Device for preparing polyglycolic acid with narrow molecular weight distribution and preparation method thereof

Abstract

The present invention discloses an apparatus suitable for preparing polyglycolic acid with a narrow molecular weight distribution and a preparation method thereof. The apparatus includes a melting and mixing kettle, a static mixer connected to the discharge port of the melting and mixing kettle, and a viscosity increasing device arranged downstream of the static mixer.

Description

Technical Field

[0001] The invention relates to the field of polymer preparation, and in particular to a device suitable for preparing polyglycolic acid with narrow molecular weight distribution and a preparation method thereof. Background Art

[0002] In today's society, with the rapid development of polymer science and technology, a wide variety of polymer materials have also penetrated into people's daily lives, greatly enriching people's choices of clothing, food, housing, and transportation, and bringing great convenience to people's lives. However, with it, there are more and more wastes after the use of polymer products, which has brought great harm to the environment. This has also led to the increasingly serious "white pollution" problem in the social environment, and environmental pollution control is imminent. In order to solve the "white pollution" problem, there are currently two main development directions, one is to study and develop the recycling and reuse of waste plastics, and the other is to develop degradable plastics. Since recycling and reuse usually have disadvantages such as low recovery rate, serious secondary processing pollution, and high labor costs, the development of degradable plastics is the main development trend today and in the future.

[0003] Polyglycolic acid, also known as polyglycolide (PGA for short), is a biodegradable aliphatic polymer that can be hydrolyzed under the catalysis of enzymes or acids and bases in microorganisms or organisms to eventually form carbon dioxide and water. It is a degradable material with great development potential. In terms of application, due to its good biocompatibility, PGA can be used to make degradable medical device materials (for example, absorbable surgical sutures, tissue engineering materials, orthopedic materials, etc.). In addition, due to its excellent gas barrier properties, PGA can also be used in the packaging field (for example, shopping bags, express bags, packaging bags, fresh-keeping bags, etc.). Furthermore, due to its strong mechanical strength and low environmental load, PGA itself can also be used to process into downhole tool components for hydrocarbon resource recovery (for example, bridge plugs, fracturing balls, etc.), temporary plugging agent materials, etc. It can be seen that PGA, as an emerging degradable material, has considerable economic benefits and good application prospects. Based on this, how to produce and process polyglycolic acid products to better replace traditional polymer materials has become an important research topic.

[0004] The research on the twin-screw reactive extrusion process of PGA materials has found that different L / D ratios of the twin-screw and differences in screw elements will lead to significant differences in the performance of the final PGA products. In addition, when the material passing through the twin-screw is in a fluid state, the conveying blocks, kneading blocks, and mixing blocks can achieve good mixing and dispersion effects. However, as the molecular weight of the material increases, the strong shearing effect of the screw elements becomes more obvious, which easily causes uneven heating of the material and local overheating. In addition, due to the limitation of the sealing conditions of the twin-screw and the influence of hydrolysis and oxidation degradation, the molecular weight distribution of the material will show an uneven state, resulting in a wider molecular weight distribution and a larger dispersion coefficient of the final material, which will seriously affect its subsequent processing performance. Moreover, PGA products manufactured by molding processes such as injection molding or blow molding may have obvious defects (for example, unqualified mechanical strength, poor anti-aging performance, etc.), seriously affecting the normal use of the products. Furthermore, due to the good crystallization ability of PGA itself and its fast crystallization speed, when the flow rate of the PGA material in the pipeline is slow, it is very easy to crystallize and solidify, resulting in pipeline blockage and inability to carry out subsequent processing normally.

[0005] Therefore, there is an urgent need in the art to provide a method for obtaining polyglycolic acid with a narrow molecular weight distribution. Summary of the Invention

[0006] The present invention aims to provide polyglycolic acid with a narrow molecular weight distribution.

[0007] In a first aspect of the present invention, there is provided an apparatus for preparing polyglycolic acid with a narrow molecular weight distribution, including a melting and mixing kettle, a static mixer connected to the discharge port of the melting and mixing kettle, and a viscosity increasing device provided downstream of the static mixer.

[0008] In another embodiment, a reaction aid storage tank is connected to the feed port of the melting and mixing kettle.

[0009] In another embodiment, the static mixer and the viscosity increasing device are connected through a material conveying assembly.

[0010] In another embodiment, the material conveying assembly includes a material conveying pipeline connecting the discharge port of the static mixer and the feed port of the viscosity increasing device, a melt pump provided on the material conveying pipeline, and a cleaning liquid discharge pipeline connected in parallel with the melt pump on the material conveying pipeline.

[0011] In another embodiment, a cleaning liquid discharge branch is provided on the material conveying pipeline between the melt pump and the feed port of the viscosity increasing device.

[0012] In another embodiment, a three-way valve is provided at the connection of the cleaning liquid discharge pipeline to the material conveying pipeline.

[0013] In another embodiment, the cleaning liquid drain branch is connected to the material conveying pipeline through a three-way valve.

[0014] In another embodiment, a melt metering pump is provided between the discharge port of the melting and mixing kettle and the static mixer.

[0015] In another embodiment, the device further includes a falling film evaporation and purification unit disposed upstream of the melting and mixing kettle and coupled to the melting and mixing kettle.

[0016] In another embodiment, the falling film evaporation and purification unit includes a falling film evaporator and a sieve, and the discharge port at the bottom of the sieve is connected to the feed port at the top of the melting and mixing kettle.

[0017] In another embodiment, a centrifugal washer connected to the discharge port of the falling film evaporator, a vacuum dryer connected to the discharge port of the centrifugal washer, and a crusher connected to the discharge port of the vacuum dryer are sequentially arranged between the falling film evaporator and the sieve.

[0018] In another embodiment, the discharge port of the crusher is connected to the feed port of the sieve.

[0019] In the second aspect of the present invention, there is provided a method for preparing polyglycolic acid with a narrow molecular weight distribution by using the device provided by the present invention as described above. The method includes the steps of:

[0020] (1) Mixing glycolide and a reaction aid evenly in a melting and mixing kettle to obtain a fluid premix;

[0021] (2) Pre-polymerizing the fluid premix obtained in step (1) in a static mixer to obtain a glycolic acid prepolymer; and

[0022] (3) Subjecting the glycolic acid prepolymer obtained in step (2) to final polymerization through a viscosity increasing device to obtain polyglycolic acid.

[0023] In another embodiment, the purity of the glycolide used in step (1) is above 98%; preferably not less than 98.5%, and the acidity does not exceed 20 mmol / kg.

[0024] In another embodiment, glycolide powder is used in step (1).

[0025] In another embodiment, the reaction aid used in step (1) includes a catalyst, an initiator, and a dehydrating agent.

[0026] In another embodiment, based on the mass of glycolide used in step (1), the dosage of the catalyst is 0.001 - 5 wt%, the dosage of the initiator does not exceed 5 wt% (for example, but not limited to, 0.1 - 4 wt%, 1 - 3 wt%, etc.), and the dosage of the dehydrating agent is 0.2 - 1.6 wt% thereof.

[0027] In another embodiment, in step (1), the reaction auxiliary agent is added dropwise into the melting and mixing kettle by an injection method.

[0028] In another embodiment, the static mixer used in step (2) adopts at least two-stage gradient temperature increase mode.

[0029] In another embodiment, the static mixer used in step (2) adopts a two - to ten-stage gradient temperature increase mode, preferably three to seven stages.

[0030] In another embodiment, the temperature range of the first stage is between 120 - 220 °C; the temperature range of the last stage is between 220 - 250 °C.

[0031] In another embodiment, the temperature of the second stage is increased by 10 - 100 °C compared with the first stage; the temperature of the last stage is increased by 0 - 30 °C compared with the previous adjacent stage.

[0032] In another embodiment, the weight-average molecular weight of the glycolic acid prepolymer obtained in step (2) is about 50,000 - 150,000.

[0033] In another embodiment, the devolatilization of the viscosity increasing device in step (3) is carried out under an absolute pressure ≤ 500 Pa and a temperature of 220 - 250 °C.

[0034] In another embodiment, the length-diameter ratio of the screw in the devolatilization section can be set to 30 - 100, for example, but not limited to, 40 - 50, 60 - 90, etc.

[0035] In another embodiment, a processing aid (such as a toughening agent, compatibilizer, plasticizer, chain extender, hydrolysis regulation promoter, hydrolysis regulation inhibitor, heat stabilizer, antioxidant, antibacterial agent, or lubricant, etc.) can be added through a loss-in-weight scale at the starting point of the devolatilization section.

[0036] In another embodiment, the glycolide in step (1) is obtained by evaporating and purifying the molten crude glycolide through a falling film evaporator.

[0037] In another embodiment, the temperature of the falling film evaporator through which the molten crude glycolide passes is less than 200 °C.

[0038] In another embodiment, the pressure in the falling film evaporator does not exceed 5 kPa, and the temperature is 5 - 30 °C higher than the boiling point of glycolide at this pressure.

[0039] In another embodiment, the pressure is 0.4 - 4 kPa.

[0040] In another embodiment, the temperature is 10 - 20 °C higher than the boiling point of glycolide under this pressure.

[0041] In another embodiment, the falling film evaporator contains n heat exchange devices, where n is an integer greater than or equal to 2; the temperatures of the n heat exchange devices increase in a gradient along the flow direction of the molten crude glycolide.

[0042] In another embodiment, n is preferably an integer from 2 to 10.

[0043] In another embodiment, the temperature difference between adjacent heat exchange devices is 1 - 10 °C; preferably 2 - 8 °C.

[0044] In another embodiment, the average film-forming thickness of the heat exchange device does not exceed 300 μm.

[0045] In another embodiment, the method of evaporating and purifying the molten crude glycolide through a falling film evaporator further includes the step of washing the evaporated and purified material with an alcohol solvent.

[0046] Accordingly, the present invention provides an apparatus and a corresponding method for obtaining polyglycolic acid with a narrow molecular weight distribution. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is a schematic diagram of the apparatus for preparing polyglycolic acid with a narrow molecular weight distribution provided in Example 1 of the present invention.

[0048] Figure 2 is a schematic diagram of the apparatus for preparing polyglycolic acid with a narrow molecular weight distribution provided in Example 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0049] Through extensive and in-depth research, the inventors found that the crude glycolide can be purified by a falling film evaporator, and its comprehensive energy consumption is significantly less than that of the conventional recrystallization purification process; then, a static mixer is arranged upstream of the viscosity increasing device, and creatively, the static mixer is used as the main place for the glycolide polymerization reaction. While strengthening the mixing effect of glycolide and the reaction aid, through a multi-stage stepwise temperature increase method, the glycolide is promoted to undergo ring-opening polymerization reaction to gradually form a glycolic acid prepolymer with a relatively high molecular weight, and the viscosity increasing device mainly plays a role in removing volatiles, removing the remaining small molecules in the glycolic acid prepolymer to promote the further polymerization of the glycolic acid prepolymer and improve the molecular weight of the final PGA. On this basis, the present invention is completed.

[0050] It should be noted here that in the present invention, the "viscosity increasing device" plays a devolatilization role, which can promote the further polymerization of the glycolic acid prepolymer, timely remove the generated small molecules from the system, so as to further increase the molecular weight of the polymer, and correspondingly, the viscosity of the polymer will also further increase; in the present technology, the "viscosity increasing device" can be, for example but not limited to, a twin-screw extruder equipped with only a devolatilization section.

[0051] Device for preparing polyglycolic acid with narrow molecular weight distribution

[0052] The present invention provides a device suitable for preparing PGA with narrow molecular weight distribution, which includes a melting and mixing kettle, a static mixer coupled with the melting and mixing kettle, and a viscosity increasing device arranged downstream of the static mixer.

[0053] A melt metering pump is arranged between the static mixer and the melting and mixing kettle, and the bottom discharge port of the melting and mixing kettle is connected to the feed port of the static mixer through the melt metering pump.

[0054] The discharge port of the static mixer is connected to the feed port of the viscosity increasing device through a material conveying assembly.

[0055] The material conveying assembly includes a material conveying pipeline connecting the discharge port of the static mixer and the feed port of the viscosity increasing device, a melt pump arranged on the material conveying pipeline, and a cleaning liquid drainage pipeline arranged in parallel with the melt pump. A cleaning liquid drainage branch is arranged on the material conveying pipeline between the melt pump and the feed port of the viscosity increasing device.

[0056] Preferably, both the liquid inlet end and the liquid discharge end of the cleaning liquid drainage pipeline are connected in parallel to both sides of the melt pump through a three-way valve I.

[0057] Preferably, the cleaning liquid drainage branch is connected to the material conveying pipeline through a three-way valve II.

[0058] Static mixers commonly used in the art can be used, such as but not limited to, SK type static mixer, SX type static mixer, SV type static mixer, etc.

[0059] The production device further includes a falling film evaporation and purification unit arranged upstream of the melting and mixing kettle and coupled with the melting and mixing kettle.

[0060] The falling film evaporation and purification unit includes a falling film evaporator, a centrifugal washing machine, a vacuum dryer, a crusher, and a sieve arranged in sequence downstream of the falling film evaporator. Among them, the discharge port at the bottom of the sieve is connected to the feed port at the top of the melting and mixing kettle.

[0061] Further, a reaction material feed port and a reaction aid feed port are also provided at the top of the melting and mixing kettle. Among them, the reaction material feed port is connected to a loss-in-weight weigher through a material feed pipe, and the reaction aid feed port is connected to a reaction aid storage tank arranged upstream of the melting and mixing kettle through an aid feed pipe. A liquid metering pump is also provided on the aid feed pipe between the reaction aid storage tank and the reaction aid feed port.

[0062] In an embodiment of the present invention, the loss-in-weight weigher is connected between the discharge port at the bottom of the screening machine and the reaction material feed port at the top of the melting and mixing kettle.

[0063] The heat exchange surface of the falling film evaporator is a multi-section heat exchange surface with the temperature increasing gradually from top to bottom in a gradient distribution.

[0064] In this technical device, the pipelines between the bottom discharge port of the melting and mixing kettle and the feed port of the static mixer, the material conveying pipeline, the cleaning liquid drainage pipeline, the melt metering pump, the melt pump, the three-way valve I and the three-way valve II are all equipped with conventional heat tracing. In addition, the model of the metering pump can be selected according to the reaction time of the material in the melting and mixing kettle and the volume of the pipeline. The models of the melt pump and the viscosity increasing equipment need to match the model of the melt metering pump.

[0065] As used in the present invention, "couple" means that two devices are operably connected to each other and there is a relationship of mutual interaction and mutual influence.

[0066] Preparation method of narrow molecular weight distribution polyglycolic acid

[0067] The present invention provides a preparation method of narrow molecular weight distribution polyglycolic acid, comprising the steps of:

[0068] In the first step, glycolide and a reaction aid are mixed evenly in a melting and mixing kettle to obtain a fluid-state premix.

[0069] In the second step, the fluid-state premix is transported to a static mixer for prepolymerization to obtain a glycolic acid prepolymer.

[0070] In the third step, the glycolic acid prepolymer is transported to a viscosity increasing equipment for final polymerization to obtain a polyglycolic acid product.

[0071] In the above first step, the purity of the glycolide used is not less than 98%; preferably not less than 98.5%, and the acidity does not exceed 20 mmol / kg.

[0072] In an embodiment of the present invention, glycolide powder is used in the first step.

[0073] In an embodiment of the present invention, in the above first step, glycolide powder with a purity of not less than 98.5% is introduced into a melting and mixing kettle, heated to 90 - 120 °C under normal pressure, and an appropriate amount of reaction aids is added while stirring to make the molten glycolide and the reaction aids mix evenly, obtaining a premix in a fluid state.

[0074] The reaction aids used in the above first step include a catalyst, an initiator, and a dehydrating agent; in terms of the dosage of the reaction aids, the dosage of the catalyst is about 0.001 - 5 wt% of the mass of the glycolide powder, the dosage of the initiator does not exceed about 5 wt% of the mass of the glycolide powder (for example, but not limited to, 0.1 - 4 wt%, 1 - 3 wt%, etc.), and the dosage of the dehydrating agent is about 0.2 - 1.6 wt% of the mass of the glycolide powder.

[0075] The catalyst can be selected from at least one of tin compounds, antimony compounds, or zinc compounds, for example, but not limited to, stannous octanoate, stannous chloride, stannous lactate, antimony trioxide, diethylzinc, or zinc acetate dihydrate.

[0076] The initiator can be selected from one or two of alkane substances with hydroxyl structures such as primary alcohols or secondary alcohols (for example, n - propanol, isopropanol, n - butanol, isobutanol, etc.) or aromatic substances with hydroxyl active groups (for example, benzyl alcohol, phenethyl alcohol, etc.).

[0077] The dehydrating agent can be selected from carbodiimide, polycarbodiimide, or carbodiimide - based compounds (for example, but not limited to, N,N'-diisopropylcarbodiimide, dicyclohexylcarbodiimide, etc.).

[0078] To prevent the local concentration of the reaction aids in the molten glycolide from being too high, in an embodiment of the present invention, the reaction aids can be added drop by drop to the melting and mixing kettle by an injection method.

[0079] The static mixer in the above second step adopts at least 2 - stage gradient heating, for example, adopts a 2 - 10 - stage gradient heating; preferably adopts 3 - 7 stages.

[0080] In an embodiment of the present invention, the temperature range of the first stage of the static mixer is between 120 - 220 °C, for example, but not limited to, 140 - 170 °C, 150 - 180 °C, 190 - 220 °C, etc.; the temperature range of the last stage is between 130 - 250 °C, for example, but not limited to, 150 - 200 °C, 190 - 230 °C, 220 - 250 °C, etc.

[0081] In one embodiment of the present invention, the temperature of the second section of the static mixer is increased by 10 - 100 °C compared to the first section, for example but not limited to 40 - 50 °C, 20 - 70 °C, 30 - 60 °C, 80 - 90 °C, etc.; the temperature of the last section is increased by 0 - 30 °C compared to the previous adjacent section, for example but not limited to 10 - 20 °C, etc.

[0082] In one example of the present invention, the static mixer has a total of four sections. The temperature of the first section is set to 120 - 180 °C, the temperature of the second section is set to 190 - 210 °C, the temperature of the third section is set to 210 - 220 °C, and the temperature of the fourth section is set to 220 - 230 °C.

[0083] In one example of the present invention, the static mixer has a total of five sections. The temperature of the first section is set to 150 - 170 °C, the temperature of the second section is set to 190 - 200 °C, the temperature of the third section is set to 200 - 210 °C, the temperature of the fourth section is set to 210 - 220 °C, and the temperature of the fifth section is set to 220 - 230 °C.

[0084] In one example of the present invention, the static mixer has a total of three sections. The temperature of the first section is set to 190 - 220 °C, the temperature of the second section is set to 200 - 240 °C, and the temperature of the third section is set to 220 - 250 °C.

[0085] In one embodiment of the present invention, in the working state, the total residence time of the material in the static mixer generally should not exceed 150 minutes, for example but not limited to 60 - 90 minutes.

[0086] The weight - average molecular weight of the glycolic acid prepolymer obtained in the above - mentioned second step is about 50,000 - 150,000.

[0087] In the actual application of the static mixer, when the viscosity of the material in a fluid state between two adjacent sections is too high, a melt pump can be installed between these two adjacent sections to promote the forward flow of the material.

[0088] In the above - mentioned third step, the viscosity - increasing device only has a devolatilization section. The absolute pressure of this devolatilization section is ≤500 Pa, and the temperature is set to 220 - 250 °C.

[0089] Preferably, the screw length - to - diameter ratio of the devolatilization section is set to 30 - 100, for example but not limited to 40 - 50, 60 - 90, etc.

[0090] During the production process, according to actual requirements, processing aids (such as toughening agents, compatibilizers, plasticizers, chain extenders, hydrolysis - regulating promoters, hydrolysis - regulating inhibitors, heat stabilizers, antioxidants, antibacterial agents, or lubricants, etc.) can be added at the starting point of the devolatilization section through a loss - in - weight feeder.

[0091] It should be noted here that in this technology, the tackifying equipment plays a role in devolatilization, which can promote the further polymerization of the glycolic acid prepolymer and timely remove the generated small molecules from the system to further increase the molecular weight of the polymer.

[0092] In a preferred embodiment of the present invention, the temperature control of each pipeline is as follows:

[0093] The temperature of the pipeline between the bottom discharge port of the melting and mixing kettle and the feed port of the melt metering pump is set to 90 - 120 °C, the temperature of the pipeline between the discharge port of the melt metering pump and the feed port of the static mixer is set to 120 - 160 °C, and the temperature of the material conveying pipeline is set to 220 - 250 °C.

[0094] In an embodiment of the present invention, the following steps are further included before the above first step to obtain the glycolide used in the above first step: introducing the molten crude glycolide into a falling film evaporator, and performing evaporation and purification at a vacuum and a temperature 5 - 30 °C (preferably 10 - 20 °C) higher than the boiling point of the glycolide corresponding to the vacuum. The obtained material is transferred to a centrifugal washing machine, and after washing and centrifugal separation, the solid is transferred to a vacuum dryer for vacuum drying. The dried solid is then pulverized by a pulverizer and screened by a sieve to obtain purified glycolide powder.

[0095] As used in the present invention, "crude glycolide" can be a mixture obtained by conventional methods in the art. Based on the total weight of the mixture, the content of glycolide therein is below 98 wt%. This mixture also contains heavy component impurities and light component impurities. Heavy component impurities generally refer to poorly soluble glycolic acid polymers, such as tetramers, pentamers, hexamers, etc. of glycolic acid. Light component impurities generally refer to glycolic acid, water, other acids (such as but not limited to, free methoxyacetic acid, free oxalic acid) or residual solvents, etc.

[0096] In an embodiment of the present invention, the falling film evaporator contains n (sections) heat exchange devices, where n is an integer greater than or equal to 2, and can be, for example, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

[0097] As used in the present invention, the "heat exchange device" can be a heat exchange tube, a heat exchange surface, etc.

[0098] The molten crude glycolide used in the present invention can be obtained by conventional methods in the art. For example, but not limited to, placing the crude glycolide at 90 ± 5 °C to completely melt it.

[0099] In the present invention, the feeding rate of the general crude glycolide into the falling film evaporator is 300 - 1000 g / hour.

[0100] In the present invention, the pressure in the falling film evaporator is generally a vacuum, for example, the absolute pressure ≤ 5 kPa, preferably 0.4 - 4 kPa.

[0101] In one embodiment of the present invention, the evaporation and purification temperature in the falling film evaporator is generally less than 200 °C, preferably not greater than 180 °C, and more preferably not greater than 160 °C.

[0102] In one embodiment of the present invention, the temperatures of the n heat exchange devices in the falling film evaporator are different and increase gradually in the flow direction of the molten crude glycolide or from top to bottom; the temperature gradient can be that the temperature difference between adjacent heat exchange devices is 1 - 10 °C; preferably 2 - 8 °C.

[0103] In one embodiment of the present invention, when the vacuum in the falling film evaporator is set to an absolute pressure of 0.4 kPa, the heat exchange surface of the falling film evaporator is set to four sections of heat exchange surfaces, and the temperatures of each section of the heat exchange surface from top to bottom are respectively set to 160 °C, 164 °C, 168 °C, and 170 °C.

[0104] In one embodiment of the present invention, when the vacuum in the falling film evaporator is set to an absolute pressure of 1.1 kPa, the heat exchange surface of the falling film evaporator is set to five sections of heat exchange surfaces, and the temperatures of each section of the heat exchange surface from top to bottom are respectively set to 170 °C, 175 °C, 178 °C, 182 °C, and 185 °C.

[0105] In one embodiment of the present invention, when the vacuum in the falling film evaporator is set to an absolute pressure of 2.0 kPa, the heat exchange surface of the falling film evaporator is set to three sections of heat exchange surfaces, and the temperatures of each section of the heat exchange surface from top to bottom are respectively set to 180 °C, 186 °C, and 192 °C.

[0106] In one embodiment of the present invention, when the vacuum in the falling film evaporator is set to an absolute pressure of 3.1 kPa, the heat exchange surface of the falling film evaporator is set to three sections of heat exchange surfaces, and the temperatures of each section of the heat exchange surface from top to bottom are respectively set to 190 °C, 194 °C, and 196 °C.

[0107] In one embodiment of the present invention, when the vacuum in the falling film evaporator is set to an absolute pressure of 4.1 kPa, the heat exchange surface of the falling film evaporator is set to four sections of heat exchange surfaces, and the temperatures of each section of the heat exchange surface from top to bottom are respectively set to 190 °C, 192 °C, 195 °C, and 198 °C.

[0108] In one embodiment of the present invention, when the vacuum in the falling film evaporator is set to an absolute pressure of 5.0 kPa, the heat exchange surface of the falling film evaporator is set to two sections of heat exchange surfaces, and the temperatures of each section of the heat exchange surface from top to bottom are respectively set to 195 °C and 198 °C.

[0109] In one embodiment of the present invention, the average film-forming thickness of the material in the falling film evaporator along the heat transfer surface of the falling film evaporator from top to bottom is ≤300 μm, preferably ≤120 μm, and more preferably ≤100 μm.

[0110] In one embodiment of the present invention, the evaporation residence time of the material on the heat transfer surface in the falling film evaporator does not exceed 5 min.

[0111] In the present invention, the washing in the centrifugal washing machine is generally carried out at room temperature (10-40 °C, preferably 20-30 °C) using an alcohol solvent; the alcohol solvents include, but are not limited to, anhydrous ethanol, methanol, n-propanol, isopropanol, n-butanol, isobutanol or tert-butanol, etc.

[0112] In one embodiment of the present invention, after the above washing, solid-liquid separation is carried out, and drying is carried out to obtain a refined glycolide product.

[0113] Solid-liquid separation can be carried out by conventional methods in the art, such as, but not limited to, filtration, suction filtration, centrifugal separation, etc.; drying can be carried out by conventional methods in the art, such as, but not limited to, vacuum drying, oven drying, desiccator drying or infrared drying, etc.; in one embodiment of the present invention, vacuum drying is adopted: drying is carried out at an absolute pressure ≤500 Pa and 40-60 °C for 1-2 hours.

[0114] The molecular weight distribution index of the polyglycolic acid product prepared by the method of the present invention is about 1.2-1.3.

[0115] As used herein, the "distribution index" is a parameter D representing the width of the molecular weight distribution, D = M(w) / M(n), where M(w) is the weight-average molecular weight and M(n) is the number-average molecular weight. When D = 1, it is a polymer with a uniform molecular weight. The larger the value of D, the wider the molecular weight distribution and the greater the degree of polydispersity. The measurement method generally uses gel permeation chromatography (Gel Permeation Chromatography, GPC) to measure M(w) and M(n) of the sample, and then calculate the ratio of the two.

[0116] In the first step of the method of the present invention, the content of glycolide in the reaction material used (i.e., the glycolide material) can be determined by gas chromatography analysis well-known in the art, and the acidity can be determined by potentiometric titration well-known in the art (for example, using an automatic potentiometric titrator).

[0117] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0118] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0119] In this article, all features defined in the form of numerical ranges or percentage ranges, such as values, quantities, contents and concentrations, are only for brevity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values within the range (including integers and fractions).

[0120] As used herein, when the term "about" is used to modify a numerical value, it indicates an error tolerance of measurement within ±5% of the numerical value.

[0121] The above-mentioned features mentioned in the present invention or the features mentioned in the embodiments can be combined in any way. All the features disclosed in the specification of this case can be used in combination with any combination form. As long as there is no contradiction in the combination of these features, all possible combinations should be considered to be within the scope of this specification. Each feature disclosed in the specification can be replaced by any alternative feature that can provide the same, equal or similar purpose. Therefore, unless otherwise specified, the disclosed features are only general examples of equal or similar features.

[0122] The main advantages of the present invention are:

[0123] 1. Compared with the conventional recrystallization purification process, the present invention purifies the crude glycolide by a falling film evaporator without using a solvent. Under the premise of shortening the evaporation and heating time of glycolide as much as possible to reduce the degree of self-polymerization of glycolide, the heavy components in the crude glycolide are removed to the maximum extent, and the light components in the crude glycolide (such as glycolic acid, water or free acid, etc.) can be removed in the subsequent washing. That is, the use of a falling film evaporator can not only significantly reduce the amount of solvent used, but also greatly reduce the comprehensive energy consumption of the crude glycolide purification process, saving production costs.

[0124] 2. The present invention couples a melting and mixing kettle with a static mixer. First, glycolide and a reaction assistant are fully and evenly mixed in the melting and mixing kettle to obtain a premix, and then the premix is introduced into the static mixer for prepolymerization. By creatively using high-efficiency mixing to achieve low-shear and high-dispersion effects on the premix, it is beneficial to prevent and eliminate heat accumulation in local areas of the material, and can effectively prevent side reactions such as local overheating and thermal degradation caused by uneven heat absorption in the material, thereby ensuring that glycolide can undergo a good prepolymerization reaction to obtain a glycolic acid prepolymer with a certain molecular weight. Then, the glycolic acid prepolymer is introduced into a viscosity-increasing device (for example, a twin-screw extruder equipped only with a devolatilization section) for final polymerization, which can effectively shorten the time for the material to be subjected to high-shear action in the viscosity-increasing device. This is not only beneficial to inhibiting the occurrence of side reactions such as thermal degradation, but also beneficial to inhibiting the occurrence of transesterification reactions, thereby reducing the content of oligomers and / or low-molecular-chain substances in the system. The prepared polyglycolic acid not only has a significantly increased molecular weight, but also has a smaller molecular weight distribution index and a more uniform molecular weight distribution.

[0125] 3. The present invention introduces the fluid-state premix of molten glycolide into a static mixer, and uses the cross-flow method to strengthen the mixing effect between glycolide and the reaction assistant, so that the reaction assistant can be more evenly dispersed in the reaction system. At the same time, by using the method of gradient temperature increase, the ring-opening polymerization reaction of glycolide is gently initiated at a relatively low temperature and for a relatively short time first, and then the temperature is appropriately increased and the time is appropriately extended to form a relatively stable and reactive glycolic acid molecular chain in the reaction system. Then, at a relatively high temperature and for a relatively long time, the further growth of the glycolic acid molecular chain is promoted to obtain a glycolic acid prepolymer with a certain molecular weight. The gradient temperature increase process of the above static mixer is beneficial to inhibiting the sharp change of the viscosity of the reaction system, and can effectively avoid the occurrence of phenomena such as excessive heat accumulation and coking and slagging caused by the sharp change of the local viscosity of the reaction system.

[0126] 4. The main polymerization reaction of the present invention is carried out in a static mixer, which minimizes the influence of oxygen and moisture on the polymerization reaction; and by using the characteristic of good uniform heat transfer of the static mixer, the molecular chain in the polymerization reaction can maintain stable growth for a longer time. In a conventional reactive twin-screw extruder, due to the high-shear action, the local temperature of the reaction system rises too fast, which is more likely to cause side reactions such as thermal degradation earlier, is not conducive to the increase of the molecular weight of the final product, and will also lead to the broadening of the molecular weight distribution of the final product, which will have a negative impact on the processing of the material and its mechanical properties and aging resistance.

[0127] 5. The present invention is suitable for scale-up production, is beneficial to saving the transformation cost of the process flow, can achieve low-carbon continuous production, and has outstanding economic practicality.

[0128] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. The experimental methods without specific conditions noted in the following embodiments are generally carried out under conventional conditions or according to the conditions recommended by the manufacturer. Unless otherwise specified, all percentages, ratios, proportions, or parts are by weight. The units in the weight / volume percentages in the present invention are well-known to those skilled in the art. For example, it refers to the weight (grams) of the solute in 100 milliliters of the solution. Unless otherwise defined, all professional and scientific terms used herein have the same meaning as those familiar to those skilled in the art. In addition, any methods and materials similar or equivalent to the described content can be applied to the method of the present invention. The preferred implementation methods and materials described herein are for illustrative purposes only.

[0129] In the following embodiments, the content of glycolide in the purified glycolide powder involved can be determined by gas chromatography analysis well-known in the art. The particle size of the glycolide powder is determined by the mesh of the sieve. For example, using a 70-mesh sieve (corresponding to about 200 μm) or a sieve with a mesh of 70 or more, the glycolide powder that can pass through the sieve has a particle size that can basically meet the requirement of "D 90 ≤ 200 μm", and the acidity is determined by potentiometric titration well-known in the art (for example, using an automatic potentiometric titrator).

[0130] Regarding the test of the molecular weight and its distribution of polyglycolic acid, gel permeation chromatography (GPC) method is used for testing, specifically as follows:

[0131] Take 0.2 g of PGA sample, dissolve it in 100 mL of hexafluoroisopropanol solution with a sodium trifluoroacetate content of 5 mmol / L, filter it through a polytetrafluoroethylene filter membrane with a pore size of 0.4 μm, take 20 μL and add it to the "LC-20AD GPC" injector manufactured by Shimadzu (Japan). The test conditions are as follows: column temperature 40 °C; eluent: hexafluoroisopropanol dissolved with 5 mmol / L sodium trifluoroacetate; flow rate 1 mL / min; detector: RI detector; calibration: use five different standard polymethyl methacrylates with molecular weights ranging from 7000 to 200000 for molecular weight calibration.

[0132] In the following embodiments, the volume of the melting and stirring kettle used is 50 L, the volume of the static mixer is 8 L, the maximum delivery flow rate of the melt metering pump is 10 L / h, and the maximum delivery flow rate of the melt pump is 12 L / h.

[0133] The static mixer used in the following embodiments is a commercially available SK-type static mixer.

[0134] Device Embodiment 1

[0135] Provide as attachedFigure 1 The preparation device of polyglycolic acid with a narrow molecular weight distribution as shown

[0136] Device A for preparing PGA with a narrow molecular weight distribution includes a melting and mixing kettle 100, a static mixer 200 coupled with the melting and mixing kettle, and a viscosity increasing device 300 (shown in the form of a twin-screw extruder) arranged downstream of the static mixer.

[0137] A melt metering pump 120 is arranged between the static mixer and the melting and mixing kettle. The bottom discharge port of the melting and mixing kettle 100 is connected to the feed port of the static mixer 200 through the melt metering pump 120.

[0138] The discharge port of the static mixer 200 is connected to the feed port of the viscosity increasing device 300 through a material conveying assembly 230.

[0139] The material conveying assembly 230 includes a material conveying pipeline 231 connecting the discharge port of the static mixer 200 to the feed port of the viscosity increasing device 300, a melt pump 232 arranged on the material conveying pipeline, a cleaning liquid drainage pipeline 233 arranged in parallel with the melt pump 232, and a cleaning liquid drainage branch 234 is arranged on the material conveying pipeline 231 between the melt pump 232 and the feed port of the viscosity increasing device 300.

[0140] Both the liquid inlet end and the discharge end of the cleaning liquid drainage pipeline 233 are connected in parallel to both sides of the melt pump 232 through a three-way valve 235-I.

[0141] The cleaning liquid drainage branch 234 is connected to the material conveying pipeline 231 through a three-way valve 235-II.

[0142] The top of the melting and mixing kettle 100 is provided with a reaction material feed port and a reaction auxiliary feed port. Among them, the reaction material feed port is connected to a loss-in-weight scale 1112 through a material feed pipe 112, and the reaction auxiliary feed port is connected to a reaction auxiliary storage tank 110 arranged upstream of the melting and mixing kettle through an auxiliary feed pipe 111. A liquid metering pump 1111 is also arranged on the feed pipe 111 between the reaction auxiliary storage tank 110 and the reaction auxiliary feed port.

[0143] The pipelines between the bottom discharge port of the melting and mixing kettle 100 and the feed port of the static mixer 200, the material conveying pipeline 231, the cleaning liquid drainage pipeline 233, the melt metering pump 120, the melt pump 232, the three-way valve 235-I, and the three-way valve 235-II are all equipped with conventional heat tracing.

[0144] Device Example 2

[0145] Provide the preparation device of polyglycolic acid with a narrow molecular weight distribution as shown in the appendix Figure 2 The preparation device of polyglycolic acid with a narrow molecular weight distribution as shown

[0146] The apparatus A' for narrow molecular weight distribution PGA includes a melt mixing kettle 100, a static mixer 200 coupled with the melt mixing kettle, and a viscosity increasing device 300 (shown in the form of a twin-screw extruder) provided downstream of the static mixer.

[0147] A melt metering pump 120 is provided between the static mixer and the melt mixing kettle. The bottom discharge port of the melt mixing kettle 100 is connected to the feed port of the static mixer 200 through the melt metering pump 120.

[0148] The discharge port of the static mixer 200 is connected to the feed port of the viscosity increasing device 300 through a material conveying assembly 230.

[0149] The material conveying assembly 230 includes a material conveying pipeline 231 connecting the discharge port of the static mixer 200 to the feed port of the viscosity increasing device 300, a melt pump 232 provided on the material conveying pipeline, a cleaning liquid drainage pipeline 233 arranged in parallel with the melt pump 232, and a cleaning liquid drainage branch 234 provided on the material conveying pipeline 231 between the melt pump 232 and the feed port of the viscosity increasing device 300.

[0150] Both the inlet end and the discharge end of the cleaning liquid drainage pipeline 233 are connected in parallel to both sides of the melt pump 232 through a three-way valve 235-I.

[0151] The cleaning liquid drainage branch 234 is connected to the material conveying pipeline 231 through a three-way valve 235-II.

[0152] The apparatus A' further includes a falling film evaporation and purification unit 400 provided upstream of the melt mixing kettle 100 and coupled with the melt mixing kettle 100.

[0153] The falling film evaporation and purification unit 400 includes a falling film evaporator 410, a centrifugal washer 420, a vacuum dryer 430, a crusher 440, and a sieve 450 arranged in sequence below the falling film evaporator. The discharge port at the bottom of the sieve 450 is connected to the feed port at the top of the melt mixing kettle 100 through a loss-in-weight weigher 1112.

[0154] The top of the melt mixing kettle 100 is provided with a reaction material feed port and a reaction aid feed port. Among them, the reaction material feed port is connected to the loss-in-weight weigher 1112 through a material feed pipe 112, and the reaction aid feed port is connected to a reaction aid storage tank 110 provided upstream of the melt mixing kettle through an aid feed pipe 111. A liquid metering pump 1111 is further provided on the feed pipe 111 between the reaction aid storage tank 110 and the reaction aid feed port.

[0155] The heat exchange surface of the falling film evaporator 410 is a multi-section heat exchange surface with the temperature increasing gradually from top to bottom and showing a gradient distribution.

[0156] The pipelines between the bottom discharge port of the melting and mixing kettle 100 and the feed port of the static mixer 200, the material conveying pipeline 231, the cleaning liquid drainage pipeline 233, the melt metering pump 120, the melt pump 232, the three-way valve 235-I and the three-way valve 235-II are all equipped with conventional heat tracing.

[0157] Preparation Example 1

[0158] The molten crude glycolide was introduced into a falling film evaporator (for example, with a height of 30 cm and a bottom diameter of 10 cm) at a feed rate of 600 g / h. The vacuum in the falling film evaporator was set to an absolute pressure of about 0.4 kPa. The heat exchange surface of the falling film evaporator was divided into four sections of heat exchange surfaces. The temperatures of each section of the heat exchange surface from top to bottom were set to about 160 °C, about 164 °C, about 168 °C, and about 170 °C respectively. The average film thickness of the material along the entire heat exchange surface from top to bottom was about 92 μm. The evaporation residence time of the material on the entire heat exchange surface was about 68 s. The glycolide material after evaporation and purification was transferred to a centrifugal washer. After washing and centrifugal separation, the solid was transferred to a vacuum dryer for vacuum drying (absolute pressure ≤ 500 Pa, dried at 60 °C for 1 hour). The dried solid was then crushed by a crusher and screened by a sieve to obtain purified glycolide powder (D 90 ≤ 200 μm, purity ≥ 98.5%, acidity ≤ 20 mmol / kg); Subsequently, the purified glycolide powder was introduced into the melting and mixing kettle, heated to about 115 °C at atmospheric pressure, and while stirring, the reaction aids [based on the mass of the purified glycolide powder, the addition amount of the catalyst (stannous octoate) was about 0.001 wt%, the addition amount of the initiator (isopropanol) was about 0.1 wt%, and the addition amount of the dehydrating agent (dicyclohexylcarbodiimide) was about 0.2 wt%] were all added to the melting and mixing kettle by an existing injection method within about 5 min. Stirring and mixing continued for about 10 min to make the molten glycolide and the reaction aids evenly mixed, obtaining a premix in a fluid state; The premix in a fluid state was transported to a static mixer through a melt metering pump for prepolymerization to obtain a glycolic acid prepolymer (sampled and measured by GPC, its weight average molecular weight was about 138,200). Subsequently, the glycolic acid prepolymer was transported to a viscosity increasing device (for example, a twin-screw extruder with only a devolatilization section) for final polymerization. The material coming out of the head of the viscosity increasing device was quenched with liquid nitrogen to obtain a polyglycolic acid product.

[0159] Preparation Example 2

[0160] The molten crude glycolide is introduced into a falling film evaporator (for example, with a height of 45 cm and a bottom diameter of 10 cm) at a feed rate of 500 g / h. The vacuum in the falling film evaporator is set to an absolute pressure of about 1.1 kPa. The heat exchange surface of the falling film evaporator consists of five sections of heat exchange surfaces. The temperatures of each section of the heat exchange surface from top to bottom are set to about 170 °C, about 175 °C, about 178 °C, about 182 °C, and about 185 °C respectively. The average film-forming thickness of the material along the entire heat exchange surface from top to bottom is about 101 μm. The evaporation residence time of the material on the entire heat exchange surface is about 134 s. The glycolide material after evaporation and purification is transferred to a centrifugal washer. After washing and centrifugal separation, the solid is transferred to a vacuum dryer for vacuum drying (absolute pressure ≤ 500 Pa, dried at 60 °C for 1 hour). The dried solid is then crushed by a crusher and screened by a sieve to obtain purified glycolide powder (D 90 ≤ 200 μm, purity ≥ 98.5%, acidity ≤ 20 mmol / kg); Subsequently, the purified glycolide powder is introduced into a melting and mixing kettle, heated to about 118 °C under normal pressure, and while stirring, all the reaction aids

based on the mass of the purified glycolide powder, the addition amount of the catalyst (a mixture of stannous chloride and antimony trioxide with a mass ratio of 1:2) is about 0.1 wt%, the addition amount of the initiator (isopropanol) is about 1.0 wt%, and the addition amount of the dehydrating agent (N,N'-diisopropylcarbodiimide) is about 0.5 wt%

[0161] Preparation Example 3

[0162] The molten crude glycolide is introduced into a falling film evaporator (for example, with a height of 60 cm and a bottom diameter of 20 cm) at a feeding rate of 700 g / h. The vacuum in the falling film evaporator is set to an absolute pressure of about 2.0 kPa. The heat exchange surface of the falling film evaporator has three sections of heat exchange surfaces, and the temperatures of each section of the heat exchange surface from top to bottom are set to about 180 °C, about 186 °C, and about 192 °C respectively. The average film-forming thickness of the material along the entire heat exchange surface from top to bottom is about 117 μm, and the evaporation residence time of the material on the entire heat exchange surface is about 296 s. The glycolide material after evaporation and purification is transferred to a centrifugal washer. After washing and centrifugal separation, the solid is transferred to a vacuum dryer for vacuum drying (absolute pressure ≤ 500 Pa, drying at 40 °C for 2 hours). The dried solid is then crushed by a crusher and screened by a sieve to obtain purified glycolide powder (D 90 ≤ 200 μm, purity ≥ 98.5%, acidity ≤ 20 mmol / kg); Subsequently, the purified glycolide powder is introduced into a melting and mixing kettle, heated to about 110 °C under normal pressure, and while stirring, all the reaction aids

based on the mass of the purified glycolide powder, the addition amount of the catalyst (a mixture of diethylzinc and stannous lactate in a mass ratio of 1:1) is about 0.01 wt%, the addition amount of the initiator (n-propanol) is about 0.2 wt%, and the addition amount of the dehydrating agent (dicyclohexylcarbodiimide) is about 1.6 wt%

[0163] Preparation Example 4

[0164] The molten crude glycolide is introduced into a falling-film evaporator (for example, with a height of 36 cm and a bottom diameter of 8 cm) at a feeding rate of 980 g / h. The vacuum in the falling-film evaporator is set to an absolute pressure of about 3.1 kPa. The heat exchange surface of the falling-film evaporator is composed of three sections of heat exchange surfaces. The temperatures of each section of the heat exchange surface from top to bottom are set to about 190 °C, about 194 °C, and about 196 °C respectively. The average film-forming thickness of the material along the entire heat exchange surface from top to bottom is about 298 μm. The evaporation residence time of the material on the entire heat exchange surface is about 129 s. The time for the material to pass through the heat exchange surface of the falling-film evaporator is controlled to be about 40 minutes. The film-forming thickness of the material along the heat exchange surface from top to bottom is ≤100 μm. The glycolide material after evaporation and purification is transferred to a centrifugal washing machine. After washing and centrifugal separation, the solid is transferred to a vacuum dryer for vacuum drying (absolute pressure ≤500 Pa, drying at 50 °C for 2 hours). The dried solid is then pulverized by a pulverizer and screened by a sieve to obtain purified glycolide powder (D 90 ≤200 μm, purity ≥98.5%, acidity ≤20 mmol / kg); Subsequently, the purified glycolide powder is introduced into a melting and mixing kettle, heated to about 120 °C under normal pressure, and while stirring, all the reaction aids [based on the mass of the purified glycolide powder, the addition amount of the catalyst (antimony trioxide) is about 0.05 wt%, the addition amount of the initiator (n-butanol) is about 0.8 wt%, and the addition amount of the dehydrating agent (carbodiimide) is about 0.3 wt%] are added to the melting and mixing kettle by the existing injection method within about 5 min. Stirring and mixing continue for about 10 min to make the molten glycolide and the reaction aids mix evenly to obtain a premix in a fluid state; The premix in a fluid state is transported to a static mixer by a melt metering pump for prepolymerization to obtain a glycolic acid prepolymer (sampled and measured by GPC, its weight-average molecular weight is about 87,600). Subsequently, the glycolic acid prepolymer is transported to a viscosity-increasing device (for example, a twin-screw extruder only equipped with a devolatilization section) for final polymerization. The material coming out of the head of the viscosity-increasing device is quenched with liquid nitrogen to obtain a polyglycolic acid product.

[0165] Preparation Example 5

[0166] The molten crude glycolide is introduced into a falling film evaporator (e.g., with a height of 20 cm and a bottom diameter of 10 cm) at a feed rate of 300 g / h. The vacuum in the falling film evaporator is set to an absolute pressure of about 4.1 kPa. The heat exchange surface of the falling film evaporator consists of four sections of heat exchange surfaces. The temperatures of each section of the heat exchange surface from top to bottom are set to about 190 °C, about 192 °C, about 195 °C, and about 198 °C respectively. The average film thickness of the material along the entire heat exchange surface from top to bottom is about 53 μm, and the evaporation residence time of the material on the entire heat exchange surface is about 52 s. The glycolide material after evaporation and purification is transferred to a centrifugal washer. After washing and centrifugal separation, the solid is transferred to a vacuum dryer for vacuum drying (absolute pressure ≤ 500 Pa, drying at 60 °C for 2 hours). The dried solid is then pulverized by a pulverizer and screened by a sieve to obtain purified glycolide powder (D 90 ≤ 200 μm, purity ≥ 98%, acidity ≤ 20 mmol / kg); Subsequently, the purified glycolide powder is introduced into a melting and mixing kettle, heated to about 112 °C under normal pressure, and while stirring, all the reaction aids [based on the mass of the purified glycolide powder, the addition amount of the catalyst (stannous octoate, antimony trioxide, and zinc acetate dihydrate in a mass ratio of 5:2:3) is about 1.9 wt%, the addition amount of the initiator (benzyl alcohol) is about 3.4 wt%, and the addition amount of the dehydrating agent (carbodiimide) is about 1.1 wt%] are added to the melting and mixing kettle by the existing injection method within about 5 min. Stirring is continued for about 10 min to uniformly mix the molten glycolide with the reaction aids to obtain a premix in a fluid state; The premix in a fluid state is transported to a static mixer by a melt metering pump for prepolymerization to obtain a glycolic acid prepolymer (sampled and measured by GPC, its weight average molecular weight is about 50,300). Subsequently, the glycolic acid prepolymer is transported to a viscosity increasing device (e.g., a twin-screw extruder with only a devolatilization section) for final polymerization, and the material coming out of the head of the viscosity increasing device is quenched with liquid nitrogen to obtain a polyglycolic acid product.

[0167] Preparation Example 6

[0168] The molten crude glycolide is introduced into a falling film evaporator (for example, with a height of 40 cm and a bottom diameter of 10 cm) at a feeding rate of 500 g / hour. The vacuum in the falling film evaporator is set to an absolute pressure of about 5.0 kPa. The heat exchange surface of the falling film evaporator is a two-stage heat exchange surface, and the temperatures of each stage of the heat exchange surface from top to bottom are set to about 195 °C and about 198 °C respectively. The average film thickness of the material along the entire heat exchange surface from top to bottom is about 110 μm, and the evaporation residence time of the material on the entire heat exchange surface is about 130 s. The glycolide material after evaporation and purification is transferred to a centrifugal washer. After washing and centrifugal separation, the solid is transferred to a vacuum dryer for vacuum drying (absolute pressure ≤ 500 Pa, drying at 60 °C for 2 hours). The dried solid is then crushed by a crusher and screened by a sieve, and purified glycolide powder (D 90 ≤ 200 μm, purity ≥ 98%, acidity ≤ 20 mmol / kg) is obtained; subsequently, the purified glycolide powder is introduced into a melting and mixing kettle, heated to about 116 °C under normal pressure, and while stirring, all the reaction aids [based on the mass of the purified glycolide powder, the addition amount of the catalyst (stannous octoate) is about 4.2 wt%, the addition amount of the initiator (a mixture of isopropanol and benzyl alcohol in a mass ratio of 4:1) is about 5.0 wt%, and the addition amount of the dehydrating agent (polycarbodiimide) is about 0.7 wt%] are added to the melting and mixing kettle by the existing injection method within about 5 min. Stirring and mixing continue for about 10 min to make the molten glycolide and the reaction aids mix evenly, and a premix in a fluid state is obtained; the premix in a fluid state is transported to a static mixer through a melt metering pump for prepolymerization to obtain a glycolic acid prepolymer (sampled and measured by GPC, its weight average molecular weight is about 56,200), and then the glycolic acid prepolymer is transported to a viscosity-increasing device (for example, a twin-screw extruder only equipped with a devolatilization section) for final polymerization. The material coming out of the head of the viscosity-increasing device is quenched with liquid nitrogen, and a polyglycolic acid product is prepared.

[0169] The temperature parameters of each section in the static mixer in the above Examples 1-6 are shown in Table 1-1 as follows:

[0170] Table 1-1

[0171] Item Static mixer The first section The second section The third section The fourth section The fifth section Preparation Example 1 A total of four sections About 120 °C About 190 °C About 213 °C About 224 °C / Preparation Example 2 A total of four sections About 180 °C About 209 °C About 220 °C About 230 °C / Preparation Example 3 A total of five sections About 150 °C About 190 °C About 202 °C About 213 °C About 220 °C Preparation Example 4 A total of five sections About 168 °C About 200 °C About 210 °C About 220 °C About 230 °C Preparation Example 5 A total of three sections About 190 °C About 205 °C About 221 °C / / Preparation Example 6 A total of three sections About 220 °C About 238 °C About 250 °C / /

[0172] The time required for the material in the static mixer to pass through each section in the above Preparation Examples 1-6 is shown in Table 1-2 as follows:

[0173] Table 1-2

[0174]

[0175]

[0176] The parameter settings of the devolatilization section in the viscosity increasing equipment in the above-mentioned preparation examples 1-6 are shown in Table 2 below:

[0177] Table 2

[0178] Item Temperature (°C) Absolute pressure (Pa) Screw length-diameter ratio Screw rotation speed (r / min) Preparation Example 1 About 235 100 52 30 Preparation Example 2 About 240 250 50 45 Preparation Example 3 About 240 120 56 60 Preparation Example 4 About 236 150 60 30 Preparation Example 5 About 221 380 50 45 Preparation Example 6 About 250 500 54 60

[0179] Note: In the above-mentioned preparation examples 1-6, the time for the glycolic acid prepolymer to be in the devolatilization section of the viscosity increasing equipment is about 15 minutes.

[0180] The temperature settings of the pipelines from the melt mixing kettle to the viscosity increasing device in the above-mentioned preparation examples 1-6 are shown in Table 3 below:

[0181] Table 3

[0182]

[0183] Note: The delivery flow rates of the melt metering pump and melt pump used in the above-mentioned Preparation Examples 1-3 were both set to about 6.7 L / h, and the delivery flow rates of the melt metering pump and melt pump used in Examples 4-6 were both set to about 5.3 L / h.

[0184] Comparative Example

[0185] The crude glycolide was purified by the following conventional recrystallization technique:

[0186] The crude glycolide is added to an appropriate amount of acetone (the mass of acetone is about 2-4 times the mass of the crude glycolide), the glycolide is dissolved at about 60°C, and then filtered using a 1-3 μm quantitative filter paper. The filtered liquid is cooled to about 20°C at a rate of about 0.2°C per minute, and then cooled to about -15°C at a rate of about 0.4-0.5°C per minute. The above cooling process takes about 5 hours in total. When the temperature drops to about -15°C, the temperature is kept constant for about 1 hour, and then filtered and solid-liquid separated. The above steps are repeated 2-4 times to obtain purified glycolide (purity ≥98.5%, acidity ≤20mmol / kg).

[0187] The purified glycolide obtained by the recrystallization technology is added to the twin-screw extruder through the main feed port, and the reaction aid is added through the side feed port to allow the glycolide to undergo a polymerization reaction in the twin-screw extruder. The material coming out of the head of the twin-screw extruder is quenched with liquid nitrogen to obtain a polyglycolic acid product.

[0188] According to the above method, the temperature settings of each section in the twin-screw extruder used in Comparative Examples 1-6 are shown in Table 4-1 below:

[0189] Table 4-1

[0190]

[0191] Note: In Comparative Examples 1-6, the reaction assistant was added in the first stage of the mixing section.

[0192] In the above Comparative Examples 1-6, in the mixing section, the residence time of the material in the first stage was about 1 min, in the second stage was about 3 min, and in the third stage was about 6 min, with a total of 10 min; in the reaction section, the residence time of the material in the first stage was about 5 min, in the second stage was about 10 min, in the third stage was about 20 min, and in the fourth stage was about 45 min, with a total of 80 min; in the devolatilization section, the residence time of the material was about 15 min.

[0193] In the above Comparative Examples 1-6, the screw length-diameter ratio, screw rotation speed of each section of the twin-screw extruder, and the pressure setting in the devolatilization section are shown in Table 4-2 below:

[0194] Table 4-2

[0195] Item Screw length-diameter ratio of the mixing section Screw length-diameter ratio of the reaction section Screw length-diameter ratio of the devolatilization section Screw rotation speed (r / min) Absolute pressure of the devolatilization section Comparative Example 1 35 77 52 30 100 Pa Comparative Example 2 32 75 50 45 250 Pa Comparative Example 3 40 80 56 60 120 Pa Comparative Example 4 35 72 60 30 150 Pa Comparative Example 5 30 70 50 45 380 Pa Comparative Example 6 32 77 54 60 500 Pa

[0196] The reaction assistant used in the above Comparative Example 1 was the same as that in Example 1, and by analogy, the reaction assistants used in Comparative Examples 2-6 were the same as those in Examples 2-6 respectively.

[0197] The molecular weights and distribution indices of the polyglycolic acid obtained in the above Preparation Examples 1-6 and Comparative Examples 1-6 were tested as shown in Table 5 below:

[0198] Table 5

[0199] Item Preparation Example 1 Preparation Example 2 Preparation Example 3 Preparation Example 4 Preparation Example 5 Preparation Example 6 Number-average molecular weight (Mn) 144754 152544 155367 133346 105497 108446 Weight-average molecular weight (Mw) 178048 190680 186440 162682 135036 142065 Molecular weight distribution index 1.23 1.25 1.20 1.22 1.28 1.31 Item Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Number-average molecular weight (Mn) 98559 96094 96884 100055 72145 82748 Weight-average molecular weight (Mw) 166565 160477 157922 160088 132026 140672 Molecular weight distribution index 1.69 1.67 1.63 1.60 1.83 1.70

[0200] The above are only the preferred embodiments of the present invention, and are not intended to limit the scope of the essential technical content of the present invention. The essential technical content of the present invention is broadly defined in the scope of the claims of the application. Any technical entity or method completed by others, if it is exactly the same as that defined in the scope of the claims of the application, or is an equivalent change, will be regarded as covered by the scope of the claims.

Claims

1. A method for preparing polyglycolic acid with a narrow molecular weight distribution, characterized in that, The method includes the steps of: (1) Mixing glycolide powder with a reaction auxiliary evenly in a melting and mixing kettle to obtain a fluid premix, wherein the purity of the glycolide is above 98% and the acidity does not exceed 20 mmol / kg; (2) Subjecting the fluid premix obtained in step (1) to prepolymerization in a static mixer to obtain a glycolic acid prepolymer; (3) Subjecting the glycolic acid prepolymer obtained in step (2) to final polymerization through a viscosity-increasing device to obtain polyglycolic acid, wherein the viscosity-increasing device is a twin-screw extruder only provided with a devolatilization section; Before step (1), the following steps are further included to obtain the glycolide powder used in step (1): introducing molten crude glycolide into a falling-film evaporator, evaporating and purifying it under vacuum and at a temperature 5 - 30°C higher than the boiling point of glycolide corresponding to the vacuum, transferring the obtained material to a centrifugal washer, washing it with an alcohol solvent and centrifugally separating it, then transferring the solid to a vacuum dryer for vacuum drying, and further pulverizing the dried solid with a pulverizer and screening it with a sieve to obtain purified glycolide powder; The falling-film evaporator contains n heat exchange devices, where n is an integer greater than or equal to 2, the temperatures of the n heat exchange devices increase in a gradient along the flowing direction of the molten crude glycolide, the average film-forming thickness of the heat exchange device does not exceed 300 μm, and the evaporation residence time of the material on the heat exchange device does not exceed 5 min; The device for implementing the method includes a melting and mixing kettle, a static mixer connected to the discharge port of the melting and mixing kettle, and a viscosity-increasing device arranged downstream of the static mixer; The device further includes a falling-film evaporation and purification unit arranged upstream of the melting and mixing kettle and coupled with the melting and mixing kettle; The falling-film evaporation and purification unit includes a falling-film evaporator and a sieve, and the discharge port at the bottom of the sieve is connected to the feed port at the top of the melting and mixing kettle.

2. The method according to claim 1, characterized in that, For the device for implementing the method, a reaction auxiliary storage tank is connected to the feed port of the melting and mixing kettle; a melt metering pump is arranged between the discharge port of the melting and mixing kettle and the static mixer.

3. The method according to claim 1, characterized in that, The static mixer is connected to the viscosity-increasing device through a material conveying assembly; the material conveying assembly includes a material conveying pipeline connecting the discharge port of the static mixer and the feed port of the viscosity-increasing device, a melt pump arranged on the material conveying pipeline, and a cleaning liquid drain pipe connected in parallel with the melt pump on the material conveying pipeline.

4. The method according to claim 3, wherein Both the liquid inlet end and the liquid discharge end of the cleaning liquid drain pipe are connected in parallel to both sides of the melt pump through a three-way valve.

5. The method according to claim 3, characterized in that A cleaning liquid discharge branch is arranged on the material conveying pipeline between the melt pump and the feed port of the viscosity-increasing device, and the cleaning liquid discharge branch is connected to the material conveying pipeline through a three-way valve.

6. The method according to claim 1, characterized in that The static mixer is selected from an SK-type static mixer, an SX-type static mixer, and an SV-type static mixer.

7. The method according to claim 1, wherein A loss-in-weight scale is provided between the discharge port at the bottom of the sieve and the feed port at the top of the melting and mixing kettle. A reaction aid feed port is also provided at the top of the melting and mixing kettle. The reaction aid feed port is connected to a reaction aid storage tank arranged upstream of the melting and mixing kettle through a aid feed pipe. A liquid metering pump is further provided on the aid feed pipe between the reaction aid storage tank and the reaction aid feed port.

8. The method according to claim 1, characterized in that, The static mixer used in step (2) adopts at least a two-stage gradient temperature increase method.

9. The method according to claim 8, wherein The temperature range of the first stage is between 120 - 220 °C; the temperature range of the last stage is between 220 - 250 °C.

10. The method according to claim 8, wherein The temperature of the second stage is increased by 10 - 100 °C compared with the first stage; the temperature of the last stage is increased by 0 - 30 °C compared with the previous adjacent stage.

11. The method according to claim 1, wherein The weight-average molecular weight of the glycolic acid prepolymer obtained in step (2) is 50,000 - 150,000.

12. The method according to claim 1, characterized in that, The devolatilization of the tackifying device in step (3) is carried out under an absolute pressure ≤ 500 Pa and a temperature of 220 - 250 °C.

13. The method according to claim 1, characterized in that, The temperature difference between adjacent heat exchange devices is 1 - 10 °C.

14. The method according to claim 1, wherein The pressure in the falling film evaporator does not exceed 5 kPa, and the temperature of the falling film evaporator through which the molten crude glycolide passes is less than 200 °C.

15. The method according to claim 1, characterized in that, The pressure in the falling film evaporator is 0.4 - 4 kPa, and the temperature is 10 - 20 °C higher than the boiling point of glycolide at this pressure.

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