the last depolymerization needs to undergo m-lactide enrichment and separation, or m-lactide enrichment and separation may not be performed.

[0035] According to one embodiment of the present invention, the depolymerization process involves three stages of splitting. Specifically, the first split is performed when the conversion rate of the lactic acid oligomer is no higher than 20 wt%; the second split is performed when the conversion rate is higher than 20 wt% but no higher than 50 wt%; and the third split is performed when the conversion rate is higher than 50 wt% but no higher than 100 wt%. This yields crude lactide with an m-lactide content of no more than 4 wt%, 4-8 wt%, and 8-50 wt%, respectively. Lactide with optical purity of 98% or higher, 96-98%, and less than 96% is collected from the three depolymerization stages, respectively. The aforementioned implementation method employs three-stage splitting and crude product collection, which can yield crude lactide with an m-lactide content of no more than 4 wt%, 4-8 wt%, and 8-50 wt%, respectively. In the crude lactide obtained from the first and second depolymerizations, the m-lactide content is less than 4% and 8%, respectively. Therefore, it is not necessary to separate and enrich m-lactide. Only the crude lactide obtained from the last depolymerization needs to be enriched and separated for m-lactide, or m-lactide enrichment and separation can be omitted.

[0036] According to the present invention, as long as the purpose of the present invention can be achieved, the conditions of the depolymerization process are not particularly limited. Optionally, the depolymerization conditions corresponding to each split independently include: a temperature of 195-240°C, preferably 205-225°C.

[0037] According to one embodiment of the present invention, the depolymerization conditions corresponding to each split independently include: a pressure of 133-13000 Pa.

[0038] In this invention, there is no special limitation on the depolymerization time. Under the premise of vacuum degree and pressure control, it can be selected according to the final required conversion rate. This invention will not be elaborated on further here.

[0039] In each depolymerization step, the present invention may, as needed, include a step of stopping the vacuum and replenishing an inert atmosphere (such as nitrogen) to atmospheric pressure between two adjacent splits.

[0040] In this invention, the optional purification of crude lactide with different contents of m-lactide can be carried out according to actual needs. Crude lactide generally exists in the form of a mixture with lactic acid, water, and oligomers (mainly dimers and trimers). Pulverization can be used to obtain lactide of polymer-grade purity. Preferably, crude lactide with different contents of m-lactide is purified separately to obtain lactide with different optical purities. The purification method is conventional in the art, such as distillation. Recrystallization, melt crystallization, and coupling separation methods can also be used. This invention does not elaborate further on conventional purification methods.

[0041] The source of the lactic acid oligomer in this invention is not particularly limited and can be prepared by conventional preparation methods in the art. According to one embodiment of this invention, the preparation method of the lactic acid oligomer includes: dehydrating and polymerizing lactic acid.

[0042] In this invention, dehydration polymerization is a conventional method in the art, which can be carried out in the presence of a catalyst. There are no special restrictions on the type of catalyst, which can be a commonly used polymerization catalyst in the art. Stannous octoate is used as an example to illustrate the advantages of this invention, but it should not be construed as a limitation of this invention. Dehydration polymerization is carried out to better remove water, and is generally carried out in a system equipped with a distillation column, condenser and fraction collection equipment. The system pressure is generally 8-30 kPa and the temperature is generally 95-200°C.

[0043] In this invention, flash evaporation can be performed as needed after dehydration polymerization. During the depolymerization of lactic acid oligomers, a small amount of water and lactic acid are generated as byproducts. Furthermore, water and lactic acid are always inevitably present in the lactic acid oligomers. Therefore, flash evaporation is used to remove some of the water and lactic acid. Flash evaporation needs to be carried out within a short residence time. If the residence time is too long, the lactic acid oligomers will continue to react and generate new water, leading to an increase in the molecular weight of the lactic acid oligomers. Both of these are negative results. Preferably, the flash evaporation conditions include: a temperature of 120-170℃, a pressure of 1-20 kPa, and a time of 1-60 s. However, water, lactic acid, and lactide are easily separated; therefore, flash evaporation is often unnecessary.

[0044] According to the present invention, lactic acid oligomers are a conventional technical term that is recognized by those skilled in the art. Preferably, the number average molecular weight of the lactic acid oligomers is 650-5000.

[0045] In this invention, the specific configuration of lactic acid can be selected as needed. According to one embodiment of this invention, the lactic acid is selected from L-lactic acid or D-lactic acid. Replacing L-lactic acid with D-lactic acid is chemically equivalent and yields corresponding equivalent effects.

[0046] According to one embodiment of the present invention, the optical purity of the lactic acid is not less than 99%.

[0047] The present invention will be described in detail below through embodiments. The following are preparation examples and embodiments:

[0048] L-lactic acid with an optical purity >99% and a chemical purity >99% was purchased from Anhui Gelat Biochemical Co., Ltd.

[0049] D-lactic acid with optical purity >99% and chemical purity >99% was purchased from Sinopharm Reagent.

[0050] The catalyst, stannous octoate, was purchased from Aladdin, reagent grade.

[0051] The contents of L-lactide, D-lactide, and m-lactide are determined according to common knowledge in the art, such as the method shown in CN112679465A. This invention uses an Rt-βDEXsm chiral capillary separation column, thus distinguishing between L-lactide and D-lactide. However, a non-chiral separation column can also be used, and mixtures of m-lactide and L / D-lactide can be distinguished.

[0052] The percentages of L-lactide, D-lactide, and m-lactide were determined by gas chromatography (GC). Calibration was performed using a standard curve. The chromatographic column and conditions were as follows:

[0053]

[0054]

[0055] Moisture was determined by Karl Fischer titration;

[0056] Lactic acid residue was determined by high performance liquid chromatography (HPLC). The chromatographic column was a BEH C18 column (2.1 mm * 100 mm * 1.7 μm). The mobile phase consisted of: Phase A: 0.02 mol / L phosphoric acid aqueous solution and Phase B: acetonitrile. The mobile phase gradient was A:B = 60:40. The column flow rate was 0.15 mL / min, the column temperature was 40 °C, and the UV detector wavelength was 210 nm. Calibration was performed using a standard curve.

[0057] Acid value testing is performed using potentiometric acid-base titration.

[0058] Unless otherwise specified, all other raw materials used in the examples and comparative examples are commercially available.

[0059] Example of preparation of lactic acid oligomers

[0060] Preparation Example 1

[0061] Preparation of Lactic Acid Oligomer 1: In a 5L glass reactor equipped with a distillation column, condenser, and fraction collection flask, 3500g of L-lactic acid and 7g of stannous octoate were added, and the reactor was purged with nitrogen for 3 minutes. The pressure was then reduced to 20kPa, and the temperature was gradually increased. Liquid condensation was observed in the condenser starting at 95℃. The temperature was gradually and slowly increased to 190℃. The pressure was then reduced from 20kPa to 10kPa within 5 minutes and maintained at 10kPa for approximately 1 hour, until a significant decrease in the fraction collection rate was observed. A total of 620g of fraction (mainly water) was collected. Lactic acid oligomer 1 was obtained in the glass reactor.

[0062] The lactic acid oligomer 1 obtained in the preparation example was divided into multiple portions and used in Example 1 and Comparative Example 1 to prepare lactide.

[0063] Preparation Example 2

[0064] Preparation of lactic acid oligomer 2: In a 5L glass reactor equipped with a distillation column, condenser and fraction collection flask, 3500g of D-lactic acid and 7g of stannous octoate were added, and the reactor was purged with nitrogen for 3 minutes; then the pressure was reduced to 20kPa, and the temperature was gradually increased. Liquid condensation was observed in the condenser from 95℃, and the temperature was gradually and slowly increased to 195℃; then the pressure was reduced from 20kPa to 10kPa within 5 minutes and maintained at 10kPa for about 1 hour. A total of 622g of fraction (mainly water) was collected, and lactic acid oligomer 2 was obtained in the glass reactor.

[0065] The lactic acid oligomer 2 obtained in the preparation example was divided into multiple portions and used in Example 2 and Comparative Example 2 to prepare lactide.

[0066] Preparation Example 3

[0067] Preparation of lactic acid oligomer 3: In a 5L glass reactor equipped with a distillation column, condenser and fraction collection bottle, 3500g of L-lactic acid and 7g of stannous octoate were added and the reactor was purged with nitrogen for 3 minutes; then the pressure was reduced to 20kPa and the temperature was gradually increased. Liquid condensation was observed in the condenser from 95℃ and the temperature was gradually and slowly increased to 180℃; then the pressure was reduced from 20kPa to 10kPa in 5 minutes and maintained at 10kPa for about 3 hours. A total of 618g of fraction (mainly water) was collected and lactic acid oligomer 3 was obtained in the glass reactor.

[0068] The lactic acid oligomer obtained in the preparation example was divided into multiple portions and used in Example 3, Example 4 and Comparative Example 3 to prepare lactide.

[0069] Example

[0070] Example 1

[0071] 1400.6g of lactic acid oligomer 1 was placed in a 1L glass bottle, which was connected to a distillation head, a condenser, and a collection bottle. The condenser was temperature-controlled using 90℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle. First, the lactic acid oligomer 1 was heated to 205℃. Then, the pressure was rapidly reduced to 2.5kPa to begin the first depolymerization step, which took approximately 100 minutes. 613.81g of product was collected, with a crude product conversion rate of approximately 43.8%, denoted as crude lactide 1A. The vacuum was then stopped, nitrogen was added to restore atmospheric pressure, and the collection bottle was replaced. The pressure was again reduced to 2.5kPa, and the lactic acid oligomer was heated to 215℃ for the second depolymerization step. The depolymerization step took approximately 100 minutes, until the temperature-controlled thermocouple in the bottle was about to detach from the liquid surface. The residue in the bottle was observed to turn yellow and its viscosity increased. 517.89g of product was collected, with a conversion rate of approximately 37.0%, denoted as crude lactide 1B.

[0072] Crude lactide 1A and 1B were weighed and analyzed. The results are shown in Table 1.

[0073] The percentage of m-lactide in Table 1 refers to the mass ratio of m-lactide in the mixture of L / D-lactide and m-lactide. The percentages of other components are obtained by subtracting the percentages of L / m / D-lactide, lactic acid, water, and other five components from 100%.

[0074] Example 2

[0075] Place 733.7g of lactic acid oligomer 2 into a 1L glass bottle. Connect the glass bottle to a distillation head, a condenser, and a collection bottle. The condenser is temperature-controlled using 70℃ circulating heat transfer oil. Connect the end of the collection bottle to a vacuum pump.

[0076] First, the lactic acid oligomer 2 was heated to 230℃, then the pressure was rapidly reduced to 8.8 kPa to begin the first depolymerization step, which took approximately 15 minutes. 263.38 g of product was collected, with a conversion rate of approximately 35.9%, and this was designated as crude lactide 2A. The vacuum was then stopped, and nitrogen was added to bring the pressure back to atmospheric pressure. The collection bottle was then replaced. The pressure was again reduced to 8.2 kPa, while the temperature was maintained at 230℃, for the second depolymerization step. This step lasted approximately 18 minutes, until the temperature-controlled thermocouple in the bottle detached from the liquid surface. The residue in the bottle was observed to turn yellow and its viscosity increased. 337.52 g of product was collected, with a conversion rate of 46.0%, and this was designated as crude lactide 2B.

[0077] Crude lactide 2A and 2B were weighed and their components were tested. The results are shown in Table 2.

[0078] Example 3

[0079] 801.23g of lactic acid oligomer 3 was placed in a 1L glass bottle, which was connected to a distillation head, a condenser, and a collection bottle. The condenser was temperature-controlled using 90℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle. First, the lactic acid oligomer 3 was heated to 195℃. Then, the pressure was rapidly reduced to 1.5kPa to begin the first depolymerization step, which took approximately 1.5 hours. 395.74g of product was collected, with a conversion rate of approximately 49.4%, and this was designated as crude lactide 3A. The vacuum was then stopped, and nitrogen was added to bring the pressure back to atmospheric pressure. The collection bottle was then replaced. The pressure was again reduced to 0.5kPa, while the temperature was maintained at 195℃, for the second depolymerization step. This step lasted approximately 1.5 hours, until the temperature-controlled thermocouple in the bottle detached from the liquid surface. The residue in the bottle was observed to turn yellow and its viscosity increased. 275.7g of product was collected, with a conversion rate of 34.4%, and this was designated as crude lactide 3B.

[0080] Crude lactide 3A and 3B were weighed and analyzed. The results are shown in Table 3.

[0081] Example 4

[0082] 970g of lactic acid oligomer 3 was flash-treated using a falling film evaporator: feed temperature 170℃, main evaporator temperature 170℃, pressure 4.5kPa. Scraper speed 400rpm, scraper area 0.1m². 2 Residence time is approximately 10 seconds (visually estimated). The primary condensate temperature is 20°C, and the secondary condenser is protected with liquid nitrogen. 18.5g of fraction was collected from the primary condenser, consisting of a mixture of approximately 13.5g of lactic acid and 5g of water. The lactic acid oligomers remaining after flash evaporation are used for subsequent depolymerization and demonopolymerization.

[0083] 779.88g of the depolymerized lactic acid oligomer 3 was placed in a 1L glass bottle, which was connected to a distillation head, a condenser, and a collection bottle. The condenser was temperature-controlled using 90℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle. First, the lactic acid oligomer 3 was heated to 195℃. Then, the pressure was rapidly reduced to 1.8kPa to begin the first depolymerization step, which took approximately 30 minutes. 137.04g of product was collected, with a conversion rate of approximately 17.6%, denoted as crude lactide 4A. The vacuum was then stopped, nitrogen was added to bring the pressure back to atmospheric pressure, and the collection bottle was replaced. The pressure was again reduced to 1.5kPa, while the temperature was maintained at 195℃, for the second depolymerization step. This step took approximately 45 minutes, and 219.53g of product was collected, with a conversion rate of approximately 28.1%, denoted as crude lactide 4B. The vacuum was then stopped, nitrogen was added to bring the pressure back to atmospheric pressure, and the collection bottle was replaced. The pressure was again reduced to 0.5kPa, while the temperature was maintained at 195℃, for the third depolymerization step. The depolymerization process takes approximately 1.5 hours, until the temperature-controlled thermocouple inside the flask detaches from the liquid surface. 303.17 g of product was collected, representing a conversion rate of approximately 38.4%, and is designated as crude lactide 4C.

[0084] Crude lactide 4A, 4B, and 4C were weighed and analyzed. The results are shown in Table 3.

[0085] Comparative Example 1

[0086] 1363.83g of lactic acid oligomer 1 was placed in a 1L glass bottle, which was connected to a distillation head, a condenser, and a collection bottle. The condenser was temperature-controlled using 90℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle. The lactic acid oligomer was heated to 215℃ and the pressure reduced to 2.5kPa in one step to initiate the depolymerization process. The depolymerization process took approximately 2.5 hours, until the temperature-controlled thermocouple in the bottle was about to detach from the liquid surface. The residue in the bottle was observed to turn yellow and its viscosity increased. 1121.07g of product was collected, with a conversion rate of 82.2%, and designated as crude lactide 1C.

[0087] The crude lactide 1C was weighed and analyzed. The results are shown in Table 1.

[0088] Comparative Example 2

[0089] 712.16g of lactic acid oligomer 2 was placed in a 1L glass bottle, which was connected to a distillation head, a condenser, and a collection bottle. The condenser was temperature-controlled using 90℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle. The lactic acid oligomer was heated to 230℃ and the pressure reduced to 8.2kPa in one step to initiate the depolymerization process. The depolymerization process lasted approximately 0.5 hours, until the temperature-controlled thermocouple in the bottle detached from the liquid surface. The residue in the bottle was observed to turn yellow and its viscosity increased. 589.67g of product was collected, with a conversion rate of 82.8%, and was designated as crude lactide 2C.

[0090] The crude lactide 2C was weighed and analyzed. The results are shown in Table 2.

[0091] Comparative Example 3

[0092] 786.59g of lactic acid oligomer 3 was placed in a 1L glass bottle, which was connected to a distillation head, a condenser, and a collection bottle. The condenser was temperature-controlled using 90℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle. The lactic acid oligomer 3 was heated to 195℃ and the pressure reduced to 0.8kPa in one step to initiate the depolymerization process. The depolymerization process took approximately 3 hours, until the temperature-controlled thermocouple in the bottle detached from the liquid surface. The residue in the bottle was observed to turn yellow and its viscosity increased. 660.73g of product was collected, with a conversion rate of 84.0%, and was designated as crude lactide 3C.

[0093] The crude lactide 3C was weighed and analyzed. The results are shown in Table 3.

[0094] Table 1 shows the composition of Example 1 and Comparative Example 1.

[0095] distillate 1A 1B 1C L-lactide % 83.59％ 77.61％ 80.48％ m-lactide % 6.06％ 17.37％ 11.60％ D-lactide % 0.52％ 1.36％ 0.88％ Lactic acid% 4.59％ 0.17％ 2.61％ water% 1.72％ 0.07％ 0.96％ Other components % 3.52％ 3.42％ 3.47％ Optical purity % 96.1％ 89.6％ 92.8％ Total mass (g) 613.81 517.89 1121.07 Conversion rate 43.8％ 37.0％ 82.2％ m-lactide as a percentage of lactide 6.8％ 18.3％ 12.6％

[0096] Table 2 shows the composition of Example 2 and Comparative Example 2.

[0097] distillate 2A 2B 2C D-lactide % 83.05％ 74.82％ 79.19％ m-lactide % 6.10％ 17.96％ 12.68％ L-lactide % 0.57％ 1.39％ 0.92％ Lactic acid% 4.28％ 0.51％ 2.05％ water% 1.61％ 0.19％ 0.72％ Other components % 4.39％ 5.13％ 4.44％ Optical purity % 96.0％ 89.0％ 92.2％ Total mass (g) 263.38 337.52 589.67 Conversion rate 35.9％ 46.0％ 82.8％ m-lactide as a percentage of lactide 6.8％ 19.4％ 13.8％

[0098] Table 3 shows the composition of Examples 3, 4 and Comparative Example 3.

[0099]

[0100]

[0101] Separation of crude lactide:

[0102] Crude lactide 1A was distilled:

[0103] The distillation apparatus is a 1L glass distillation column with an inner diameter of 25mm. The packing material is glass spring packing with a packing section height of 1.2m. The distillation column is heated, with an initial heating temperature of 120℃, requiring preheating. The condensate in the top condenser is ethylene glycol monomethyl ether. Three interchangeable collection bottles are provided at the top of the column, each equipped with a piston to prevent cross-contamination.

[0104] 611.1g of crude lactide 1A was placed in a distillation apparatus and purged with nitrogen three times. Initially, the temperature of the condensate at the top of the column was controlled at 25°C, and a vacuum of 1.2 kPa was applied. The bottom temperature was then rapidly raised to 140°C and refluxed for 8 minutes. After the top temperature stabilized, the reflux ratio was reduced to total recovery. During this period, approximately 29g of distillate was distilled off, denoted as fraction 1A-1. Fraction 1A-1 contains approximately 24% water and 71% lactic acid, with the remainder being lactide and oligomers.

[0105] The bottom temperature was then maintained at 140℃, while the top condensate temperature was raised to 78℃. A vacuum of 1.2 kPa was applied, with a reflux ratio of 4:1. 61.7 g of distillate was collected, designated as fraction 1A-2. Fraction 1A-2 contained 74.62% L-lactide, 10.81% m-lactide, 0.47% D-lactide, 9.12% lactic acid, and 4.61% water.

[0106] The bottom temperature was then raised to 150℃, and the reflux ratio was maintained at 4:1. 403.9 g of distillate was collected, designated as fraction 1A-3. This fraction contained 92.03% L-lactide, 6.53% m-lactide, and 0.57% D-lactide. Lactic acid was below the HPLC detection limit (0.01%), and moisture was below the Karl Fischer detection limit (100 ppm). The total carboxyl content in lactic acid and oligomers was determined to be 63 mol / t using acid value titration. The first distillation was completed.

[0107] 400g of fraction 1A-3 was reloaded into the distillation apparatus and purged with nitrogen three times. The bottom of the column was then rapidly heated to 150°C, and the condensate temperature at the top was raised to 78°C. A vacuum of 1.2 kPa was applied, with a reflux ratio of 4:1. 11.6g of fraction was collected, designated as fraction 1A-4. Fraction 4 contained 81.97% L-lactide, 17.46% m-lactide, and 0.51% D-lactide. Lactic acid levels were below the HPLC detection limit (0.01%), and water content was below the Karl Fischer detection limit (100 ppm).

[0108] The reflux ratio was then maintained at 8:1, and 268.8 g of fraction was collected, designated as fraction 1A-5. Fraction 1A-5 contained 92.88% L-lactide, 6.46% m-lactide, and 0.58% D-lactide. Lactic acid was below the HPLC detection limit (0.01%), and moisture was below the Karl Fischer detection limit (100 ppm). Using acid value titration, the total carboxyl content was determined to be 4.6 mol / t. Fraction 5 meets the requirements for polymerization-grade lactide.

[0109] Separation of crude lactide 1B:

[0110] The distillation apparatus is a 1L glass distillation column with an inner diameter of 25mm. The packing material is glass spring packing with a packing section height of 1.2m. The distillation column is heated, with an initial heating temperature of 120℃, requiring preheating. The condensate in the top condenser is ethylene glycol monomethyl ether. Three interchangeable collection bottles are provided at the top of the column, each equipped with a piston to prevent cross-contamination.

[0111] 515.6 g of crude lactide 1B was placed in a distillation apparatus and purged with nitrogen three times. Initially, the temperature of the condensate at the top of the column was controlled at 58°C, and a vacuum of 1.2 kPa was applied. The bottom temperature was then rapidly raised to 140°C and subjected to total reflux for 10 minutes. After the top temperature stabilized, the reflux ratio was adjusted to 4:1. During this period, approximately 11.5 g of distillate was distilled off, which was designated as fraction 1B-1. Fraction 1B-1 contained 56.4% L-lactide, 31.55% m-lactide, 0.99% D-lactide, 7.71% lactic acid, and 4.89% water.

[0112] The bottom of the column was then rapidly heated to 150°C, and the condensate temperature at the top was raised to 72°C. A vacuum of 1.2 kPa was applied, with a reflux ratio of 10:1. 326 g of distillate was collected, designated as fraction 1B-2. Fraction 1B-2 contained 80.76% L-lactide, 17.71% m-lactide, and 1.42% D-lactide. Lactic acid content was below the HPLC detection limit (0.01%), and moisture content was below the Karl Fischer detection limit (100 ppm). Using acid value titration, the total carboxyl content in lactic acid and oligomers was determined to be 6.7 mol / t. With only one distillation, fraction 1B-2 met the requirements for polymer-grade lactide.

[0113] Separation of crude lactide 1C:

[0114] The distillation apparatus is a 1L glass distillation column with an inner diameter of 25mm. The packing material is glass spring packing with a packing section height of 1.2m. The distillation column is heated, with an initial heating temperature of 120℃, requiring preheating. The condensate in the top condenser is ethylene glycol monomethyl ether. Three interchangeable collection bottles are provided at the top of the column, each equipped with a piston to prevent cross-contamination.

[0115] 798g of crude lactide 1C was placed in a distillation apparatus and purged with nitrogen three times. Initially, the temperature of the condensate at the top of the column was controlled at 25°C, and a vacuum of 1.2 kPa was applied. The bottom temperature was then rapidly raised to 140°C and refluxed for 8 minutes. After the top temperature stabilized, the reflux ratio was reduced to total recovery. During this period, approximately 17g of distillate was distilled off, denoted as fraction 1C-1. Fraction 1C-1 contains approximately 24% water and 72% lactic acid, with the remainder being lactide and oligomers.

[0116] The bottom of the column was then rapidly heated to 150°C, and the condensate temperature at the top was raised to 78°C. A vacuum of 1.2 kPa was applied, with a reflux ratio of 1:1. 78 g of distillate was collected, designated as fraction 1C-2. Fraction 1C-2 contained 66.3% L-lactide, 19.11% m-lactide, 0.73% D-lactide, 9.42% lactic acid, and 4.11% water.

[0117] The reflux ratio was then changed to 4:1, and 542g of fraction was collected, designated as fraction 1C-3. This fraction contained 85.9% L-lactide, 12.13% m-lactide, and 0.94% D-lactide. Lactic acid was below the HPLC detection limit (0.01%), and moisture was below the Karl Fischer detection limit (100ppm). The total carboxyl content in lactic acid and oligomers was determined to be 63 mol / t using acid value titration. The first distillation was completed.

[0118] 533g of the rectified fraction 1C-3 was reloaded into the distillation apparatus and purged with nitrogen three times. The bottom of the column was then rapidly heated to 150°C, and the condensate temperature at the top was raised to 78°C. A vacuum of 1.2 kPa was applied, with a reflux ratio of 1:1. 10g of the fraction was collected and designated as rectified fraction 1C-4. Rectified fraction 1C-4 contained 69.62% L-lactide, 29.5% m-lactide, and 0.76% D-lactide. Lactic acid levels were below the HPLC detection limit (0.01%), and water content was below the Karl Fischer detection limit (100 ppm).

[0119] The reflux ratio was then changed to 8:1, and 347g of fraction was collected, designated as fraction 1C-5. Fraction 1C-5 contained 86.92% L-lactide, 12.03% m-lactide, and 0.95% D-lactide. Lactic acid was below the HPLC detection limit (0.01%), and moisture was below the Karl Fischer detection limit (100ppm). Using acid value titration, the total carboxyl content was determined to be 5.6 mol / t, and the moisture content was <100ppm (100ppm is the lower limit of equipment stability test results). Fraction 5 meets the requirements for polymerization-grade lactide.

[0120] As can be seen from the results in Table 1, the lactide in fraction 1A has higher optical purity (or lower m-lactide content) compared to fractions 1B and 1C. The results in Tables 2 and 3 are similar.

[0121] As shown in Table 1, fraction 1B contains less lactic acid and water than fraction 1C. Therefore, fraction 1B is easier to purify by distillation than fraction 1C. Tables 2 and 3 show similar results. Surprisingly, the content of other components (expected to be oligomers) in fraction 1A was not reduced compared to fractions 1B and 1C. This is likely because fraction 1A contains relatively more water and lactic acid, which caused some lactide hydrolysis during sample storage, resulting in oligomers. The distillation difficulty of fractions 1A and 1C is not significantly different.

[0122] As shown in Table 3, the contents of lactic acid and water in 4A and 4B after flash evaporation were reduced compared to 3A, especially in 4B where the contents of lactic acid and water were significantly reduced compared to 3A. Therefore, adding a flash evaporation step helps reduce the difficulty of subsequent distillation separation.

[0123] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A method for preparing lactide of different optical purities, characterized in that, The method includes: The lactic acid oligomer is depolymerized, wherein at least two splits are performed during the depolymerization process to obtain crude lactide with different contents of m-lactide. Optionally, the crude lactide with different contents of m-lactide is purified separately.

2. The method according to claim 1, wherein, The depolymerization process involves at least two splits to obtain crude lactide with an increased m-lactide content.

3. The method according to claim 2, wherein, When the conversion rate of lactic acid oligomers is not higher than 50 wt%, the first split is performed to obtain crude lactide with an m-lactide content not higher than 8 wt%.

4. The method according to claim 3, wherein, When the conversion rate of lactic acid oligomers is 20-50 wt%, the first split is performed to obtain crude lactide with an m-lactide content of 4-8 wt%.

5. The method according to claim 3, wherein, When the conversion rate of lactic acid oligomers is less than 20 wt%, the first split is performed to obtain crude lactide with an m-lactide content of no more than 4 wt%.

6. The method according to claim 2, wherein, The depolymerization process involves 2-5 splits.

7. The method according to any one of claims 1-6, wherein, The depolymerization process involves two splits. The first split occurs when the conversion rate of lactic acid oligomers is no higher than 50 wt%, and the second split occurs when the conversion rate of lactic acid oligomers is higher than 50 wt% but no higher than 100 wt%.

8. The method according to any one of claims 1-6, wherein, The depolymerization process involves three splits: the first split occurs when the conversion rate of lactic acid oligomers is no higher than 20 wt%; the second split occurs when the conversion rate of lactic acid oligomers is higher than 20 wt% but no higher than 50 wt%; and the third split occurs when the conversion rate of lactic acid oligomers is higher than 50 wt% but no higher than 100 wt%.

9. The method according to any one of claims 1-6, wherein, Each depolymerization condition corresponding to each split independently includes: a temperature of 195-240℃; and / or a pressure of 133-13000Pa.

10. The method according to any one of claims 1-6, wherein The method for preparing the lactic acid oligomer includes: Lactic acid undergoes dehydration polymerization.

11. The method according to claim 10, wherein, The lactic acid is selected from L-lactic acid or D-lactic acid; and / or The optical purity of the lactic acid is >99%.

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