A continuous method for preparing glycolide and the glycolide obtained therefrom

Abstract

This invention discloses a continuous method for preparing glycolide and the obtained glycolide. The method includes a step of depolymerizing glycolic acid oligomers in the presence of inorganic phosphorus to obtain glycolide. This method solves the problem that during bulk depolymerization, the depolymerization reaction easily produces decarboxylation side reactions, generating formic acid and acetic acid byproducts, leading to a decrease in glycolide yield, purity, and an increase in acid value. Using the method of this invention, the glycolide yield is >95.5%, and the glycolide specifications are: purity >95.0%, acid value <300 μmol / g, formic acid content <10 μg / g, and acetic acid content <20 μg / g, achieving good technical results.

Description

Technical Field

[0001] This invention belongs to the field of glycolide preparation, and particularly relates to a continuous method for preparing glycolide and the glycolide obtained therefrom. Background Technology

[0002] Polyglycolic acid (PEG), an aliphatic polyester, possesses high biodegradability and good biocompatibility. It can be hydrolyzed in organisms and metabolized by microorganisms in natural environments, ultimately decomposing into water and carbon dioxide. Furthermore, PEG exhibits good mechanical properties such as heat resistance and tensile strength, and demonstrates excellent gas barrier properties when used in films and sheets. Therefore, PEG is expected to replace commonly used biodegradable polymers in medical polymer materials, agricultural resource materials, and various packaging or container materials. PEG has already found applications in surgical sutures, artificial skin and blood vessels, bone fixation and repair, controlled drug release, and tissue engineering.

[0003] Glycolide is a compound with a cyclic dimer structure formed by removing two water molecules from two molecules of glycolic acid. Glycolide is not only a monomer for synthesizing polyglycolide, but it can also be copolymerized with other cyclic monomers through ring-opening to obtain random copolymers or block copolymers. Typically, the synthesis of glycolide monomers involves two steps: first, the condensation polymerization of glycolic acid to form a low molecular weight polymer, called glycolic acid oligomer; second, the glycolic acid oligomer is depolymerized by high-temperature distillation to extract the gaseous crude glycolide product from the reaction system.

[0004]

[0005] Depolymerization can occur in the presence of a solvent (solution depolymerization) or in the absence of a solvent (bulk depolymerization). The key to controlling the quality of glycolide lies in two aspects: the selection of the depolymerization reactor and the effective control of impurities through the depolymerization process.

[0006] Among the published patents on bulk depolymerization, US5347020 discloses a method for preparing and purifying heat-sensitive compounds: glycolide is prepared from low molecular weight polyglycolic acid or haloacetates through depolymerization in the presence of a catalyst. The depolymerization reaction is carried out in a vacuum-capable, self-cleaning twin-screw extruder equipped with an independently controllable heating section. The reactants undergo continuous depolymerization at a temperature of 15–280°C via forced conveying. Although screw extruders are the conventional reactor type used in bulk depolymerization, many related patents in recent years have optimized them by focusing on temperature control and the placement of vacuum vents. CN205575974U optimizes the placement of vacuum vents, limiting the length-to-diameter ratio of the screw extruder to 25–110, and providing at least one vacuum vent at intervals from the top of the screw at length-to-diameter ratios of 2–20. CN109438411A optimizes the location of the feed hopper and the vacuum port. The twin-screw extruder includes a first zone, a second zone and a third zone connected in sequence. The raw material feed hopper is located at the front end of the first zone, and the catalyst feed hopper is located at the connection between the first zone and the second zone. The connection between the second zone and the third zone is provided with a vacuum port that communicates with the product collection system.

[0007] WO1993018019A1 describes heating oligomers under reduced pressure at temperatures below the decomposition temperature of cyclic esters, in the absence of a gaseous entrainer stream of cyclic esters, wherein the improvement includes the following steps: (a) providing a multi-stage depolymerization column having a heating device, an oligomer feed inlet near the top of the column, a vapor product stream outlet above the oligomer feed inlet, and a receiving container at the bottom of the column; (b) feeding oligomers into the top of the column; (c) heating the column to approximately 185°C to 270°C; (d) maintaining the temperature of the receiving container below the depolymerization temperature of the oligomers; (e) coordinating the oligomer feed rate into the column with the column temperature such that a portion of the oligomers is converted into a vapor product stream containing cyclic esters, while another portion of the oligomers exits the column at its bottom and flows to the receiving container; and (f) discharging the vapor product stream through the top outlet of the column and recovering the cyclic esters therefrom. This patent describes a depolymerization reaction carried out in a tower reactor. By feeding oligomers to the top of the tower and coordinating the feed rate with the depolymerization temperature, a portion of the oligomers is converted into a vapor product stream containing cyclic esters, while the other portion of the oligomers leaves the bottom of the tower and enters the receiver container. This improves production efficiency and reduces the tendency for unconverted oligomers to turn into acidic byproducts due to heating.

[0008] CN112958030A uses a falling film reactor to prepare glycolide. Its working principle is as follows: Glycolic acid / glycolic acid ester liquid enters the reactor body through the inlet and is first dispersed by a liquid pre-distributor. The dispersed liquid then enters a distribution plate, where it undergoes pre-distribution through liquid distribution holes. Any excess liquid overflows directly through a weir. The pre-distributed liquid enters the falling film tubes through the liquid phase inlet on uniformly arranged film tubes. Inside the falling film tubes, the liquid film flows in a film-like manner, with one side contacting the inner wall of the falling film tube and exchanging heat, while the other side is the gas phase. After heating, the glycolide produced in the reaction can easily pass through the gas-liquid mass transfer interface, escaping from the liquid phase and entering the gas phase space. Driven by a vacuum pump, it flows out of the vaporization chamber and into the condenser for condensation and collection. Because the pressure drop loss of gas flow in the gas phase is much lower than that in the liquid phase, the pressure inside the falling film tube is close to the pressure in the vaporization chamber. This allows the depolymerization reaction to proceed at a lower pressure. At lower pressures, the boiling point of glycolide is lower, allowing it to escape from the reaction liquid into the gas phase at lower temperatures, thus promoting the reaction towards glycolide formation. Furthermore, the falling film reactor has a larger gas-liquid mass transfer interface, accelerating the evaporation rate of glycolide and resulting in a faster reaction rate. Therefore, the reaction temperature of glycolic acid oligomers is lower, the heating time is shorter, and there are fewer side reactions using a falling film reactor. Heating (or heat transfer) during the reaction process is controlled by a hot (cold) medium outside the falling film tube.

[0009] In their research on the bulk depolymerization of glycolic acid oligomers, the inventors discovered that glycolide prepared from glycolic acid oligomers through bulk depolymerization contains certain amounts of impurities such as formic acid and acetic acid. This is mainly because glycolide contains a certain amount of water, and glycolide reacts with water to form glycolic acid dimers. Under the temperature and pressure conditions required for depolymerization, glycolic acid dimers undergo decarboxylation reactions at both the molecular chain ends and within the molecular chain, i.e., the removal of carboxyl groups (-COOH) and the release of carbon dioxide. After the removal of carboxyl groups at the molecular chain ends of the glycolic acid dimer, it reacts with water to produce formaldehyde, which is then oxidized to formaldehyde. After the removal of carboxyl groups within the glycolic acid dimer molecular chain, acetic acid is directly produced.

[0010] Glycolide prepared by depolymerization typically contains small amounts of glycolic acid, glycolic acid oligomers, water, and other impurities. The additional formation of formic acid and acetic acid further increases the content of acidic impurities. These acidic impurities act as polymerization initiators, causing the depolymerized glycolide to polymerize in pipelines, generating large amounts of low-molecular-weight polyglycolic acid (PEG). This low-molecular-weight PEG adheres to the pipeline inner wall, leading to blockages during prolonged continuous operation. Furthermore, it significantly affects the molecular weight of the polymer during the ring-opening polymerization of glycolide, failing to meet the requirements of polyglycolic acid for applications such as surgical sutures, which require an intrinsic viscosity of at least 1 and a weight-average molecular weight of at least 100,000. Therefore, reducing the formation of formic acid and acetic acid is crucial for controlling the acid value of the depolymerized glycolide. Current bulk depolymerization technologies do not simultaneously address the control of glycolide purity, acid value, and formic acid and acetic acid content. Summary of the Invention

[0011] To overcome the problems existing in the prior art, the present invention provides a continuous method for preparing glycolide and the glycolide obtained therefrom. This method solves the problem that during bulk depolymerization, the depolymerization reaction easily produces decarboxylation side reactions, generating formic acid and acetic acid as byproducts, leading to a decrease in glycolide yield, purity, and an increase in acid value.

[0012] One of the objectives of this invention is to provide a continuous method for preparing glycolide, comprising the step of depolymerizing glycolic acid oligomers in the presence of inorganic phosphorus to obtain glycolide.

[0013] In a preferred embodiment, the glycolic acid oligomer and the inorganic phosphorus are mixed by a mixing device and then continuously added to a depolymerization reactor (to carry out a depolymerization reaction).

[0014] The depolymerization reactor can be any depolymerization reactor disclosed in the prior art, such as a depolymerization vessel.

[0015] In a further preferred embodiment, the mixing is carried out at 200-240°C, preferably 210-220°C, for example, 200°C, 210°C, 220°C, 230°C or 240°C.

[0016] In a further preferred embodiment, the mixing device is an extruder, such as a single-screw extruder or a twin-screw extruder. Preferably, the extruder temperature is 200-240°C, more preferably 210-220°C, for example, 200°C, 210°C, 220°C, 230°C, or 240°C. More preferably, the glycolic acid oligomer directly enters the extruder, and the inorganic phosphorus enters the extruder through an additive feed port on the extruder. In this way, the glycolic acid oligomer and the inorganic phosphorus can be continuously added to the depolymerization reactor (for depolymerization reaction) through the single-screw extruder.

[0017] In a preferred embodiment, the inorganic phosphorus is selected from at least one of sodium polyphosphate, sodium hexametaphosphate, sodium pyrophosphate, trisodium phosphate, disodium hydrogen phosphate, dicalcium hydrogen phosphate, dipotassium hydrogen phosphate, diammonium hydrogen phosphate, potassium pyrophosphate, and calcium pyrophosphate.

[0018] In a further preferred embodiment, the inorganic phosphorus is selected from at least one of sodium tripolyphosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, and diammonium hydrogen phosphate.

[0019] Through extensive experiments, the inventors discovered that the depolymerization reaction of glycolic acid oligomers in the presence of inorganic phosphorus can inhibit the decarboxylation reaction of glycolic acid dimers and reduce the generation of impurities such as formic acid and acetic acid.

[0020] Under depolymerization conditions, the PGA backbone may undergo various forms of free radical reactions, starting with the homolytic cleavage of acyl-oxygen or alkoxy bonds to form several oxygen- or carbon-centered free radicals. In all molecular bonding processes, electrons tend to pair, and unpaired electrons in free radicals also tend to pair; therefore, most free radicals are highly reactive and readily react to form stable molecules. Free radicals possess properties such as initiation, chain transfer, and mutual reaction termination, increasing the possible reaction pathways. A large number of free radicals can promote decarboxylation reactions. Through extensive experimental research, the inventors discovered that adding inorganic phosphorus can reduce the generation of free radicals to a certain extent, thus inhibiting deacidification reactions and reducing the formation of impurities such as formic acid and acetic acid.

[0021] In a further preferred embodiment, the amount of inorganic phosphorus is 0.3 to 1.0 wt% of the weight of the glycolic acid oligomer, preferably 0.3 to 0.6 wt%, for example 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, or 1.0 wt%.

[0022] In a preferred embodiment, the conditions for the depolymerization reaction include: heating to 230–290°C (e.g., 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, or 290°C) and pressure of 0.5–8 kPa (e.g., 0.5 kPa, 1 kPa, 2 kPa, 4 kPa, 6 kPa, or 8 kPa).

[0023] In a further preferred embodiment, the conditions for the depolymerization reaction include: heating to 250–280°C and pressure of 0.5–5 kPa.

[0024] In a preferred embodiment, the glycolic acid oligomer is obtained as follows: an aqueous glycolic acid solution is reacted in the presence of a catalyst to obtain the glycolic acid oligomer.

[0025] In a preferred embodiment, when preparing the glycolic acid oligomer: the glycolic acid aqueous solution is a 50-70 wt% glycolic acid aqueous solution, for example, 50 wt%, 55 wt%, 60 wt%, 65 wt%, or 70 wt% glycolic acid aqueous solution.

[0026] In a preferred embodiment, when preparing the glycolic acid oligomer, the catalyst is selected from at least one of stannous octoate, stannous chloride, antimony trioxide, zinc oxide, and zinc acetylacetonate.

[0027] In a further preferred embodiment, when preparing the glycolic acid oligomer, the ratio of the catalyst to the glycolic acid is 0.05-1 wt%, preferably 0.1-1 wt%, more preferably 0.1-0.5 wt%, for example 0.05%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, or 1 wt%.

[0028] In a preferred embodiment, the reaction for preparing the glycolic acid oligomer includes the following conditions: first heating to 110-220°C, and then reacting under vacuum for 0.5-5 hours, preferably 2-5 hours.

[0029] In a further preferred embodiment, the vacuum condition refers to 0.1 to 10 kPa.

[0030] For example, the temperature is first raised from room temperature to 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, 210℃ or 220℃ for pre-reaction. Then (while maintaining the system temperature), the vacuum degree of the reaction system is controlled to 0.1kPa, 0.5kPa, 1kPa, 2kPa, 3kPa, 4kPa, 5kPa, 6kPa, 7kPa, 8kPa, 9kPa or 10kPa to continue the reaction for 0.5h, 1h, 2h, 3h, 4h, 5h, 6h, 8h or 10h.

[0031] In a further preferred embodiment, when preparing the glycolic acid oligomer, the reaction includes the following conditions: after heating to 110-220°C, reacting at 2-10 kPa for 0.5-5 hours, and then reacting under reduced vacuum for 0.5-5 hours.

[0032] In a preferred embodiment, the glycolic acid oligomer is cooled and pelletized / crushed after preparation and before depolymerization.

[0033] A second objective of this invention is to provide glycolide obtained using the method described in one objective of this invention.

[0034] Preferably, the glycolide has a purity >95.0%, an acid value <300 μmol / g, a formic acid content <10 μg / g, and an acetic acid content <20 μg / g.

[0035] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values; 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. In the following, various technical solutions can, in principle, be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0036] Compared with existing technologies, the present invention has the following beneficial effects: In the method of the present invention, the glycolic acid oligomer prepared using the glycolic acid aqueous solution is mixed with an inorganic phosphorus compound and continuously added to the depolymerization reactor through a single-screw extruder. Using the method of the present invention, the yield of glycolide is >95.5%, and the glycolide indicators are: purity >95.0%, acid value <300 μmol / g, formic acid content <10 μg / g, and acetic acid content <20 μg / g, achieving better technical results. Detailed Implementation

[0037] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0038] It should also be noted that the various specific technical features described in the following embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the various possible combinations will not be described separately in this invention.

[0039] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0040] Unless otherwise specified, the raw materials used in the examples and comparative examples are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0041] The analytical determination method for glycolic acid oligomers and glycolide products in this invention:

[0042] (1) Determination of end carboxyl group content in glycolic acid oligomers

[0043] The content of terminal carboxyl groups in glycolic acid oligomers was determined by acid-base titration. The sample was heated to 150°C and dissolved in 40 mL of dry dimethyl sulfoxide. After dissolution, the solution was cooled to room temperature, and a few drops of bromophenol blue indicator solution were added, resulting in a yellow color. Titration was then performed using a standard concentration of sodium hydroxide in benzyl alcohol solution. The endpoint was reached when the solution color changed from yellow to green. The content of terminal carboxyl groups in the sample was calculated by calculating the volume of sodium hydroxide solution used to reach the titration endpoint.

[0044] (2) Determination of the purity of glycolide

[0045] The analysis was performed using a gas chromatograph 7890B. 0.5 g of sample was placed in a centrifuge tube and dissolved thoroughly in 25 mL of dimethyl sulfoxide. The mixture was centrifuged at 10000 rpm for 5 min. The supernatant was then analyzed. The chromatographic column was DB-FFAP 30 m × 0.25 mm × 0.25 μm. A standard curve of (250–5000) μg / g was used for calculation.

[0046] (3) Determination of glycolide acid value

[0047] The acid value of crude glycolide was determined using acid-base titration. The sample was dissolved in 20 mL of dry dimethyl sulfoxide. After dissolution, a few drops of bromophenol blue indicator solution were added, resulting in a yellow solution. Titration was performed using a standard concentration of sodium hydroxide in benzyl alcohol. The endpoint was reached when the solution color changed from yellow to green. The acid value was calculated by determining the volume of sodium hydroxide solution used to reach the titration endpoint.

[0048] (4) Determination of formic acid and acetic acid content

[0049] Analysis was performed using headspace-GC-MS. Extracted ions (formic acid m / z 46, acetic acid m / z 60) were quantified using a single-point external standard method. 2g of sample was purged with nitrogen. Headspace conditions: 120℃, equilibration for 30 min. Standards were aqueous solutions of formic acid and acetic acid, with a concentration of approximately 0.8%. 0.1g of the aqueous solution was transferred to a 20mL headspace vial, capped, and analyzed using the same method as the sample.

[0050]

Example 1

[0051] 114 kg of a 70% aqueous glycolic acid solution and 399 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 5 kPa under vacuum, and the reaction was continued for 2 hours. The system pressure was then reduced to 1 kPa, and the reaction was continued for 3 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 61.2 kg of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 269 μmol / g.

[0052] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder. Sodium tripolyphosphate is added to the extruder through the additive feed port at a rate of 40 g / h (0.4% based on glycolic acid oligomers). The single-screw extruder temperature is set to 215℃.

[0053] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 290℃ after feeding, while simultaneously evacuating the system to reduce the pressure to 5 kPa. 58.8 kg of glycolide was collected, with a yield of 96.1%. The glycolide purity was measured to be 95.7%, with an acid value of 267 μmol / g, formic acid content of 6 μg / g, and acetic acid content of 15 μg / g.

[0054]

Example 2

[0055] 114 kg of a 70% aqueous glycolic acid solution and 798 g of stannous octoate (1.0 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 5 kPa under vacuum, and the reaction was carried out for 2 hours. The system pressure was then reduced to 0.1 kPa, and the reaction was continued for 1.5 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 60.8 kg of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 254 μmol / g.

[0056] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder. Disodium hydrogen phosphate is added to the extruder through the additive feed port at a rate of 60 g / h (0.6% based on glycolic acid oligomers). The single-screw extruder temperature is set to 220℃.

[0057] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 280℃ after feeding, while simultaneously evacuating to reduce the system pressure to 3 kPa. 58.2 kg of glycolide was collected, with a yield of 95.7%. The glycolide purity was measured to be 96.0%, with an acid value of 239 μmol / g, formic acid content of 3 μg / g, and acetic acid content of 11 μg / g.

[0058]

Example 3

[0059] 114 kg of a 70% aqueous glycolic acid solution and 79.8 g of stannous octoate (0.1 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 8 kPa under vacuum, and the reaction was continued for 3 hours. The system pressure was then reduced to 0.1 kPa, and the reaction was continued for 2 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 61.7 kg of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 306 μmol / g.

[0060] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder. Dipotassium hydrogen phosphate is added to the extruder through the additive feed port at a rate of 100 g / h (1.0% based on glycolic acid oligomers). The single-screw extruder temperature is set to 210℃.

[0061] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 270℃ after feeding, while simultaneously evacuating to reduce the system pressure to 2 kPa. 59.0 kg of glycolide was collected, with a yield of 95.6%. The glycolide purity was measured to be 95.8%, with an acid value of 263 μmol / g, formic acid content of 5 μg / g, and acetic acid content of 12 μg / g.

[0062]

Example 4

[0063] 114 kg of a 70% aqueous glycolic acid solution and 239.4 g of stannous octoate (0.3 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 10 kPa under vacuum, and the reaction was carried out for 1 hour. The system pressure was then reduced to 3 kPa, and the reaction was continued for 3 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 61.4 kg of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 245 μmol / g.

[0064] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder. Diammonium hydrogen phosphate is added to the extruder through the additive feed port at a rate of 50 g / h (0.5% based on glycolic acid oligomers). The single-screw extruder temperature is set to 220℃.

[0065] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 230℃ after feeding, while simultaneously evacuating to reduce the system pressure to 0.5 kPa. 58.8 kg of glycolide was collected, with a yield of 95.8%. The glycolide purity was measured to be 95.1%, with an acid value of 292 μmol / g, formic acid content of 6 μg / g, and acetic acid content of 18 μg / g.

[0066]

Example 5

[0067] 114 kg of a 70% aqueous glycolic acid solution and 319.2 g of stannous octoate (0.4 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 2 kPa under vacuum, and the reaction was continued for 1 hour. The system pressure was then reduced to 0.5 kPa, and the reaction was continued for another hour to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 61.2 kg of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 203 μmol / g.

[0068] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder. Hexametaphosphate is added to the extruder through the additive feed port at a rate of 30 g / h (0.3% based on glycolic acid oligomers). The single-screw extruder temperature is set to 215℃.

[0069] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 280℃ after feeding, while simultaneously evacuating to reduce the system pressure to 3 kPa. 58.9 kg of glycolide was collected, with a yield of 96.2%. The glycolide purity was measured to be 95.5%, with an acid value of 278 μmol / g, formic acid content of 8 μg / g, and acetic acid content of 14 μg / g.

[0070]

Example 6

[0071] 114 kg of a 70% aqueous glycolic acid solution and 239.4 g of stannous octoate (0.3 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 8 kPa under vacuum, and the reaction was continued for 2 hours. The system pressure was then reduced to 3 kPa, and the reaction was continued for another 2 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 61.4 kg of oligomers. The terminal carboxyl group content of the glycolic acid oligomers was measured to be 193 μmol / g.

[0072] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder. Sodium pyrophosphate is added to the extruder through the additive feed port at a rate of 40 g / h (0.4% based on glycolic acid oligomers). The single-screw extruder temperature is set to 215℃.

[0073] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 260℃ after feeding, while simultaneously evacuating the system to reduce the pressure to 1 kPa. 58.7 kg of glycolide was collected, with a yield of 95.6%. The glycolide purity was measured to be 95.9%, with an acid value of 257 μmol / g, formic acid content of 5 μg / g, and acetic acid content of 11 μg / g.

[0074]

Comparative Example 1

[0075] 114 kg of a 70% aqueous glycolic acid solution and 399 g of stannous octoate (0.5 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 5 kPa under vacuum, and the reaction was continued for 2 hours. The system pressure was then reduced to 1 kPa, and the reaction was continued for 3 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 60.8 g of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 258 μmol / g.

[0076] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder, with the temperature of the single-screw extruder set at 215°C.

[0077] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 290℃ after feeding, while simultaneously evacuating to reduce the system pressure to 5 kPa. 54.8 kg of glycolide was collected, with a yield of 90.1%. The glycolide purity was measured to be 87.9%, with an acid value of 529 μmol / g, formic acid content of 127 μg / g, and acetic acid content of 208 μg / g.

[0078] [Comparative Example 2]

[0079] 114 kg of a 70% aqueous glycolic acid solution and 798 g of stannous octoate (1.0 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 5 kPa under vacuum, and the reaction was carried out for 2 hours. The system pressure was then reduced to 0.1 kPa, and the reaction was continued for 1.5 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 61.2 kg of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 263 μmol / g.

[0080] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder, with the temperature of the single-screw extruder set at 220°C.

[0081] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 280℃ after feeding, while simultaneously evacuating to reduce the system pressure to 3 kPa. 56.1 kg of glycolide was collected, with a yield of 91.7%. The glycolide purity was measured to be 88.3%, with an acid value of 547 μmol / g, formic acid content of 106 μg / g, and acetic acid content of 189 μg / g.

[0082] [Comparative Example 3]

[0083] 114 kg of a 70% aqueous glycolic acid solution and 79.8 g of stannous octoate (0.1 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 8 kPa under vacuum, and the reaction was continued for 3 hours. The system pressure was then reduced to 0.1 kPa, and the reaction was continued for 2 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 61.5 kg of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 289 μmol / g.

[0084] Glycolic acid oligomers were fed into the depolymerization reactor at a rate of 10 kg / h via a single-screw extruder. Bis(2,4-di-tert-butylphenyl) pentaerythritol diphosphite was added to the extruder through the additive feed port at a rate of 100 g / h (1.0% based on glycolic acid oligomers). The single-screw extruder temperature was set to 210°C.

[0085] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 270℃ after feeding, while simultaneously evacuating the system to reduce the pressure to 2 kPa. 57.7 kg of glycolide was collected, with a yield of 93.8%. The glycolide purity was measured to be 90.4%, with an acid value of 503 μmol / g, formic acid content of 96 μg / g, and acetic acid content of 166 μg / g.

[0086] [Comparative Example 4]

[0087] 114 kg of a 70% aqueous glycolic acid solution and 79.8 g of stannous octoate (0.1 wt%, based on glycolic acid) were added to a reactor. The temperature was gradually increased from room temperature to 220 °C, and free water in the raw materials and water generated during the reaction were removed by distillation. At this temperature, the system pressure was reduced to 8 kPa under vacuum, and the reaction was continued for 3 hours. The system pressure was then reduced to 0.1 kPa, and the reaction was continued for 2 hours to obtain glycolic acid oligomers. The oligomers were discharged, cooled, and crushed into powder, yielding 61.6 kg of oligomers. The end carboxyl group content of the glycolic acid oligomers was measured to be 292 μmol / g.

[0088] Glycolic acid oligomers are fed into the depolymerization reactor at a rate of 10 kg / h through a single-screw extruder. Sodium dihydrogen phosphate is added at a rate of 50 g / h (0.5% based on glycolic acid oligomers) through the additive feed port on the extruder. The single-screw extruder temperature is set to 210℃.

[0089] The depolymerization reactor was preheated to 220℃ before feeding and gradually increased to 270℃ after feeding, while simultaneously evacuating to reduce the system pressure to 2 kPa. The depolymerization reactor employed a twin-shaft, co-rotating, variable-speed stirrer. The crude glycolate product was collected after condensation, and the heavy material at the bottom of the reactor was continuously discharged through a twin-screw extruder. 55.1 kg of glycolate was collected, with a yield of 89.4%. The glycolate purity was measured to be 85.5%, with an acid value of 729 μmol / g, formic acid content of 148 μg / g, and acetic acid content of 261 μg / g.

[0090] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A continuous method for preparing glycolide, comprising the step of depolymerizing glycolic acid oligomers in the presence of inorganic phosphorus to obtain glycolide, wherein the inorganic phosphorus is selected from at least one of sodium polyphosphate, sodium hexametaphosphate, sodium pyrophosphate, trisodium phosphate, disodium hydrogen phosphate, dicalcium hydrogen phosphate, dipotassium hydrogen phosphate, diammonium hydrogen phosphate, potassium pyrophosphate, and calcium pyrophosphate; wherein the glycolic acid oligomers are mixed with the inorganic phosphorus and continuously added to a depolymerization reactor for depolymerization reaction, wherein the mixing is carried out at 200-240°C; and the conditions for the depolymerization reaction include: Heat to 230~290℃ and apply pressure of 0.5~8kPa.

2. The method according to claim 1, characterized in that, The mixing is carried out at 210–220°C.

3. The method according to claim 1, characterized in that, The amount of inorganic phosphorus used is 0.3 to 1.0 wt% of the weight of the glycolic acid oligomer.

4. The method according to claim 1, characterized in that, The amount of inorganic phosphorus used is 0.3 to 0.6 wt% of the weight of the glycolic acid oligomer.

5. The method according to claim 1, characterized in that, The conditions for the depolymerization reaction include: heating to 250~280℃ and pressure of 0.5~5kPa.

6. The method according to any one of claims 1 to 5, characterized in that, The glycolic acid oligomers are obtained as follows: an aqueous glycolic acid solution is reacted in the presence of a catalyst to obtain glycolic acid oligomers.

7. The method according to claim 6, characterized in that, The catalyst is selected from at least one of stannous octoate, stannous chloride, antimony trioxide, zinc oxide, and zinc acetylacetonate.

8. The method according to claim 6, characterized in that, The ratio of the catalyst to the glycolic acid is 0.05-1 wt%.

9. The method according to claim 6, characterized in that, The ratio of the catalyst to the glycolic acid is 0.1 to 1 wt%.

10. The method according to claim 6, characterized in that, The ratio of the catalyst to the glycolic acid is 0.1 to 0.5 wt%.

11. The method according to claim 6, characterized in that, The reaction includes the following conditions: first, the temperature is raised to 110~220℃, and then the reaction is carried out under vacuum for 0.5~5 hours.

12. The method according to claim 6, characterized in that, The reaction includes the following conditions: after heating to 110~220℃, the reaction is carried out at 2~10kPa for 0.5~5h, and then the vacuum is reduced for another 0.5~5h.

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