Preparation method for lactide, and direct lactide and depolymerized lactide

Abstract

The present invention provides a preparation method for lactide, and direct lactide and depolymerized lactide. The preparation method comprises: carrying out a prepolymerization reaction on lactic acid to obtain a prepolymerized product; demonomerizing the prepolymerized product; the components separated from the prepolymerized product after demonomerization including direct lactide; and carrying out a depolymerization reaction on the remaining prepolymerized product after demonomerization to obtain depolymerized lactide. The preparation method of the present invention involves a simple process, and lactide having high optical purity can be obtained without melt crystallization.

Description

A method for preparing lactide and direct lactide and depolymerized lactide Technical Field

[0001] This invention relates to the field of organic chemical technology, and more specifically, to a method for preparing lactide and direct lactide and depolymerized lactide. 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 uniquely 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 through oligomerization and depolymerization of lactide, and then carry out ring-opening polymerization of lactide as a monomer to obtain PLA. This method can obtain 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 PLA, the synthesis process and key equipment of lactide are a major research focus in the industry. Currently, the industrial synthesis of lactide generally involves the pyrolysis and cyclization reaction of polylactic acid with a certain degree of polymerization in the presence of a catalyst to obtain lactide products, as disclosed in technologies such as US 5247058, US 5338822, US 5521278, and EP 98203427.4.

[0005] To prepare high-quality PLA, it is essential to use high-purity lactide. Therefore, lactide needs to be purified to a high degree before it can undergo ring-opening polymerization to produce high-quality PLA. Various lactide purification processes are known, which typically include one or more integrated distillation, condensation, and melt crystallization steps, as described in US 5521278, US 5357034, and US 5214159.

[0006] Racemic reaction refers to the inversion of a chiral carbon atom from the L-configuration to the D-configuration, or vice versa. Commercially available PLA primarily uses L-lactic acid as a raw material, corresponding to the 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, maintaining its original L-configuration, the resulting lactide is L-lactide, the primarily desired product. When one of the two chiral carbon atoms races, the resulting lactide is meso-lactide (m-lactide), named so because the chiralities of the two chiral carbons are exactly opposite. This renders the entire molecule optically inactive. Racemic reactions that produce m-lactide occur during the oligomerization-depolymerization process in lactide synthesis. Currently, even when using optically pure L-lactic acid (typically requiring an L-lactic acid content >99.5%) as raw material and lactide as an intermediate to produce PLA, a large amount of m-lactide is inevitably generated during the lactide production process due to racemic reactions. However, current applications involving L-lactide typically require very high optical purity. To obtain L-lactide of such purity, m-lactide needs to be separated to a very high degree.

[0007] Known methods for separating m-lactide typically employ distillation to obtain m-lactide-rich and L-lactide-rich streams. For example, the method in CN105324164A uses distillation to separate the feed, yielding a feed containing 2.89% m-lactide and 88.28% L-lactide. A stream with 24.96% m-lactide is obtained at the top of the column, and a stream with 2.5% m-lactide is obtained in the side stream. However, because m-lactide and L-lactide have the same molecular weight and differ only in the chirality of one carbon atom, their separation is extremely difficult and energy-intensive.

[0008] If it is necessary to further improve the optical purity of lactide, or in other words, reduce the content of m-lactide, melt crystallization is usually used. For example, in CN105324164A, a stream with an m-lactide content of 2.5% and an L-lactide content of 97.27% obtained by distillation is fed into a melt crystallization unit and purified by three consecutive falling film crystallizations, finally yielding a product containing 99.83% L-lactide and 0.14% m-lactide.

[0009] In fact, crystallization is the only known and industrially feasible method for obtaining high optical purity lactide from a mixture of m-lactide and L-lactide, specifically, a lactide content >99% for L-lactide and <1% for m-lactide. The main differences between existing technologies lie in the different crystallizers and crystallization conditions. However, crystallization methods suffer from high energy consumption and limited yield.

[0010] Therefore, it is necessary to develop a simpler and more efficient method for obtaining high-purity lactide. Summary of the Invention

[0011] To address the problems in the prior art, the present invention aims to provide a method for preparing lactide and the product thereof. The preparation method of the present invention is simple, does not require melt crystallization, and can yield lactide with high optical purity.

[0012] To achieve the above objectives, according to a first aspect of the present invention, a method for preparing lactide is provided, comprising:

[0013] Lactic acid is subjected to a prepolymerization reaction to obtain a prepolymer product, which includes lactic acid oligomers and first-batch lactide, and the first-batch lactide is referred to as direct lactide.

[0014] The prepolymer product is subjected to a single-polymer removal process;

[0015] The component separated from the prepolymer product after the demonopolymerization process is referred to as the demonopolymerized component, which contains the direct lactide.

[0016] The remaining prepolymer product after the depolymerization process is referred to as the remaining component. The remaining component is subjected to a depolymerization reaction to obtain a second batch of lactide, which is referred to as depolymerized lactide.

[0017] This invention first discovers that during the process of lactic acid oligomerization into lactic acid oligomers, a small amount of lactic acid molecules undergo a dimerization cyclization reaction, directly yielding a small amount of lactide, namely the aforementioned direct lactide. This invention also surprisingly discovers that when optically pure lactic acid is used as a raw material, for example, L-lactic acid with an L-lactic acid content >99.5% (it should be noted that using D-lactic acid as a raw material has the same effect), the optical purity of the aforementioned direct lactide is often higher than that of the lactide obtained through subsequent depolymerization steps. In other words, the proportion of m-lactide (meta-lactide) in the direct lactide that appears as a byproduct in the lactic acid prepolymerization stage is relatively small.

[0018] Unfortunately, in traditional lactide synthesis processes, lactic acid prepolymers containing high-optical-purity lactide (i.e., direct lactide) are fed into a depolymerization step. Because a large amount of lactide is generated in this depolymerization step, and numerous racemic reactions occur, the resulting lactide is of low optical purity. Furthermore, this low-optical-purity lactide is collected together with the direct lactide, leading to a low optical purity in the final product. In existing technologies, the aforementioned direct lactide has not been adequately addressed or separated.

[0019] Another possible reason why the aforementioned direct lactide has not received sufficient attention and has not been isolated is that lactide is usually removed from lactic acid prepolymers by vaporization under high temperature and high vacuum conditions. However, high temperature and high vacuum are also conditions for the depolymerization of lactic acid prepolymers. Therefore, the commonly used temperature and pressure conditions may cause oligomers to depolymerize and continuously generate new lactide while removing the aforementioned direct lactide. Consequently, researchers have not been able to notice that the aforementioned direct lactide has higher optical purity, and even if they have, they lack effective separation methods.

[0020] Therefore, simply finding that direct lactide has high optical purity is not enough to obtain lactide products with high optical purity. Existing technologies require further effective methods to separate direct lactide while simultaneously inhibiting the depolymerization of lactic acid oligomers.

[0021] This invention, through extensive and precise experiments, demonstrates that direct lactide can be effectively separated from lactic acid prepolymers without causing depolymerization of lactic acid oligomers. Since this separation involves removing existing lactide monomers from the lactic acid prepolymer without significantly or completely removing oligomers, this step is referred to as "monomer removal treatment" in this invention. It is important to note that the so-called "monomer removal treatment" in this invention refers to the removal of lactide as a polylactic acid monomer, not the removal of lactic acid, as lactic acid is not directly used as a monomer in the synthesis of polylactic acid.

[0022] The preparation method of the present invention starts from source control, preferentially separating direct lactide with high optical purity, thus overcoming the defects in the prior art.

[0023] Depolymerized lactide is a common type of lactide with low optical purity in the prior art. Numerous patents describe the process of this depolymerization, so it will not be repeated here. For information on equipment, depolymerization temperature, and pressure control, please refer to the relevant descriptions in Example 7 of CN112898266A and other known patent literature.

[0024] In some preferred embodiments of the present invention, the temperature of the depolymerization treatment is 180–200°C, and the absolute pressure is 500–3500 Pa. The depolymerization treatment of the present invention, through the control of temperature and pressure, allows direct lactide to be removed without oligomer depolymerization and racemic reactions. A series of experiments conducted in this invention have shown that depolymerization reactions (because they are chemical processes involving the breaking and rejoining of chemical bonds) require relatively high temperatures. Therefore, it is preferable to limit the temperature of the depolymerization treatment within the above-mentioned range, which is more conducive to the removal of direct lactide without oligomer depolymerization and racemic reactions.

[0025] In some preferred embodiments of the present invention, the depolymerization treatment temperature is 185–195°C, and the absolute pressure is 800–2500 Pa. A series of experiments conducted by the present invention show that depolymerization occurs more significantly above 195°C; therefore, it is further preferred to limit the depolymerization treatment temperature to 185–195°C. However, on the other hand, the depolymerization treatment temperature cannot be too low, because at lower temperatures the vapor pressure of lactide is lower, making effective removal difficult.

[0026] In some preferred embodiments of the present invention, the temperature of the prepolymerization reaction is not higher than 195°C. Preferably, controlling the prepolymerization temperature within the above-mentioned range is more conducive to suppressing the formation of m-lactide during the prepolymerization stage.

[0027] More preferably, the temperature of the prepolymerization reaction is not higher than 190°C.

[0028] More preferably, the temperature of the prepolymerization reaction is not higher than 185°C.

[0029] When the oligomerization stage reaction temperature is controlled below 190°C, direct lactide contains almost no m-lactide (<1%). Even when the prepolymerization stage temperature is increased to 195°C, the m-lactide content in direct lactide remains below 4%.

[0030] Lactic acid oligomers can be prepared according to known information, for example, according to US5142023. The lactic acid oligomers used in this invention are prepared through the following steps:

[0031] L-lactic acid or D-lactic acid is mixed with a catalyst (e.g., stannous octoate) at a mass ratio of 100:0.1-1 (e.g., 100:0.2), and then purged with nitrogen.

[0032] The pressure is reduced to 10-60 kPa (e.g., 20 kPa), and the temperature is gradually increased to 100-195℃ (preferably 180-195℃) to separate the fraction (mainly water). The pressure is gradually reduced and the temperature is increased according to the fraction extraction requirements to obtain lactic acid oligomers.

[0033] Other process conditions for prepolymerizing lactic acid to obtain lactic acid oligomers are well known to those skilled in the art. Apart from the temperature control mentioned above, other preparation conditions can be easily designed by those skilled in the art based on common knowledge, and will not be elaborated here.

[0034] In some preferred embodiments of the present invention, the demonomeric component accounts for no more than 12% by weight of the lactic acid oligomer.

[0035] More preferably, the demonopolymer component accounts for no more than 10% of the weight percentage of the prepolymer product.

[0036] More preferably, the de-monomer component accounts for no more than 8% by weight of the prepolymer product.

[0037] Controlling the yield during the depolymerization process is more conducive to avoiding depolymerization and obtaining direct lactide with high optical purity.

[0038] In some preferred embodiments of the invention, the direct lactide accounts for no more than 10% by weight of the prepolymer. Another key control point of the invention is the amount of direct lactide removed. Blindly pursuing high yields in the lactic acid oligomerization step may result in the depolymerization of lactic acid oligomers to form low-light-purity lactide. In some applications, the final required optical purity of the lactide is 96%, therefore, a small amount of low-light-purity lactide mixed with direct lactide is acceptable.

[0039] More preferably, the direct lactide accounts for no more than 8% of the weight of the prepolymer.

[0040] More preferably, the direct lactide accounts for no more than 6% of the weight of the prepolymer.

[0041] In some preferred embodiments of the present invention, the percentage of m-lactide in the de-monolithic component is no more than 4% of the total lactide.

[0042] More preferably, in the de-monolithic component, the weight percentage of m-lactide is not higher than 2%.

[0043] More preferably, in the de-monolithic component, the weight percentage of m-lactide is not higher than 1%.

[0044] Controlling m-lactide within the above-mentioned range is more conducive to improving the optical strength of the product.

[0045] In actual production, the depolymerized components usually also contain water and lactic acid. In some preferred embodiments of the present invention, before the depolymerization treatment, the preparation method further includes: flash evaporation treatment of the prepolymerized product, wherein the flash evaporation conditions include: temperature of 120-170°C, time of 1-60 s, and pressure of 1-20 kPa.

[0046] More preferably, the flash evaporation conditions include: a temperature of 160-170°C, a time of 1-15 seconds, and a pressure of 4-10 kPa.

[0047] Prepolymers typically contain water and lactic acid. Therefore, flash evaporation can be performed before depolymerization to remove some of the water and lactic acid, reducing the difficulty of subsequent separation and purification. Compared to depolymerization, the flash evaporation process of this invention is carried out at a lower temperature and a higher absolute pressure. Its purpose is to selectively remove water and lactic acid without removing lactide. Preferably, flash evaporation is performed 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.

[0048] Since flash evaporation requires additional equipment investment and water, lactic acid, and lactide are easily separated, flash evaporation may be omitted if subsequent separation processes allow.

[0049] It will be readily understood by those skilled in the art that the aforementioned demonomeric components and depolymerized lactide are obtained in the form of mixtures, which typically also include lactic acid, water, and oligomers (mainly dimers and trimers). Therefore, in some preferred embodiments of the present invention, the demonomeric components and depolymerized lactide are purified separately to obtain polymer-grade lactide.

[0050] Methods for purifying crude products such as deisomeric components and depolylactide have been extensively discussed in the prior art, and purification can typically be achieved using methods including distillation. In some cases, recrystallization, melt crystallization, and coupling separation can also be used for purification, but these methods are well known to have drawbacks such as high energy consumption and limited yield. Therefore, in practical applications, purification is preferably carried out by distillation.

[0051] Direct lactide has a low demonopolymerization temperature, a small removal rate, and contains relatively few oligomers, but more water and lactic acid. Therefore, it can be purified by washing and drying. This purification method is widely known to those skilled in the art and will not be described in detail here.

[0052] In some preferred embodiments of the present invention, the temperature of the depolymerization reaction is 195–240°C and the pressure is 133–13000 Pa absolute pressure.

[0053] Another surprising and unexpected benefit is that the preparation method of the present invention performs a depolymerization process before the traditional depolymerization process, or performs a flash evaporation and depolymerization process. Therefore, the water and lactic acid content in the crude depolymerized lactide product obtained by the present invention is reduced, which further reduces the difficulty of subsequent purification (e.g., distillation separation purification).

[0054] According to another aspect of the present invention, a direct lactide obtained according to the above preparation method is also provided.

[0055] In some preferred embodiments of the present invention, the mass percentage of m-lactide in the direct lactide is ≤4%. Preferably, the mass percentage of m-lactide in the direct lactide is ≤2%, and more preferably ≤1%. The mass percentage of m-lactide refers to m-lactide / (m-lactide + L-lactide + D-lactide), excluding lactic acid, water, oligomers, etc.

[0056] According to another aspect of the present invention, a depolymerized lactide obtained according to the above preparation method is also provided.

[0057] Compared with existing technologies, this invention provides a new method for preparing lactide. This method starts from the source control and avoids the awkward situation of mixing and then separating high-optical-purity lactide and low-optical-purity lactide. This invention can obtain high-optical-purity lactide using a relatively simple de-monolithography process without the need for a melt crystallization process. Furthermore, the method provided by this invention has the advantages of simple operation and the ability to utilize existing equipment, and has broad application prospects and huge economic benefits. Attached Figure Description

[0058] Figure 1 shows the NMR spectrum of lactic acid prepolymer 1 in Preparation Example 1. Detailed Implementation

[0059] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0060] In the following examples and comparative examples, some of the raw materials were sourced as follows:

[0061] L-lactic acid with an optical purity >99% and a chemical purity >99% was purchased from Anhui Gelat Biochemical Co., Ltd.

[0062] D-lactic acid with optical purity >99% and chemical purity >99% was purchased from Sinopharm Reagent.

[0063] The catalyst, stannous octoate, was purchased from Aladdin, reagent grade.

[0064] Unless otherwise specified, all other raw materials used in the examples or comparative examples are commercially available.

[0065] In the following examples and comparative examples, the NMR test solvent was deuterated chloroform.

[0066] The percentages of L-lactide, D-lactide, and m-lactide were determined by gas chromatography (GC). Standards were used.

[0067] Quasi-curve correction. The chromatographic column and conditions are shown in Table 1 below:

[0068] Table 1

[0069] When the main component is L-lactide, the optical purity = (L-lactide + m-lactide / 2) / (L-lactide + m-lactide + D-lactide) × 100%.

[0070] When the main component is D-lactide, the optical purity = (D-lactide + m-lactide / 2) / (L-lactide + m-lactide + D-lactide) × 100%.

[0071] The moisture content was determined by Karl Fischer titration.

[0072] The residual lactic acid in the fraction 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 a 0.02 mol / L aqueous solution of phosphoric acid in phase A and acetonitrile in phase B, with a mobile phase gradient of 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. The sample was calibrated using a standard curve.

[0073] Unless otherwise specified, all percentages referred to below are calculated by mass.

[0074] Preparation Example 1

[0075] This preparation example provides a method for preparing lactic acid oligomers from lactic acid, comprising:

[0076] 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 from 95°C, and the temperature was gradually and slowly increased to 190°C. The pressure was then reduced from 20kPa to 10kPa within 5 minutes and maintained at 10kPa for about 1 hour. A total of 620g of fraction (mainly water) was collected, and prepolymer product 1 was obtained in the glass reactor.

[0077] Sampling and testing of prepolymer product 1 1 The 1-2 ppm range of H NMR is shown in Figure 1. As can be seen from Figure 1, a double peak was observed at 1.67 ppm, which was confirmed to be L-lactide; no peak corresponding to m-lactide was clearly observed at 1.72 ppm.

[0078] Preparation Example 2

[0079] This preparation example provides a method for preparing lactic acid oligomers from lactic acid, comprising:

[0080] 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. The pressure was then reduced to 20kPa, and the temperature was gradually increased. Liquid condensation was observed in the condenser starting from 95℃, and the temperature was gradually and slowly increased to 195℃. The pressure was then 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 prepolymer product 2 was obtained in the glass reactor.

[0081] Preparation Example 3

[0082] This preparation example provides a method for preparing lactic acid oligomers from lactic acid, comprising:

[0083] 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 from 95°C, and the temperature was gradually and slowly increased to 180°C. The pressure was then reduced from 20kPa to 10kPa within 5 minutes and maintained at 10kPa for about 3 hours. A total of 618g of fraction (mainly water) was collected, and prepolymer 3 was obtained in the glass reactor.

[0084] Example 1

[0085] This embodiment provides a method for preparing lactide, which includes the following steps:

[0086] 696.2g of prepolymer product 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 70℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle.

[0087] First, the prepolymer product 1 is heated to 195°C, and then the pressure is rapidly reduced to 2.5 kPa to start the demonization step, which takes about 15 minutes, until there is no obvious distillate in the condenser. 47.86 g of demonized component is obtained. The demonized component accounts for about 6.9% of the mass of the prepolymer product 1 and is denoted as fraction 1A.

[0088] Then, the vacuum was stopped, nitrogen was added to bring the pressure to atmospheric pressure, and the collection bottle was replaced. The remaining prepolymer product was further heated to 215°C and the pressure was reduced to 2.5 kPa to carry out the depolymerization step. The depolymerization step lasted for about 2 hours, until the temperature control thermocouple in the bottle was about to detach from the liquid surface. It was observed that the residue in the bottle turned yellow and the viscosity increased. 514.48 g of the depolymerized product obtained in the collection bottle accounted for about 73.9% of the mass of the prepolymer product 1, and was recorded as fraction 1B.

[0089] Fractions 1A and 1B were weighed and their components were analyzed, and the results are shown in Table 2. The proportions of other components were obtained by subtracting the proportions of four components, including L-lactide, from 100%.

[0090] Examples of further purification are as follows:

[0091] The distillation unit is a 1L glass distillation column with an inner diameter of 25mm. The packing is glass spring packing with a packing section height of 1.2m. The distillation column is heated with an initial heating temperature of 120℃ and requires preheating. The condensate in the top condenser is ethylene glycol monomethyl ether. Three switchable collection bottles are provided at the top of the column, each with a piston to prevent cross-contamination.

[0092] 512.2g of fraction 1B was placed into a distillation apparatus and purged with nitrogen three times. Then, the temperature of the condensate at the top of the column was controlled at 25°C, the vacuum degree at 1.2kPa, and the reflux ratio at full recovery. The bottom of the column was rapidly heated to about 100°C. During this period, about 9.1g of fraction was distilled off, which was recorded as fraction 1 (it is estimated that it did not condense completely). The main components of fraction 1 were water and lactic acid.

[0093] Then the bottom of the column is rapidly heated to 150°C, the temperature of the condensate at the top of the column is raised to 88°C, the vacuum degree is 1.2 kPa, the reflux ratio is 1:1, and 24.2 g of fraction is collected, which is recorded as rectification fraction 2 (it is estimated that it was not completely condensed); rectification fraction 2 is a mixture of m-lactide, L-lactide, water, and lactic acid.

[0094] The reflux ratio was then changed to 5:1, and 387.06 g of fraction was collected, designated as fraction 3. GC analysis showed that the fraction contained 85.1% L-lactide, 12.93% m-lactide, and 1.0% D-lactide. The lactic acid content was below the detection limit (0.01%). The moisture content was <100 ppm (100 ppm is the lower limit of the equipment stability test result). Since the lactic acid content in fraction 3 was difficult to determine accurately using chromatography, acid value titration was used to determine the total carboxyl content in lactic acid and oligomers to be 80 mol / t; the first distillation was completed.

[0095] 380g of distillate fraction 3 was reloaded into the distillation apparatus and purged with nitrogen three times. Then, the bottom of the column was rapidly heated to 135°C, the temperature of the condensate at the top of the column was raised to 88°C, the vacuum degree was 1.0 kPa, the reflux ratio was 1:1, and 14g of fraction was collected, which was recorded as distillate fraction 4. Distillate fraction 4 was a mixture of m-lactide, L-lactide, and trace amounts of lactic acid and oligomers.

[0096] The reflux ratio was then changed to 5:1, and 300g of fraction was collected, designated as fraction 5. GC analysis showed that the fraction contained 85.82% L-lactide, 13.04% m-lactide, and 1.01% D-lactide. The oligomer content was estimated to be <0.2wt% using the difference method. Acid value titration was used to determine the total carboxyl content to be 6.2mol / t, and the moisture content to be <100ppm (100ppm is the lower limit of the equipment stability test result). Fraction 5 met the requirements for polymer-grade lactide.

[0097] Example 2

[0098] This embodiment provides a method for preparing lactide, which includes the following steps:

[0099] 777.1g of prepolymer product 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 70℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle.

[0100] First, the prepolymer product 2 is heated to 200°C, and then the pressure is rapidly reduced to 1.2 kPa to start the demonopolymerization step, which takes about 15 minutes, yielding 87.35 g of demonopolymerized component. The demonopolymerized component accounts for about 11.3% of the mass of the prepolymer product 2 and is denoted as fraction 2A.

[0101] Then, the vacuum was quickly stopped, nitrogen was added to bring the pressure back to normal, and the collection bottle was replaced. The remaining prepolymer product was then heated to 230°C, and the pressure was reduced to 8.6 kPa to carry out the depolymerization step, while the pressure was gradually reduced to 3.5 kPa. The depolymerization step lasted for about 0.5 hours, until the temperature control 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. 553.11 g of the depolymerized product obtained in the collection bottle accounted for about 71.6% of the mass of the prepolymer product 2, and was recorded as fraction 2B.

[0102] Fractions 2A and 2B were weighed and their components were analyzed, and the results are shown in Table 3. The proportions of other components were obtained by subtracting the proportions of four components, including D-lactide, from 100%.

[0103] Example 3

[0104] This embodiment provides a method for preparing lactide, which includes the following steps:

[0105] 707.8g of prepolymer product 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 70℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle.

[0106] First, the prepolymer product 3 is heated to 180°C, and then the pressure is rapidly reduced to 0.5 kPa to start the demonization step, which takes about 45 minutes, until there is no obvious distillate in the condenser. 41.04 g of demonized component is obtained. The demonized component accounts for about 5.8% of the mass of the prepolymer product 3 and is denoted as fraction 3A.

[0107] Then, the vacuum was stopped, nitrogen was added to bring the pressure to atmospheric pressure, and the collection bottle was replaced. The remaining prepolymer product was further heated to 195°C and the pressure was reduced to 0.5 kPa to carry out the depolymerization step. The depolymerization step lasted for about 3 hours, until the temperature control thermocouple in the bottle was about to detach from the liquid surface. It was observed that the residue in the bottle turned yellow and the viscosity increased. 512.46 g of the depolymerized product obtained in the collection bottle accounted for about 72.4% of the mass of the prepolymer product 3, and was recorded as fraction 3B.

[0108] Fractions 3A and 3B were weighed and their components were analyzed, and the results are shown in Table 4. The proportions of other components were obtained by subtracting the proportions of four components, including L-lactide, from 100%.

[0109] Example 4

[0110] This embodiment provides a method for preparing lactide, which involves flash evaporation before demonstration, including the following steps:

[0111] 968.4 g of prepolymer product 3 was flash-treated using a falling film evaporator. The feed temperature was 170℃, the main evaporator temperature was 170℃, and the pressure was 4.5 kPa. The scraper speed was 400 rpm, and the scraper area was 0.1 m². 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 prepolymerized product after flash evaporation is used for subsequent depolymerization and demonopolymerization.

[0112] 701g of the flash-distilled prepolymer product 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 70℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle.

[0113] First, the prepolymer product 3 after flash evaporation is heated to 195°C, and then the pressure is rapidly reduced to 1.8 kPa to start the demonopolymerization step, which takes about 30 minutes, and 44.35 g of demonopolymerized component is obtained. The demonopolymerized component accounts for about 6.3% of the mass of the prepolymer product 3 after flash evaporation, and is referred to as fraction 4A.

[0114] Then, the vacuum was stopped, nitrogen was added to bring the pressure to atmospheric pressure, and the collection bottle was replaced. The remaining prepolymer product was further heated to 205°C and the pressure was reduced to 1.2 kPa to carry out the depolymerization step. The depolymerization step lasted for about 2 hours, until the temperature control thermocouple in the bottle was about to detach from the liquid surface. It was observed that the residue in the bottle turned yellow and the viscosity increased. 517.86 g of the depolymerized product obtained in the collection bottle accounted for about 73.9% of the mass percentage of the flash-evaporated prepolymer product 3, and was recorded as fraction 4B.

[0115] Fractions 4A and 4B were weighed and their components were analyzed, and the results are shown in Table 4. The proportions of other components were obtained by subtracting the proportions of four components, including L-lactide, from 100%.

[0116] Comparative Example 1

[0117] This comparative example provides a method for preparing lactide, which includes the following steps:

[0118] Place 681.9g of prepolymer product 1 into a 1L glass bottle, connect the glass bottle to the distillation head, condenser and collection bottle; use 70℃ circulating heat transfer oil to control the temperature of the condenser; connect the end of the collection bottle to a vacuum pump.

[0119] The prepolymer product 1 was heated to 215°C and the pressure was reduced to 2.5 kPa to carry out the depolymerization step. The depolymerization step lasted for about 2 hours, until the temperature control thermocouple in the bottle was about to detach from the liquid surface. The residue in the bottle was observed to turn yellow and the viscosity increased. 560.54 g of the depolymerization product was collected, which accounted for about 82.2% of the mass of the prepolymer product 1, and was recorded as fraction 1C.

[0120] The fraction 1C was weighed and its components were tested. The results are shown in Table 2.

[0121] Comparative Example 2

[0122] This comparative example provides a method for preparing lactide, which includes the following steps:

[0123] 781.3g of prepolymer product 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 70℃ circulating heat transfer oil. A vacuum pump was connected to the end of the collection bottle.

[0124] The prepolymer product 2 was heated to 230°C and the pressure was reduced to 8.6 kPa to carry out the depolymerization step, and the pressure was gradually reduced to 3.5 kPa. The depolymerization step lasted for about 0.5 hours, until the temperature control thermocouple in the bottle was about to detach from the liquid surface. The residue in the bottle was observed to turn yellow and the viscosity increased. 635.22 g of the depolymerization product was collected, accounting for about 81.3% of the mass of the prepolymer product 2, and was recorded as fraction 2C.

[0125] The fraction 2C was weighed and its components were tested. The results are shown in Table 3.

[0126] Comparative Example 3

[0127] This comparative example provides a method for preparing lactide, which includes the following steps:

[0128] Place 693.7g of prepolymer product 3 into a 1L glass bottle, connect the glass bottle to the distillation head, condenser and collection bottle; use 70℃ circulating heat transfer oil to control the temperature of the condenser; connect the end of the collection bottle to a vacuum pump;

[0129] The prepolymer product 3 was heated to 195°C and the pressure was reduced to 0.5 kPa to carry out the depolymerization step. The depolymerization step lasted for about 4 hours, until the temperature control thermocouple in the bottle was about to detach from the liquid surface. The residue in the bottle was observed to turn yellow and the viscosity increased. 550.0 g of the depolymerized product was collected, accounting for about 79.3% of the mass of the prepolymer product 3, and was recorded as fraction 3C. Fraction 3C was weighed and its components were tested. The results are shown in Table 4.

[0130] Table 2

[0131] Table 3

[0132] Table 4

[0133] As can be seen from the results in Table 2, lactide in fraction 1A has high optical purity. The results in Tables 3 and 4 are similar.

[0134] As shown in Table 2, fraction 1B contains less lactic acid and water than fraction 1C, therefore fraction 1B is easier to purify by distillation than fraction 1C. Tables 3 and 4 show similar results.

[0135] As can be seen from the results in Table 4, the contents of lactic acid and water in fraction 4A after flash evaporation were significantly reduced compared to fraction 3A without flash evaporation, and the contents of fraction 4B were significantly reduced compared to fraction 3B. This indicates that adding the flash evaporation step helps to reduce the difficulty of subsequent distillation separation.

[0136] The above embodiments are provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing lactide, wherein, include: Lactic acid is subjected to a prepolymerization reaction to obtain a prepolymer product, which includes lactic acid oligomers and first-batch lactide, and the first-batch lactide is referred to as direct lactide. The prepolymer product is subjected to a single-polymer removal process; The component separated from the prepolymer product after the demonopolymerization process is referred to as the demonopolymerized component, which contains the direct lactide. The remaining prepolymer product after the depolymerization process is referred to as the remaining component. The remaining component is subjected to a depolymerization reaction to obtain a second batch of lactide, which is referred to as depolymerized lactide.

2. The preparation method according to claim 1, wherein, The temperature for the single-person removal process is 180–200℃, and the absolute pressure is 500–3500Pa.

3. The preparation method according to claim 2, wherein, The temperature for the single-single treatment is 185–195°C, and the absolute pressure is 800–2500 Pa.

4. The production method according to claim 1, wherein The temperature of the prepolymerization reaction is not higher than 195°C.

5. The preparation method according to claim 1, wherein, The weight percentage of the de-monomer component in the prepolymer product is no more than 12%.

6. The production method according to claim 1, wherein The direct lactide accounts for no more than 10% of the weight of the prepolymer.

7. The production method according to claim 1, wherein In the single-derived component, m-lactide accounts for no more than 4% of the total lactide.

8. The preparation method according to claim 1, wherein, Prior to the aforementioned single-single treatment, the preparation method further includes: The prepolymer product is subjected to flash evaporation treatment at a temperature of 120–170°C for 1–60 seconds and a pressure of 1–20 kPa.

9. The preparation method according to claim 1, wherein, The depolymerization reaction occurs at a temperature of 195–240°C and a pressure of 133–13000 Pa.

10. A direct lactide, wherein, The direct lactide is obtained by the preparation method according to any one of claims 1 to 9.

11. The direct propylene lactone of claim 10, wherein, In the direct lactide, the mass percentage of m-lactide is ≤4%.

12. A depolymerization of lactide, wherein, The depolymerized lactide is obtained by the preparation method according to any one of claims 1 to 9.

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