Preparation method of glycolide

Abstract

The invention provides a preparation method of glycolide, which is a polyglycolide three-stage depolymerization method. According to the method, the cracking conversion rate of the glycolide generated by cracking the polyglycolide (PGA polymer) is controlled to be less than or equal to 90%, so that almost no or only a small amount of coking and carbonization residues are generated in the cyclization reaction kettle, the coking amount is less than 0.5%, the residues can be subjected to two-stage depolymerization in the later stage to form glycolic acid monomers, and the yield of the glycolic acid monomers is improved. The glycollic acid monomer can be used as a raw material to return to a polycondensation system to synthesize a PGA polymer, and is continuously cracked to generate glycolide, so that the yield of glycolide in the whole production process is further improved, a cyclization reaction kettle does not need to be regularly cleaned, and continuous production can be carried out.

Description

Technical Field

[0001] The invention belongs to the field of degradable materials, and particularly relates to a method for preparing glycolide. Background Art

[0002] PGA polymer is a synthetic polymer material with good biodegradability. Different from traditional polymer materials with stable performance such as plastics and rubber, PGA polymer as a material gradually degrades after a certain period of use and eventually turns into water and carbon dioxide that are harmless to humans, animals, plants and the natural environment. It is one of the polymer materials with the best known degradation performance.

[0003] One of the synthetic routes of glycolide is the "two-step method", which first uses glycolic acid or glycolate esters as raw materials to prepare PGA polymers with a molecular weight of 2W to 5W through polycondensation, and then generates glycolide through the cracking and cyclization of the PGA polymer. In the "two-step method" process, even if a high-boiling point solvent is added, as the viscosity of the material system increases at high temperature, solidification and / or coking and carbonization will occur at the bottom of the cyclization reactor, making it difficult to further increase the yield of glycolide; on the other hand, the cyclization reactor needs to be cleaned intermittently to discharge the coked and carbonized polymer from the system along with the high-boiling point solvent. Summary of the invention

[0004] The present invention provides a method for preparing glycolide, which is a three-stage depolymerization method of polyglycolide (PGA polymer), and the preparation method specifically comprises the following steps:

[0005] (1) First stage depolymerization: polymer I is subjected to first stage depolymerization at a first temperature and vacuum conditions, and the pyrolysis conversion rate of polymer I is controlled to be ≤90%, thereby obtaining glycolide and polymer II; the first temperature is T 1 , the polymer I is the polyglycolide;

[0006] (2) Second stage depolymerization: Performing second stage depolymerization on the polymer II in step (1) under a second temperature and pressure condition to obtain polymer III, wherein the second temperature is T 2 ;

[0007] (3) Third stage depolymerization: The polymer III in step (2) is subjected to a third stage depolymerization at a third temperature and pressure to obtain a depolymerization solution containing glycolic acid monomers, wherein the third temperature is T 3 ;

[0008] (4) separating the depolymerization solution in step (3) to obtain glycolic acid monomers;

[0009] Among them, the T 1 、T 2 and T3 The following relationship is satisfied: T3<T2<T1.

[0010] The third temperature T3 in the reaction temperature of the present invention can be controlled at above 100°C, and the first temperature T 1 Can be controlled below 290℃.

[0011] Further, the T 1 、T 2 and T 3 The following relations are also satisfied: 220℃≤T 1 ≤290℃, 150℃≤T 2 ≤250℃, 100℃≤T 3 ≤230℃.

[0012] In the present invention, the polymer I is thermally cracked at a first depolymerization temperature to produce glycolide and the polymer II. The polymer II continues to depolymerize at a second depolymerization temperature. Since the depolymerization temperature is lower than the first depolymerization temperature and it cannot be cracked into rings under a pressurized environment, almost no glycolide is generated, but its molecular weight is further reduced to generate the polymer III. The polymer III continues to depolymerize at a third depolymerization temperature until glycolic acid monomers are generated.

[0013] The present invention controls the cracking conversion rate of polymer I in the cyclization reactor by graded depolymerization, thereby avoiding coking of materials in the cyclization reactor. At the same time, polymer II obtained in the cyclization reactor can be directly transferred to the next stage depolymerization reactor in the form of melt for depolymerization reaction, without the need to reheat and melt polymer II.

[0014] The polyglycolide and PGA polymer mentioned in the present invention are the same substance.

[0015] In the second stage depolymerization process of the present invention, the reaction conditions are properly controlled and generally no glycolide is generated. The calculation formula of the cleavage conversion rate of polymer I in the present invention is:

[0016]

[0017] or,

[0018]

[0019] The calculation formula of the total yield of glycolide in the present invention is:

[0020] (Amount of glycolide in the gas phase obtained after the cleavage reaction is completed / (mass of polymer I at the beginning of the cleavage reaction-theoretical mass of polymer I obtained by the polycondensation reaction of the recovered glycolic acid monomers))*100%.

[0021] The glycolide described in the present invention is generally crude glycolide, and the crude glycolide may contain pure glycolide, linear PGA oligomers, glycolic acid and other substances.

[0022] Furthermore, the vacuum condition of step (1) of the present invention is an absolute pressure of 0.01 to 2 kPa, and the reaction time is 0.5 to 12 hours; the pressurization condition of step (2) is a gauge pressure of 0.5 to 5 MPa, and the reaction time is 0.5 to 3 hours; the pressurization condition of step (3) is a gauge pressure of 0.5 to 5 MPa, and the reaction time is 1 to 12 hours.

[0023] The reaction time in step (2) is preferably 1 to 1.5 hours; the reaction time in step (3) is preferably 1.5 to 3 hours.

[0024] Furthermore, the process of step (1) in the present invention further includes: adding a catalyst to the polymer I; the added mass of the catalyst is less than or equal to 5wt% of the mass of the polymer I; and the catalyst is selected from at least one of tin compounds, antimony compounds or zinc compounds.

[0025] The tin compound, antimony compound or zinc compound may be selected from stannous octoate, stannous chloride, tin lactate, antimony trioxide, diethyl zinc or zinc acetate dihydrate, but is not limited to the above substances.

[0026] The molecular weight of polymer II in the present invention may be higher than that of polymer I, or lower than that of polymer I. After a period of cracking, the molecular weight of polymer I must first decrease, but in the later stage of high-temperature cracking, the polyester segments may repolymerize, resulting in a molecular weight higher than that of polymer I. The molecular weight of polymer I is generally between 2W and 5W, the molecular weight of polymer II is generally between 1W and 10W, and the molecular weight of polymer III is generally greater than or equal to 1W.

[0027] Furthermore, the pyrolysis conversion rate of the polymer I in step (1) of the present invention is greater than or equal to 50% and less than or equal to 80%.

[0028] Furthermore, the method for preparing glycolide of the present invention further comprises: subjecting the glycolic acid monomers described in step (4) to a condensation polymerization reaction to obtain a polymer I, and recycling the polymer to step (1) for use.

[0029] Furthermore, the preparation method of the present invention further comprises: adding methyl glycolate to the polymer II obtained in step (1); the mass ratio of the added mass of the methyl glycolate to the polymer II is 0.2:1 to 3:1.

[0030] Furthermore, after step (1) in the present invention, the mixture of polymer II and methyl glycolate is transferred from the cyclization reactor of step (1) to a depolymerization reactor to carry out step (2).

[0031] Furthermore, after step (1) in the present invention, the mixture of polymer II and methyl glycolate is transferred from the cyclization reaction kettle of step (1) to a depolymerization kettle to carry out step (2); and after step (2), the polymer III is transferred to a next-stage depolymerization kettle to carry out step (3).

[0032] The above-mentioned addition of a certain amount of methyl glycolate can reduce the viscosity of polymer II, thereby reducing the industrial pressure gradient, making it easier to transfer the melt of polymer II to the next level of depolymerization kettle. At the same time, the appropriate amount of methyl glycolate can also reduce the melting point of polymer II, avoiding the polymer II from solidifying into a larger block due to the temperature reduction, affecting the efficiency and effect of the second-level depolymerization. Exemplarily, after adding a small amount of methyl glycolate, the polymer with a melting point of 225°C can be reduced to a melting point of 180°C. Exemplarily, after adding an appropriate amount of methyl glycolate, the polymer with a melting point of 225°C can be reduced to a melting point of 145°C.

[0033] Methyl glycolate is added to polymer II, and after the second stage depolymerization, polymer III obtained in the depolymerization reactor is a mixture of the thermal decomposition product of polymer II and methyl glycolate.

[0034] Preferably, the mass ratio of the added mass of methyl glycolate to the mass ratio of polymer II is 0.5:1 to 1.75:1. In the case where the first temperature and the second temperature differ greatly, if the amount of methyl glycolate added is too little, the effect of reducing the melting point of polymer II is limited. In the process of transferring polymer II to the next stage depolymerization reactor through the pipeline, there is a risk of solidification of the material in the pipeline; when the amount is too much, the effect of reducing the melting point and viscosity of polymer II is not obvious. Among them, the amount of polymer II is a theoretical amount assuming that the coking rate is 0, and its value can be obtained by the weighing device of the cyclization reactor, or it can be obtained by the amount of polymer I and the amount of gas phase produced. For example: the amount of polymer I = 100, the amount of gas phase produced = 60, then it can be known that the amount of polymer II is 100-60 = 40.

[0035] Furthermore, the third stage depolymerization in the present invention is alcoholysis, and the alcohol substance added in the third stage depolymerization is a C1-30 saturated monohydric alcohol or polyhydric alcohol.

[0036] Furthermore, the alcohol substance in the present invention is a C1-4 saturated monohydric alcohol or polyhydric alcohol, such as methanol, ethanol, isopropanol, butanol, etc., and the mass ratio of the alcohol substance to the polymer III is 1:1 to 10:1.

[0037] Preferably, the alcohol substance is methanol, and the mass ratio of methanol to the polymer III is 2:1 to 6:1.

[0038] In addition, the third stage depolymerization in the present invention may also be hydrolysis.

[0039] The mass of the polymer III is the theoretical mass of the polymer II without mass loss (ie, without gas generation) during the depolymerization process, and its mass value is equal to the mass value of the polymer II.

[0040] If methanol is used in the alcoholysis process, step (4) specifically comprises the following steps:

[0041] 1) performing a solid-liquid separation operation on the depolymerization liquid, which may be gravity centrifugal separation or cake filtration separation at high temperature (not exceeding 230° C.) or room temperature, to remove solid impurities in the depolymerization liquid;

[0042] 2) The depolymerization liquid from which the solid impurities have been removed is subjected to a liquid phase separation operation such as distillation, negative pressure distillation or rectification to distill / evaporate methanol and methyl glycolate respectively.

[0043] Compared with the prior art, the present invention has the following beneficial technical effects:

[0044] The preparation method of glycolide proposed by the present invention does not require the addition of a high boiling point solvent, overcomes the problem of increased viscosity of the material system at high temperature, and produces almost no or only a small amount of coking and carbonized residues in the cyclization reactor. Experiments have shown that the amount of coking is less than 0.5%. The glycolic acid monomers (for example, glycolic acid monomers, glycolic acid lipid monomers) obtained by depolymerization are finally returned to the polycondensation system as raw materials to synthesize polyglycolide PGA, and continue to be cracked to generate glycolide. In the whole process, the yield of glycolide is also improved.

[0045] The present invention does not need to clean the cyclization reactor regularly. The cyclization reactor can continue to perform the cracking cyclization reaction, and the residue in the reactor is transferred to the next-stage depolymerization reactor for depolymerization reaction to generate a low-molecular-weight PGA polymer. Similarly, the low-molecular-weight PGA polymer can be transferred to the next-stage depolymerization reactor for depolymerization reaction to generate glycolic acid monomers. The entire production process can be carried out continuously. DETAILED DESCRIPTION

[0046] The inventors of the present invention have found in practice that in the process of cracking PGA polymers to form glycolide, by controlling the reaction degree of the cracking of PGA polymers to form glycolide, that is, controlling the cracking conversion rate of the PGA polymers in the cracking stage, the coking and carbonization of the residues in the ring-forming reactor can be avoided; the residues are further depolymerized at a lower temperature (compared to the temperature of cracking to form glycolide) to form low molecular weight PGA polymers, and then the low molecular weight PGA polymers are depolymerized by hydrolysis or alcoholysis to form glycolic acid monomers, which can be returned to the polycondensation system as a monomer raw material for synthesizing PGA polymers. Although the yield of glycolide is reduced in the cracking ring-forming stage, almost no or only a small amount of coking and carbonization residues are generated in the ring-forming reactor (experiments show that the coking amount is less than 0.5%), and the residues can be further depolymerized into glycolic acid monomers in the later stage, and the glycolic acid monomers can be returned to the polycondensation system as a raw material to synthesize PGA polymers and continue to crack to form glycolide. For the entire stage, the total yield of glycolide is increased.

[0047] The technology of the present invention does not need to regularly clean the cyclization reactor. The cyclization reactor can continue to carry out the cracking cyclization reaction, and the residue in the reactor is transferred to the next-stage depolymerization reactor for depolymerization reaction to generate low-molecular-weight PGA polymers. Similarly, the low-molecular-weight PGA polymer can be transferred to the next-stage depolymerization reactor for depolymerization reaction to generate glycolic acid monomers. The entire production process can be carried out continuously.

[0048] The present invention discloses a method for preparing glycolide, which is specifically a three-stage depolymerization method of polyglycolide. The preparation method specifically comprises the following steps:

[0049] (1) First stage depolymerization: polymer I is subjected to first stage depolymerization at a first temperature and under vacuum conditions, and the pyrolysis conversion rate of polymer I is controlled to be ≤90%, thereby obtaining glycolide and polymer II; the first temperature is T 1 , polymer I is the above-mentioned polyglycolide;

[0050] (2) Second stage depolymerization: Under the second temperature and pressure conditions, the polymer II in step (1) is subjected to second stage depolymerization to obtain polymer III. The second temperature is T 2 ;

[0051] (3) Third stage depolymerization: The polymer III in step (2) is subjected to a third stage depolymerization under a third temperature and pressure condition to obtain a depolymerization liquid containing glycolic acid monomers. The third temperature is T 3 ;

[0052] (4) separating the depolymerization liquid in step (3) to obtain glycolic acid monomers;

[0053] Among them, T1, T2 and T3 satisfy the following relationship: T3<T2<T1.

[0054] T 1 、T 2 and T 3 The following relations are also satisfied: 220℃≤T 1 ≤290℃, 150℃≤T 2 ≤250℃, 100℃≤T 3 ≤230℃.

[0055] The vacuum condition of the above step (1) is an absolute pressure of 0.01 to 2 kPa, and the reaction time is 0.5 to 12 h; the reaction pressure in the above step (2) is a gauge pressure of 0.5 to 5 MPa, preferably 0.5 to 1.5 MPa, and the reaction time is 0.5 to 3 hours, preferably 1 to 1.5 hours; the reaction pressure in the above step (3) is a gauge pressure of 0.5 to 5 MPa, preferably 0.5 to 2.5 MPa, and the reaction time is 1 to 12 hours, preferably 1.5 to 3 hours.

[0056] The above step (1) process also includes: adding a catalyst to the polymer I; the added mass of the catalyst is less than or equal to 5wt% of the mass of the above polymer I; the catalyst is selected from at least one of tin compounds, antimony compounds or zinc compounds.

[0057] The tin compound, antimony compound or zinc compound may be selected from stannous octoate, stannous chloride, tin lactate, antimony trioxide, and diethyl zinc or zinc acetate dihydrate, but is not limited to the above substances.

[0058] In step (1), the pyrolysis conversion rate of polymer I is greater than or equal to 50% and less than or equal to 80%.

[0059] The glycolic acid monomers in step (4) are subjected to a polycondensation reaction to obtain the above-mentioned polymer I, which is then recycled to step (1) for use.

[0060] The preparation method further comprises: adding methyl glycolate to the polymer II in step (1); the mass ratio of the added methyl glycolate to the polymer II is 0.2:1 to 3:1.

[0061] After step (1), the mixture of polymer II and methyl glycolate is transferred from the cyclization reactor of step (1) to a depolymerization reactor to carry out step (2); after step (2), polymer III is transferred to a next-stage depolymerization reactor to carry out step (3).

[0062] The third stage depolymerization is alcoholysis, and the alcohol substance added in the third stage depolymerization is a C1-30 saturated monohydric alcohol or polyhydric alcohol.

[0063] If the alcohol substance is a C1-4 saturated monohydric alcohol or polyhydric alcohol, the mass ratio of the alcohol substance to the polymer III is 1:1 to 10:1.

[0064] Exemplarily, the alcohol substance is methanol, and the alcoholysis process of step (4) above specifically comprises the following steps:

[0065] 1) performing solid-liquid separation on the depolymerized liquid, which may be gravity centrifugal separation or cake filtration separation at high temperature (not exceeding 230° C.) or room temperature, to remove solid impurities in the depolymerized liquid;

[0066] 2) The depolymerization liquid from which the solid impurities have been removed is subjected to a liquid phase separation operation such as distillation, negative pressure distillation or rectification to distill / evaporate methanol and methyl glycolate respectively.

[0067] In the present invention, for materials with a molecular weight less than 1000, the molecular weight is measured based on end group analysis, and for materials with a molecular weight greater than 1000, the molecular weight is measured based on gel permeation chromatography (GPC).

[0068] The end group analysis method is a conventional method for testing molecular weight in the art, and what is measured is the number average molecular weight. The relevant test method can be as follows: take the sample to be tested (mass M), use acetonitrile as a solvent, mix the sample to be tested with acetonitrile at a mass ratio of 1:20, dissolve the sample to be tested in the acetonitrile solvent, use phenolphthalein as an indicator, and titrate with a 0.1 mol / L standard NaOH solution. When the titration end point is reached, the molar amount N of -COOH in the sample can be converted by the amount of standard NaOH solution consumed, and the number average molecular weight Mn of the sample to be tested can be calculated by the ratio of M / N. Based on the above method, the number average molecular weight of materials with a molecular weight less than 1000 can be measured more accurately.

[0069] Due to the measurement accuracy of GPC instruments, for materials with a molecular weight less than 1000, if GPC is used to measure, the measured results often have large fluctuations, that is, it is not easy to measure accurately.

[0070] For materials with a molecular weight greater than 1000, a GPC instrument can be used for measurement. The specific test method is as follows: Take 0.2g of PGA sample, dissolve it in 100mL of hexafluoroisopropanol solution with a sodium trifluoroacetate content of 5mmol / L, filter it through a polytetrafluoroethylene filter membrane with a pore size of 0.4μm, take 20μL and add it to the gel permeation chromatography (GPC) injector to determine the molecular weight of the sample. Five standard molecular weights of methyl methacrylate with different molecular weights are used for molecular weight calibration.

[0071] It is understandable that in the present invention, the actual molecular weights of polymer I, polymer II and polymer III are all above 1000, and the molecular weight values ​​are measured by GPC instrument; in some embodiments, methyl glycolate is added to polymer II obtained in step (1), and after the second stage depolymerization, polymer III obtained in the depolymerization reactor is a mixture of the thermal decomposition product of polymer II and methyl glycolate. Practice has shown that in this case, the molecular weight of the mixture is below 1000, and the molecular weight value is determined by end group analysis.

[0072] The present invention is further described in detail below with reference to specific embodiments.

[0073] Embodiment 1:

[0074] The preparation method of glycolide of the present invention is a three-stage depolymerization method of polyglycolide, and the total yield of glycolide is greater than 96%. The preparation method specifically comprises the following steps:

[0075] (1) First stage depolymerization: adding a catalyst to polymer I, performing first stage depolymerization on polymer I under first temperature and vacuum conditions, wherein the vacuum conditions are an absolute pressure of 0.01 to 2 kPa, the reaction time is 0.5 to 12 h, and the cracking conversion rate of polymer I is controlled to be ≤ 90%, to obtain glycolide and polymer II; the added mass of the catalyst is less than or equal to 5 wt % of the mass of the polymer I, and the catalyst is selected from at least one of tin compounds, antimony compounds, or zinc compounds; the first temperature is T 1 , polymer I is polyglycolide;

[0076] (2) Second stage depolymerization: Under the second temperature and pressure conditions, the polymer II in step (1) is subjected to second stage depolymerization to obtain polymer III. The second temperature is T 2 , the pressure condition is 0.5-5MPa, the reaction time is 0.5-3 hours, and no glycolide is generated in the secondary depolymerization process;

[0077] (3) Third stage depolymerization: The polymer III in step (2) is subjected to a third stage depolymerization under a third temperature and pressure condition to obtain a depolymerization liquid containing glycolic acid monomers, wherein the pressure condition is a gauge pressure of 0.5 to 5 MPa, the reaction time is 1 to 12 hours, and the third temperature is T 3 ;

[0078] (4) separating the depolymerization liquid in step (3) to obtain glycolic acid monomers;

[0079] Among them, T 1 、T 2 and T 3 The following relationship is satisfied: T3<T2<T1.

[0080] Embodiment 2:

[0081] The difference between this embodiment and embodiment 1 is that T 1 、T 2 and T 3 The following relations are also satisfied: 220℃≤T 1 ≤290℃, 150℃≤T 2 ≤250℃, 100℃≤T 3 ≤230℃.

[0082] Embodiment 3:

[0083] The only difference between this embodiment and embodiment 2 is that the pyrolysis conversion rate of polymer I in step (1) of this embodiment 3 is greater than or equal to 50% and less than or equal to 80%.

[0084] Embodiment 4:

[0085] The only difference between this embodiment and embodiment 3 is that the glycolic acid monomers in step (4) of this embodiment 4 are subjected to polycondensation reaction to obtain polymer I, which is then recycled to step (1) for use.

[0086] Embodiment 5:

[0087] The only difference between this embodiment and embodiment 4 is that the preparation method of this embodiment 5 further includes: adding methyl glycolate to the polymer II obtained in step (1); the mass ratio of the added mass of methyl glycolate to the mass ratio of the polymer II is: 0.2:1 to 3:1.

[0088] After step (1), the mixture of polymer II and methyl glycolate is transferred from the cyclization reactor of step (1) to a depolymerization reactor to carry out step (2); after step (2), polymer III is transferred to a next-stage depolymerization reactor to carry out step (3).

[0089] Embodiment 6:

[0090] The difference between this embodiment and embodiment 5 is that the third stage depolymerization in this embodiment 6 is hydrolysis.

[0091] Embodiment 7:

[0092] The only difference between this embodiment and embodiment 5 is that the third-stage depolymerization of this embodiment 7 is alcoholysis, and the alcohol substance added in the third-stage depolymerization is a C1-4 saturated monohydric alcohol or polyhydric alcohol, wherein the amount ratio of the alcohol substance to the polymer III is 1:1 to 10:1.

[0093] Embodiment 8:

[0094] A method for preparing glycolide of the present invention is a three-stage depolymerization method of polyglycolide, which specifically comprises the following steps:

[0095] (1) First stage depolymerization: 10 kg of polymer I (PGA polymer) with a molecular weight of 2.6 W was placed in a cyclization reactor, and about 50 g of catalyst-stannous octoate was added. 1 =220°C and vacuum-absolute pressure 100Pa, polymer I was subjected to first-stage depolymerization, and cracking and ring-forming reaction occurred. The reaction time was 9h, and glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 5.96kg glycolide and 4kg polymer II; the cracking conversion rate of polymer I was controlled to be 60%.

[0096] (2) Second stage depolymerization: polymer II is transferred to the depolymerization reactor through the melt and heated to the second temperature T 2 =215°C and pressurized - gauge pressure 0.5MPa, the polymer II in step (1) was subjected to second stage depolymerization, the reaction time was 2h, and after the reaction was completed, the temperature was naturally cooled to room temperature to obtain polymer III; the molecular weight of polymer III was 1.3W.

[0097] (3) Third stage depolymerization: at the third temperature T 3 =120° C. and pressurized-gauge pressure 1.2 MPa, the polymer III in step (2) is subjected to third-stage depolymerization-alcoholysis, 12 kg of methanol is added to the depolymerization kettle, the reaction time is 6 h, and after the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate;

[0098] (4) Separating the depolymerization liquid in step (3) to obtain methyl glycolate. The solid impurities in the depolymerization liquid are removed by centrifugal separation, and then the depolymerization liquid is first distilled and concentrated at 90°C and normal pressure to obtain a concentrated liquid. The evaporated methanol can be recovered and recycled for the next alcoholysis reaction; the remaining concentrated liquid is then subjected to reduced pressure distillation at an absolute pressure of 80 kPa and 140°C, and the front fraction is methanol. 6.022 kg of methyl glycolate is collected in the back stage. The methyl glycolate polycondensation reaction can obtain about 3.88 kg of polymer I.

[0099] The total yield of glycolide obtained in Example 8 is about 5.96 / (10-3.88)=97.38%.

[0100] Embodiment 9:

[0101] A method for preparing glycolide of the present invention is a three-stage depolymerization method of polyglycolide, which specifically comprises the following steps:

[0102] (2) First stage depolymerization: 10 kg of polymer I (PGA polymer) with a molecular weight of 2.6 W was placed in a cyclization reactor, and about 50 g of catalyst-stannous octoate was added. 1=230°C and vacuum-absolute pressure 500Pa, polymer I was subjected to first-stage depolymerization, and cracking and ring-forming reaction occurred. The reaction time was 5h, and glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 4.95kg glycolide and 5kg polymer II; the cracking conversion rate of polymer I was controlled to be 50%.

[0103] (2) Second stage depolymerization: polymer II is transferred to the depolymerization reactor through the melt and heated to the second temperature T 2 =220°C and pressurized-gauge pressure 1.0MPa, the polymer II in step (1) was subjected to second-stage depolymerization, the reaction time was 2h, and after the reaction was completed, the temperature was naturally cooled to room temperature to obtain polymer III; the molecular weight of polymer III was 1.5W.

[0104] (3) Third stage depolymerization: at the third temperature T 3 =150°C and pressurized-gauge pressure 1.5MPa, the polymer III in step (2) is subjected to third-stage depolymerization-alcoholysis, 25kg of ethanol is added to the depolymerization kettle, the reaction time is 5h, and after the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing ethyl glycolate;

[0105] (4) Separating the depolymerization liquid in step (3) to obtain ethyl glycolate. The solid impurities in the depolymerization liquid are removed by centrifugal separation, and then the depolymerization liquid is first distilled and concentrated at 95° C. and normal pressure to obtain a concentrated liquid. The evaporated ethanol can be recovered and recycled for the next alcoholysis reaction; the remaining concentrated liquid is then subjected to reduced pressure distillation at an absolute pressure of 80 kPa and 140° C., and the front fraction is ethanol. 8.822 kg of ethyl glycolate is collected in the back stage. The ethyl glycolate polycondensation reaction can obtain about 4.92 kg of polymer I.

[0106] The total yield of glycolide obtained in Example 9 is about 4.95 / (10-4.92)=97.44%.

[0107] Embodiment 10:

[0108] A method for preparing glycolide of the present invention is a three-stage depolymerization method of polyglycolide, which specifically comprises the following steps:

[0109] (2) First stage depolymerization: 10 kg of polymer I (PGA polymer) with a molecular weight of 3.0 W was placed in a cyclization reactor, and about 50 g of catalyst-stannous octoate was added. 1 =240°C and vacuum-absolute pressure 1000Pa, polymer I was subjected to first-stage depolymerization, and cracking and ring-forming reaction occurred. The reaction time was 6.5h, and glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 6.97kg glycolide and 3kg polymer II; the cracking conversion rate of polymer I was controlled to be 70%.

[0110] (2) Second stage depolymerization: polymer II is transferred to the depolymerization reactor through the melt and heated to the second temperature T 2 =230°C and pressurized-gauge pressure 1.1MPa, the polymer II in step (1) is subjected to a second-stage depolymerization, the reaction time is 2h, and the temperature is naturally lowered to about 55°C after the reaction to obtain polymer III. No glycolide is generated in the secondary depolymerization process; the molecular weight of polymer III is 1.5W.

[0111] (3) Third stage depolymerization: polymer III is transferred to the next stage depolymerization reactor at the third temperature T 3 =165°C and pressurized-gauge pressure 2.0MPa, the polymer III in step (2) is subjected to third-stage depolymerization-alcoholysis, 12kg of methanol is added to the depolymerization kettle, the reaction time is 3h, and after the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate;

[0112] (4) Separating the depolymerization liquid in step (3) to obtain methyl glycolate. The solid impurities in the depolymerization liquid are removed by centrifugal separation, and then the depolymerization liquid is first distilled and concentrated at 95°C and normal pressure to obtain a concentrated liquid. The evaporated methanol can be recovered and recycled for the next alcoholysis reaction; the remaining concentrated liquid is then subjected to reduced pressure distillation at an absolute pressure of 80 kPa and 140°C, and the front fraction is ethanol. 4.516 kg of methyl glycolate is collected in the back stage. The methyl glycolate polycondensation reaction can obtain about 2.91 kg of polymer I.

[0113] The total yield of glycolide obtained in Example 10 is about 6.97 / (10-2.91)=98.30%.

[0114] Embodiment 11:

[0115] A method for preparing glycolide of the present invention is a three-stage depolymerization method of polyglycolide, which specifically comprises the following steps:

[0116] (1) First stage depolymerization: 10 kg of polymer I (PGA polymer) with a molecular weight of 3.5 W was placed in a cyclization reactor, and about 80 g of catalyst-stannous octoate was added. 1 =250°C and vacuum-absolute pressure 2000Pa, polymer I was subjected to first-stage depolymerization, and cracking and ring-forming reaction occurred. The reaction time was 3h, and glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 5.57kg glycolide and 4.4kg polymer II; the cracking conversion rate of polymer I was controlled to be 56%.

[0117] Afterwards, 6.6 kg of methyl glycolate was added into the cyclization reactor at a mass ratio of methyl glycolate to polymer II of 1.5:1.

[0118] (2) Second stage depolymerization: The mixture of polymer II and methyl glycolate is transferred to a depolymerization reactor through a melt, and then heated to a temperature T 2 =215°C and pressurized-gauge pressure 1.0MPa, polymer II was subjected to second-stage depolymerization for 1.5h. After the reaction was completed, the temperature was naturally lowered to room temperature to obtain polymer III. No glycolide was generated in the second-stage depolymerization process. The molecular weight of polymer III was 300.

[0119] (3) Third stage depolymerization: polymer III is transferred to the next stage depolymerization reactor at the third temperature T 3 =170°C and pressurized-gauge pressure 2.1MPa, the polymer III in step (2) is subjected to third-stage depolymerization-alcoholysis, 13.2kg of methanol is added to the depolymerization kettle, the reaction time is 4h, and after the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate;

[0120] (4) Separate the depolymerization liquid in step (3) to obtain methyl glycolate. Remove solid impurities in the depolymerization liquid by centrifugal separation, then distill and concentrate the depolymerization liquid at 90°C and normal pressure to obtain a concentrated liquid. The evaporated methanol can be recovered and recycled for the next alcoholysis reaction. The remaining concentrated liquid is then subjected to reduced pressure distillation at an absolute pressure of 80 kPa and 140°C. The front fraction is methanol, and 13.158 kg of methyl glycolate is collected in the rear stage (including 6.6 kg of methyl glycolate added in step (1) and 6.558 kg of methyl glycolate recovered in this step). The methyl glycolate recovered in this step can be subjected to polycondensation to obtain about 4.22 kg of polymer I.

[0121] The total yield of glycolide obtained in this Example 11 is about 5.57 / (10-4.22)=96.36%.

[0122] Embodiment 12:

[0123] A method for preparing glycolide of the present invention is a three-stage depolymerization method of polyglycolide, which specifically comprises the following steps:

[0124] (1) First stage depolymerization: 10 kg of polymer I (PGA polymer) with a molecular weight of 4W was placed in a cyclization reactor, and about 80 g of catalyst-stannous octoate was added. 1 =275°C and vacuum-absolute pressure 500Pa, polymer I was subjected to first-stage depolymerization, and cracking and ring-forming reaction occurred. The reaction time was 2h, and glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 7.74kg glycolide and 2.2kg polymer II; the cracking conversion rate of polymer I was controlled to be 78%.

[0125] Afterwards, 1.1 kg of methyl glycolate was added into the cyclization reactor at a ratio of 0.5:1 between the mass of methyl glycolate and polymer II.

[0126] (2) Second stage depolymerization: The mixture of polymer II and methyl glycolate is transferred to a depolymerization reactor through a melt, and then heated to a temperature T 2 =200°C and pressurized - gauge pressure 0.50MPa, the polymer II in step (1) is subjected to a second stage depolymerization, the reaction time is 1h, and after the reaction is completed, the temperature is naturally cooled to room temperature to obtain a polymer III. No glycolide is generated in the secondary depolymerization process; the molecular weight of the polymer III is 700.

[0127] (3) Third stage depolymerization: polymer III is transferred to the next stage depolymerization reactor at the third temperature T 3 =140°C and pressurized-gauge pressure 2.0MPa, the polymer III in step (2) is subjected to third-stage depolymerization-alcoholysis, 11.0kg of methanol is added to the depolymerization kettle, the reaction time is 1h, and after the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate;

[0128] (4) Separate the depolymerization liquid in step (3) to obtain methyl glycolate. Remove solid impurities in the depolymerization liquid by centrifugal separation, then distill and concentrate the depolymerization liquid at 90°C and normal pressure to obtain a concentrated liquid. The evaporated methanol can be recovered and recycled for the next alcoholysis reaction. The remaining concentrated liquid is then subjected to reduced pressure distillation at an absolute pressure of 80 kPa and 140°C. The front fraction is methanol, and 4.30 kg of methyl glycolate is collected in the rear stage (including 1.1 kg of methyl glycolate added in step (1) and 3.20 kg of methyl glycolate recovered in this step). The methyl glycolate recovered in this step is subjected to polycondensation reaction to obtain about 2.06 kg of polymer I.

[0129] The total yield of glycolide obtained in this Example 12 is about 7.74 / (10-2.06)=97.48%.

[0130] Embodiment 13:

[0131] A method for preparing glycolide of the present invention is a three-stage depolymerization method of polyglycolide, which specifically comprises the following steps:

[0132] (1) First stage depolymerization: 10 kg of polymer I (PGA polymer) with a molecular weight of 5W was put into a cyclization reactor, and about 80 g of catalyst-stannous octoate was added. 1=260°C and vacuum-absolute pressure 100Pa, polymer I was subjected to first-stage depolymerization, and cracking and ring-forming reaction occurred. The reaction time was 2h, and glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 6.98kg glycolide and 3kg polymer II; the cracking conversion rate of polymer I was controlled to be 70%.

[0133] Afterwards, 6.0 kg of methyl glycolate was added into the cyclization reactor in a ratio of methyl glycolate to polymer II of 2:1.

[0134] (2) Second stage depolymerization: The mixture of polymer II and methyl glycolate is transferred to a depolymerization reactor through a melt, and then heated to a temperature T 2 =180°C and pressurized-gauge pressure 0.60MPa, the polymer II in step (1) is subjected to a second-stage depolymerization, the reaction time is 2h, and the temperature is naturally cooled to room temperature after the reaction is completed to obtain a polymer III. No glycolide is generated in the secondary depolymerization process; the molecular weight of the polymer III is 400.

[0135] (3) Third stage depolymerization: at the third temperature T 3 =170°C and pressurized-gauge pressure 2.2MPa, the polymer III in step (2) is subjected to third-stage depolymerization-alcoholysis, 15.0g of methanol is added to the depolymerization kettle, the reaction time is 1h, and after the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate;

[0136] (4) Separate the depolymerization liquid in step (3) to obtain methyl glycolate. Remove solid impurities in the depolymerization liquid by centrifugal separation, then distill and concentrate the depolymerization liquid at 90°C and normal pressure to obtain a concentrated liquid. The evaporated methanol can be recovered and recycled for the next alcoholysis reaction. Then, the remaining concentrated liquid is subjected to reduced pressure distillation at an absolute pressure of 80 kPa and 140°C. The front fraction is methanol, and 10.369 kg of methyl glycolate is collected in the back stage (including 6.0 kg of methyl glycolate added in step (1) and 4.369 kg of methyl glycolate recovered in this step). The methyl glycolate recovered in this step is subjected to polycondensation reaction to obtain about 2.816 kg of polymer I.

[0137] The total yield of glycolide obtained in this Example 13 is about 6.98 / (10-2.816)=97.16%.

[0138] Comparative Example 1:

[0139] The difference between this comparative example and Example 11 is that the reaction time in step (1) is 5 hours, 9.11 kg of glycolide and 0.80 kg of polymer II are obtained, and the cracking conversion rate of polymer I is controlled to be 92%; 1.2 kg of methyl glycolate is added to the cyclization reactor in a mass ratio of methyl glycolate to polymer II of 1.5:1; the molecular weight of polymer III obtained in step (2) is 900; 2.4 kg of methanol is added to the depolymerization reactor in step (3); 1.68 kg of methyl glycolate is collected in the latter stage of step (4) (including 1.2 kg of methyl glycolate added in step (1) and 0.48 kg of methyl glycolate recovered in this step), and the methyl glycolate recovered in this step can be polymerized to obtain about 0.309 kg of polymer I.

[0140] The total yield of glycolide in Comparative Example 1 is about 9.11 / (10-0.309)=94.00%.

[0141] Comparative Example 2

[0142] The prior art polyglycolide cleavage ring formation process is used to prepare glycolide, and the specific steps are as follows:

[0143] 8 kg of polymer I (PGA polymer) with a weight average molecular weight of about 35,000 was put into a ring-forming reactor, and about 40 g of stannous octoate catalyst and 2 kg of tridodecane, a high boiling point solvent, were added. The cracking and ring-forming reaction was carried out at a reaction temperature of 250°C and a reaction pressure of 2000 Pa in absolute pressure. The generated glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser. After no gas phase was generated in the ring-forming reactor, the reaction was stopped to obtain 7.55 kg of crude glycolide. The mass of the residue (polymer II) in the ring-forming reactor was 0.34 kg. The cracking conversion rate of polymer I was 95.75%, and the reaction time was 6 h.

[0144] The total yield of glycolide in Comparative Example 2 is about 7.55 / 8=94.37%.

[0145] The preparation method of glycolide proposed in the above-mentioned embodiments 1 to 13 of the present invention does not require the addition of a high-boiling point solvent, overcomes the problem of increased viscosity of the material system at high temperature, and produces almost no or only a small amount of coking and carbonized residues in the cyclization reactor. Experiments have shown that the amount of coking is less than 0.5%. The glycolic acid monomers obtained by depolymerization are finally returned to the polycondensation system as raw materials to synthesize polyglycolide PGA, and continue to be cracked to generate glycolide. In the whole process, the yield of glycolide is also improved.

[0146] The preparation method of glycolide proposed in the above-mentioned embodiments 1 to 13 of the present invention does not need to regularly clean the cyclization reaction kettle. The cyclization reaction kettle can continuously carry out the cracking cyclization reaction, and the residue in the reactor is transferred to the next-stage depolymerization kettle for depolymerization reaction to generate low-molecular-weight PGA polymers; similarly, the low-molecular-weight PGA polymers can be transferred to the next-stage depolymerization kettle for depolymerization reaction to generate glycolic acid monomers; the entire production process can be carried out continuously.

[0147] The present invention is described by the above specific embodiments, and those skilled in the art should understand that various changes and equivalent substitutions can be made to the present invention without departing from the scope of the present invention. The parts not described in detail in the present specification are well-known technologies to those skilled in the art. In addition, various modifications can be made to the present invention for specific situations or specific circumstances without departing from the scope of the present use. Therefore, the present invention is not limited to the specific embodiments disclosed, but should include all implementation methods falling within the scope of the claims of the present invention.

Claims

1. A method for preparing glycolide, characterized in that: The preparation method is a three-stage depolymerization method of polyglycolide, and the preparation method specifically comprises the following steps: (1) First stage depolymerization: performing first stage depolymerization on polymer I at a first temperature and vacuum conditions, controlling the pyrolysis conversion rate of polymer I to be ≤90%, to obtain glycolide and polymer II; the first temperature is T1, and the polymer I is the polyglycolide; (2) Second stage depolymerization: performing a second stage depolymerization on the polymer II in step (1) at a second temperature and pressure to obtain a polymer III, wherein the second temperature is T2; (3) third stage depolymerization: performing a third stage depolymerization on the polymer III in step (2) at a third temperature and pressure to obtain a depolymerization solution containing glycolic acid monomers, wherein the third temperature is T3; (4) separating the depolymerization solution in step (3) to obtain glycolic acid monomers; Wherein, T1, T2 and T3 satisfy the following relationship: T3<T2<T1.

2. The method for preparing glycolide according to claim 1, characterized in that: The T1, T2 and T3 also satisfy the following relationship: 220°C≤T1≤290°C, 150°C≤T2≤250°C, 100°C≤T3≤230°C.

3. The method for preparing glycolide according to claim 2, characterized in that: The vacuum condition of step (1) is an absolute pressure of 0.01 to 2 kPa, and the reaction time is 0.5 to 12 h; The pressurization condition in step (2) is a gauge pressure of 0.5 to 5 MPa, and the reaction time is 0.5 to 3 hours; The pressurization condition in the step (3) is a gauge pressure of 0.5 to 5 MPa, and the reaction time is 1 to 12 hours.

4. The method for preparing glycolide according to claim 1, characterized in that: The process of step (1) further includes: adding a catalyst to the polymer I; the added mass of the catalyst is less than or equal to 5wt% of the mass of the polymer I; and the catalyst is selected from at least one of tin compounds, antimony compounds or zinc compounds.

5. The method for preparing glycolide according to claim 1, characterized in that: The pyrolysis conversion rate of the polymer I in the step (1) is greater than or equal to 50% and less than or equal to 80%.

6. The method for preparing glycolide according to claim 1, characterized in that: Also includes: The glycolic acid monomers described in step (4) are subjected to a condensation reaction to obtain polymer I, which is then recycled to step (1) for use.

7. The method for preparing glycolide according to any one of claims 1 to 6, characterized in that: The preparation method further comprises: adding methyl glycolate to the polymer II obtained in step (1); the mass ratio of the added mass of the methyl glycolate to the polymer II is 0.2:1 to 3:

1.

8. The method for preparing glycolide according to claim 7, characterized in that: After the step (1), the mixture of the polymer II and the methyl glycolate is transferred from the cyclization reaction kettle of the step (1) to a depolymerization kettle to carry out the step (2); After step (2), the polymer III is transferred to the next stage depolymerization reactor to carry out step (3).

9. The method for preparing glycolide according to claim 1, characterized in that: The third stage depolymerization is alcoholysis, and the alcohol substance added in the third stage depolymerization is a C1-30 saturated monohydric alcohol or polyhydric alcohol.

10. The method for preparing glycolide according to claim 9, characterized in that: The alcohol substance is a C1-4 saturated monohydric alcohol or polyhydric alcohol, and the mass ratio of the alcohol substance to the polymer III is 1:1 to 10:1.

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