Preparation method of glycolide

Abstract

The invention provides a preparation method of glycolide, which is a secondary depolymerization method of polyglycolide (PGA polymer). By controlling the cracking conversion rate of the polyglycolide, coking and carbonization of residues in the cyclization reaction kettle are avoided, a high-boiling-point solvent does not need to be added, and the problem that the viscosity of a material system is increased at a high temperature is solved. According to the invention, almost no or only a small amount of coking and carbonization residues are generated in the cyclization reaction kettle, and experiments prove that the coking amount is lower than 0.5%. Meanwhile, the polymer II is subjected to second-stage depolymerization to form a glycolic acid monomer, the glycolic acid monomer serves as a raw material and is returned to a polycondensation system to synthesize polyglycolide, and the glycolide is generated through continuous cracking, in the whole process, the yield of the glycolide is further increased, the cyclization reaction kettle does not need to be cleaned regularly, 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 polyglycolide PGA polymers. 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 polymers 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 secondary depolymerization method of polyglycolide (PGA polymer); specifically, the method comprises the following steps:

[0005] (1) First-stage depolymerization: performing first-stage depolymerization on polymer I under first depolymerization conditions, controlling the pyrolysis conversion rate of polymer I to be less than or equal to 90%, to obtain glycolide and polymer II, wherein polymer I is the polyglycolide;

[0006] (2) second-stage depolymerization: performing a second-stage depolymerization on the polymer II under a second depolymerization condition to obtain a depolymerization solution containing glycolic acid monomers, wherein the second-stage depolymerization is hydrolysis or alcoholysis;

[0007] (3) separating the depolymerization solution in step (2) to obtain glycolic acid monomers;

[0008] in,

[0009] The first depolymerization conditions include: temperature of 220-290°C, absolute pressure of 0.01 kPa-2 kPa, and reaction time of 0.5-12 hours;

[0010] The second depolymerization conditions include: temperature of 100 to 250° C., gauge pressure of 1 to 7 MPa, and reaction time of 1 to 12 hours.

[0011] The reaction time in the above step (2) is preferably 2 to 6 hours.

[0012] The present invention controls the cracking conversion rate of the polymer I in the cyclization reactor by graded depolymerization, thereby avoiding coking of materials in the cyclization reactor. At the same time, the glycolic acid monomers obtained in step (3) can be further condensed to form PGA polymers, and further cracked to form glycolide, thereby improving the total yield of the glycolide.

[0013] The calculation formula of the pyrolysis conversion rate of polymer I in the present invention is:

[0014]

[0015] or,

[0016]

[0017] The calculation formula of the total yield of glycolide in the present invention is:

[0018] (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%.

[0019] The glycolide described in the present invention is generally crude glycolide, which may include pure glycolide, linear PGA oligomers, glycolic acid and the like.

[0020] Furthermore, in the method for preparing glycolide of the present invention, after step (1), the polymer II is transferred from the cyclization reaction kettle of step (1) to a pulverizer, and the polymer II is pulverized into particles with a particle size of 2 to 8 mm; then the pulverized polymer II is transferred from the pulverizer to a depolymerization kettle to perform step (2).

[0021] After the first stage depolymerization, if the first depolymerization temperature and the second depolymerization temperature differ greatly, the polymer II tends to solidify into agglomerates when the temperature decreases. The particle size of the polymer II can be reduced by mechanical crushing to improve the effect of the second stage depolymerization.

[0022] Furthermore, in the method for preparing glycolide of the present invention, the step (1) further comprises: adding methyl glycolate to the polymer II; the mass ratio of the methyl glycolate to the polymer II is 0.2:1 to 3:1.

[0023] Furthermore, in the method for preparing glycolide of the present invention, 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).

[0024] Adding a certain amount of methyl glycolate to polymer II can reduce the viscosity of polymer II and the melting point of polymer II. The polymer II obtained in the cyclization reactor can be directly transferred to the next-stage depolymerization reactor in the form of a melt, and the depolymerization reaction is directly carried out in a molten state without reheating the polymer II to melt; the reduction in viscosity also makes it easier to transfer the melt of polymer II to the next-stage depolymerization reactor. Exemplarily, after adding a small amount of methyl glycolate, the melting point of a 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 melting point of a polymer with a melting point of 225°C can be reduced to a melting point of 145°C.

[0025] If methyl glycolate is not added to polymer II, polymer II will solidify into large blocks when the temperature drops significantly after the reaction, affecting the efficiency and effect of the second-stage depolymerization. After a certain amount of methyl glycolate is added to polymer II, the solidification of polymer II is avoided without mechanical treatment of polymer II.

[0026] Preferably, the theoretical amount ratio of methyl glycolate to polymer II is 0.5:1 to 1.75:1. In the case where the first depolymerization temperature and the second depolymerization 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 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 the 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 inferred that the amount of polymer II is 100-60 = 40.

[0027] Furthermore, in the method for preparing glycolide described in the present invention, the step (1) also includes: adding a catalyst to the polymer I, wherein the amount of the catalyst added 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.

[0028] Furthermore, in the method for preparing glycolide of the present invention, the tin compound is at least one of stannous octoate, stannous chloride, and tin lactate, the antimony compound is antimony trioxide, and the zinc compound is diethyl zinc or zinc acetate dihydrate.

[0029] Furthermore, the method for preparing glycolide of the present invention further comprises: subjecting the glycolic acid monomers described in step (3) to a condensation reaction to obtain the polymer I, and recycling the polymer I to step (1) for use.

[0030] The polymer obtained by polycondensation reaction of the glycolic acid monomer separated in step (3) can be returned to step (1) for use.

[0031] Furthermore, in the method for preparing glycolide of the present invention, the molecular weight of the polymer I is 2W to 5W, and the molecular weight of the polymer II is 1W to 10W.

[0032] After a period of cracking, the molecular weight of polymer I will inevitably decrease first. However, in the later stage of high-temperature cracking, the polyester segments may repolymerize, resulting in the molecular weight of the obtained polymer II being higher than that of polymer I.

[0033] Furthermore, in the method for preparing glycolide of the present invention, the pyrolysis conversion rate of the polymer I in step (1) is greater than or equal to 50% and less than or equal to 80%.

[0034] Furthermore, in the method for preparing glycolide of the present invention, the second stage depolymerization of step (2) is alcoholysis, and an alcohol substance is added to the polymer II, and the alcohol substance is a C1-30 saturated monohydric alcohol or polyhydric alcohol.

[0035] 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 II is 1:1 to 10:1.

[0036] Preferably, the alcohol substance is methanol, and the mass ratio of methanol to the polymer II is 2:1 to 6:1.

[0037] If the second stage depolymerization uses methanolysis, step (3) may further include the following steps:

[0038] (1) firstly subjecting the depolymerization liquid to a solid-liquid separation operation, for example, 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;

[0039] (2) The depolymerization liquid from which the solid impurities have been removed is subjected to a liquid phase separation operation such as distillation, vacuum distillation or rectification to distill / evaporate methanol and methyl glycolate respectively.

[0040] Compared with the prior art, the present invention has the following beneficial technical effects:

[0041] The preparation method of glycolide of the present invention does not require the addition of a high boiling point solvent, thereby overcoming the problem of increased viscosity of the material system at high temperature. By controlling the cracking conversion rate of polyglycolide, the coking and carbonization of the residue in the cyclization reactor is avoided. At the same time, the residue is further depolymerized into glycolic acid monomers (for example, glycolic acid monomers, glycolic acid lipid monomers) in the later stage, and the glycolic acid monomers can be returned to the polycondensation system as raw materials to synthesize polyglycolide, and continue to be cracked to generate glycolide. In the cyclization reactor of the present invention, there is almost no or only a small amount of coking and carbonization residues, and experiments have shown that the coking amount is less than 0.5%. The whole process further improves the yield of glycolide.

[0042] The present invention does not need to clean the cyclization reaction kettle regularly, the cyclization reaction kettle can continuously carry out the cracking cyclization reaction, and the entire production process can be carried out continuously. DETAILED DESCRIPTION

[0043] The inventor of the present invention has found in the process of practical application that in the process of polyglycolide cracking and ring-forming glycolide, by controlling the reaction degree of polyglycolide cracking and forming glycolide, i.e. controlling the cracking conversion rate of polyglycolide in the cracking stage, the coking carbonization of the residue in the ring-forming reactor can be avoided. Then the residue can be further depolymerized at a lower temperature (compared to the temperature of cracking and forming glycolide) and under a pressurized environment to generate glycolic acid monomers, which can be returned to the polycondensation system as a monomer raw material for synthesizing polyglycolide. Although the yield of glycolide is reduced in the cracking ring-forming stage, there is almost no or only a small amount of coking carbonized residues in the ring-forming reactor. Experiments have shown that the coking amount is less than 0.5%, and the residue can be further depolymerized into glycolic acid monomers in the later stage, and glycolic acid monomers can be returned to the polycondensation system as a raw material to synthesize polyglycolide, and continue to crack and form glycolide. For the entire stage, the yield of glycolide is further improved. In addition, the present invention does not need to clean the cyclization reactor regularly, and the cyclization reactor can continue to perform the cracking cyclization reaction, while the residue in the reactor is transferred to the next-stage depolymerization reactor for depolymerization reaction.

[0044] A method for preparing glycolide of the present invention is a secondary depolymerization method of polyglycolide (PGA polymer), which specifically comprises the following steps:

[0045] (1) First-stage depolymerization: adding a catalyst to polymer I, performing first-stage depolymerization on polymer I under first depolymerization conditions, controlling the cracking conversion rate of polymer I to be less than or equal to 90%, to obtain glycolide and polymer II, wherein polymer I is polyglycolide;

[0046] (2) second stage depolymerization: performing second stage depolymerization on polymer II under second depolymerization conditions to obtain a depolymerization solution containing glycolic acid monomers, wherein the second stage depolymerization is hydrolysis or alcoholysis;

[0047] (3) separating the depolymerization liquid in step (2) to obtain glycolic acid monomers;

[0048] in,

[0049] The first depolymerization conditions include: temperature of 220-290°C, absolute pressure of 0.01 kPa-2 kPa, and reaction time of 0.5-12 hours;

[0050] The second depolymerization conditions include: temperature of 100 to 250° C., gauge pressure of 1 to 7 MPa, and reaction time of 1 to 12 hours.

[0051] After the above step (1), the polymer II is transferred from the cyclization reaction kettle of the step (1) to a pulverizer, and the polymer II is pulverized into particles with a particle size of 2 to 8 mm; the pulverized polymer II is then transferred from the pulverizer to a depolymerization kettle to perform the step (2);

[0052] or,

[0053] In the above step (1), methyl glycolate is added to polymer II, and the mass ratio of methyl glycolate to polymer II is 0.2:1 to 3:1. After step (1), the mixture of polymer II and methyl glycolate is transferred from the cyclization reactor of step (1) to the depolymerization reactor to carry out step (2).

[0054] The amount of the catalyst added is less than or equal to 5wt% of the mass of the polymer I. The catalyst is selected from at least one of a tin compound, an antimony compound or a zinc compound; the tin compound can be at least one of stannous octoate, stannous chloride and tin lactate, the antimony compound can be antimony trioxide, and the zinc compound can be diethyl zinc or zinc acetate dihydrate.

[0055] The glycolic acid monomers in step (3) are subjected to polycondensation reaction to obtain polymer I, which is then recycled to step (1) for use.

[0056] Preferably, the pyrolysis conversion rate of polymer I in step (1) is greater than or equal to 50% and less than or equal to 80%.

[0057] Preferably, the second stage depolymerization in step (2) is alcoholysis, and a C1-4 saturated monohydric alcohol or polyhydric alcohol is added to the polymer II, and the mass ratio of the alcohol substance to the polymer II is 1:1 to 10:1.

[0058] The present invention is further described in detail below in conjunction with specific embodiments.

[0059] Embodiment 1:

[0060] The preparation method of glycolide of the present invention is a two-stage depolymerization method of polyglycolide (PGA polymer), wherein the total yield of glycolide is greater than or equal to 96%; specifically comprising the following steps:

[0061] (1) First-stage depolymerization: adding a catalyst to polymer I, performing first-stage depolymerization on polymer I under first depolymerization conditions, controlling the cracking conversion rate of polymer I to be less than or equal to 90%, to obtain glycolide and polymer II, wherein polymer I is polyglycolide;

[0062] (2) second stage depolymerization: performing second stage depolymerization on polymer II under second depolymerization conditions to obtain a depolymerization solution containing glycolic acid monomers, wherein the second stage depolymerization is hydrolysis or alcoholysis;

[0063] (3) separating the depolymerization liquid in step (2) to obtain glycolic acid monomers;

[0064] in,

[0065] The first depolymerization conditions include: temperature of 220-290°C, absolute pressure of 0.01 kPa-2 kPa, and reaction time of 0.5-12 hours;

[0066] The second depolymerization conditions include: temperature of 100 to 250° C., gauge pressure of 1 to 7 MPa, and reaction time of 1 to 12 hours;

[0067] The amount of the catalyst added is less than or equal to 5wt% of the mass of the polymer I; the catalyst is selected from at least one of a tin compound, an antimony compound or a zinc compound; the tin compound is at least one of stannous octoate, stannous chloride and tin lactate, the antimony compound is antimony trioxide, and the zinc compound is diethyl zinc or zinc acetate dihydrate.

[0068] The glycolic acid monomers in step (3) are subjected to polycondensation reaction to obtain polymer I, which is then recycled to step (1) for use.

[0069] In step (1), methyl glycolate is added to polymer II, and the mass ratio of methyl glycolate to polymer II is 0.2:1 to 3:1. 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).

[0070] Embodiment 2:

[0071] The only difference between this embodiment and embodiment 1 is that the pyrolysis conversion rate of polymer I in step (1) is greater than or equal to 50% and less than or equal to 80%.

[0072] Embodiment 3:

[0073] The difference between this embodiment and embodiment 1 is that in step (1), methyl glycolate is added to polymer II, and the mass ratio of methyl glycolate to polymer II is 0.2:1 to 3:1; after step (1), the mixture of polymer II and methyl glycolate is transferred from the cyclization reactor of step (1) to the depolymerization reactor for step (2); replaced by: after step (1), the polymer II is transferred from the cyclization reactor of step (1) to a pulverizer, and the polymer II is pulverized into particles with a particle size of 2 to 8 mm; and the pulverized polymer II is transferred from the pulverizer to the depolymerization reactor for step (2).

[0074] Embodiment 4:

[0075] The only difference between this embodiment and embodiment 1 is that the second stage depolymerization in step (2) is alcoholysis, and the alcohol substance added to polymer II is a C1-4 saturated monohydric alcohol or polyhydric alcohol, wherein the mass ratio of the alcohol substance to the polymer II is 1:1 to 10:1.

[0076] Embodiment 5:

[0077] A method for preparing glycolide of the present invention is a secondary depolymerization method of polyglycolide (PGA polymer), which specifically comprises the following steps:

[0078] (1) First-stage depolymerization: 10 kg of polymer I (PGA polymer) with a weight average molecular weight of about 2.6 W was placed in a ring-forming reaction kettle, and about 50 g of stannous octoate catalyst was added to polymer I. The first-stage depolymerization of polymer I was carried out under the first depolymerization conditions (temperature of 220° C., absolute pressure of 100 Pa, reaction time of 9 h), and a cracking ring-forming reaction occurred. The cracking conversion rate of polymer I was controlled to be 60%, and the generated glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 5.96 kg of glycolide and 4 kg of polymer II.

[0079] (2) Second stage depolymerization: polymer II is transferred to a depolymerization kettle through the melt, 12 kg of methanol is added to the depolymerization kettle, and polymer II is subjected to second stage depolymerization under the second depolymerization conditions (temperature of 215°C, gauge pressure of 5.3 MPa, reaction time of 4 h). After the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate.

[0080] (3) Separating the depolymerization liquid in step (2), removing solid impurities in the depolymerization liquid by centrifugal separation, and then distilling and concentrating the depolymerization liquid at 90° C. and normal pressure to obtain a concentrated liquid, wherein the evaporated methanol can be recovered and recycled for the next alcoholysis reaction; and then performing reduced pressure distillation on the remaining concentrated liquid at an absolute pressure of 80 kPa and 140° C., wherein the front fraction is methanol and 6.08 kg of methyl glycolate is collected in the rear stage, and the recovered methyl glycolate is subjected to a condensation reaction to obtain a polymer I with a theoretical mass of 3.92 kg.

[0081] The total yield of glycolide in Example 5 is about 5.96 / (10-3.92)=98.02%.

[0082] Embodiment 6:

[0083] A method for preparing glycolide of the present invention is a secondary depolymerization method of polyglycolide (PGA polymer), which specifically comprises the following steps:

[0084] (1) First-stage depolymerization: 10 kg of polymer I (PGA polymer) with a weight average molecular weight of about 2.6 W was put into a ring-forming reaction kettle, and about 50 g of stannous octoate catalyst was added to polymer I. The first-stage depolymerization of polymer I was carried out under the first depolymerization conditions (temperature of 230° C., absolute pressure of 500 Pa, reaction time of 5 h), and a cracking ring-forming reaction occurred. The cracking conversion rate of polymer I was controlled to be 50%, and the generated glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 4.95 kg of glycolide and 5 kg of polymer II.

[0085] (2) Second stage depolymerization: polymer II is transferred to a depolymerization kettle through the melt, 25 kg of ethanol is added to the depolymerization kettle, and polymer II is subjected to second stage depolymerization under the second depolymerization conditions (temperature of 225°C, gauge pressure of 5.0 MPa, reaction time of 3 h). After the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing ethyl glycolate.

[0086] (3) separating the depolymerization liquid in step (2), removing solid impurities in the depolymerization liquid by centrifugal separation, and then distilling and concentrating the depolymerization liquid at 95° C. and normal pressure to obtain a concentrated liquid, wherein the evaporated ethanol can be recovered and recycled for the next alcoholysis reaction; and then performing reduced pressure distillation on the remaining concentrated liquid at an absolute pressure of 80 kPa and 140° C., wherein the front fraction is ethanol, and 8.867 kg of ethyl glycolate is collected in the rear stage, and the recovered ethyl glycolate is subjected to a condensation reaction to obtain a polymer I with a theoretical mass of 4.945 kg.

[0087] The total yield of glycolide in Example 6 is about 4.95 / (10-4.945)=97.92%.

[0088] Embodiment 7:

[0089] A method for preparing glycolide of the present invention is a secondary depolymerization method of polyglycolide (PGA polymer), which specifically comprises the following steps:

[0090] (1) First-stage depolymerization: 10 kg of polymer I (PGA polymer) with a weight average molecular weight of about 3.0 W was placed in a ring-forming reaction kettle, and about 50 g of stannous octoate catalyst was added to polymer I. The first-stage depolymerization of polymer I was carried out under the first depolymerization conditions (temperature of 240° C., absolute pressure of 1000 Pa, reaction time of 6.5 h), and a cracking ring-forming reaction occurred. The cracking conversion rate of polymer I was controlled to be 70%, and the generated glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 6.97 kg of glycolide and 3 kg of polymer II.

[0091] The polymer II in the cyclization reaction kettle of step (1) is transferred to a pulverizer, and the polymer II is processed into particles with a particle size of 4 to 8 mm.

[0092] (2) Second stage depolymerization: The crushed polymer II is transferred from the crusher to a depolymerization kettle, 12 kg of methanol is added to the depolymerization kettle, and the polymer II is subjected to second stage depolymerization under the second depolymerization conditions (temperature of 225° C., gauge pressure of 6.2 MPa, reaction time of 3 h). After the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate.

[0093] (3) Separating the depolymerization liquid in step (2), removing solid impurities in the depolymerization liquid by centrifugal separation, and then distilling and concentrating the depolymerization liquid at 90° C. and normal pressure to obtain a concentrated liquid, wherein the evaporated methanol can be recovered and recycled for the next alcoholysis reaction; and then performing reduced pressure distillation on the remaining concentrated liquid at an absolute pressure of 80 kPa and 140° C., wherein the front fraction is ethanol, and 4.493 kg of methyl glycolate is collected in the rear stage, and the recovered methyl glycolate is subjected to a condensation reaction to obtain a polymer I with a theoretical mass of 2.895 kg.

[0094] The total yield of glycolide in Example 7 is about 6.97 / (10-2.895)=98.10%.

[0095] Embodiment 8:

[0096] A method for preparing glycolide of the present invention is a secondary depolymerization method of polyglycolide (PGA polymer), which specifically comprises the following steps:

[0097] (1) First stage depolymerization: 10 kg of polymer I (PGA polymer) with a weight average molecular weight of about 3.5 W was put into a ring-forming reactor, and about 80 g of stannous octoate catalyst was added to polymer I. The first stage depolymerization of polymer I was carried out under the first depolymerization conditions (temperature of 250° C., absolute pressure of 2000 Pa, reaction time of 3 h), and a cracking ring-forming reaction occurred. The cracking conversion rate of polymer I was controlled to be 56%, and the generated glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 5.57 kg of glycolide and 4.4 kg of polymer II (residue in the ring-forming reactor).

[0098] Then, 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.

[0099] (2) Second stage depolymerization: The mixture of polymer II and methyl glycolate is transferred to a depolymerization kettle through melt transfer, 13.2 kg of methanol is added to the depolymerization kettle, and polymer II is subjected to second stage depolymerization under the second depolymerization conditions (temperature of 190° C., gauge pressure of 3.3 MPa, reaction time of 5 h). After the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate.

[0100] (3) The depolymerization liquid in step (2) is separated, and solid impurities in the depolymerization liquid are removed by centrifugation. The depolymerization liquid is then 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. 13.125 kg of methyl glycolate is collected in the rear stage (including 6.6 kg of methyl glycolate added in step (1) and 6.525 kg of methyl glycolate recovered in this step). The recovered methyl glycolate is subjected to polycondensation reaction to obtain a polymer I with a theoretical mass of 4.205 kg.

[0101] The total yield of glycolide in Example 8 is about 5.57 / (10-4.205)=96.11%.

[0102] Embodiment 9:

[0103] A method for preparing glycolide of the present invention is a secondary depolymerization method of polyglycolide (PGA polymer), which specifically comprises the following steps:

[0104] (1) First-stage depolymerization: 10 kg of polymer I (PGA polymer) with a weight average molecular weight of about 4W was put into a ring-forming reaction kettle, and about 80 g of stannous octoate catalyst was added to polymer I. The first-stage depolymerization of polymer I was carried out under the first depolymerization conditions (temperature of 275° C., absolute pressure of 500 Pa, reaction time of 2 h), and a cracking ring-forming reaction occurred. The cracking conversion rate of polymer I was controlled to be 78%, and the generated glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 7.74 kg of glycolide and 2.2 kg of polymer II.

[0105] Then, 1.1 kg of methyl glycolate was added into the cyclization reactor at a mass ratio of methyl glycolate to polymer II of 0.5:1.

[0106] (2) Second stage depolymerization: The mixture of polymer II and methyl glycolate is transferred to a depolymerization kettle through melt transfer, 11 kg of methanol is added to the depolymerization kettle, and polymer II is subjected to second stage depolymerization under the second depolymerization conditions (temperature of 200° C., gauge pressure of 4.0 MPa, reaction time of 2.5 h). After the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate.

[0107] (3) The depolymerization liquid in step (2) is separated, and solid impurities in the depolymerization liquid are removed by centrifugation. The depolymerization liquid is then 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. 4.336 kg of methyl glycolate is collected in the rear stage (including 1.1 kg of methyl glycolate added in step (1) and 3.236 kg of methyl glycolate recovered in this step). The recovered methyl glycolate is subjected to polycondensation reaction to obtain a polymer I with a theoretical mass of 2.085 kg.

[0108] The total yield of glycolide in Example 9 is about 7.74 / (10-2.085)=97.78%.

[0109] Embodiment 10:

[0110] A method for preparing glycolide of the present invention is a secondary depolymerization method of polyglycolide (PGA polymer), which specifically comprises the following steps:

[0111] (1) First stage depolymerization: 10 kg of polymer I (PGA polymer) with a weight average molecular weight of about 5W was put into a ring-forming reactor, and about 80 g of stannous octoate catalyst was added to polymer I. The first stage depolymerization of polymer I was carried out under the first depolymerization conditions (temperature of 2260° C., absolute pressure of 100 Pa, reaction time of 2 h), and a cracking ring-forming reaction occurred. The cracking conversion rate of polymer I was controlled to be 70%, and the generated glycolide vapor was collected in turn through a hot water heat exchanger and a cold water condenser to obtain 6.98 kg of glycolide and 3.0 kg of polymer II (residue in the ring-forming reactor).

[0112] The polymer II in the cyclization reaction kettle is transferred to a pulverizer, and the polymer II is processed into particles with a particle size of 2 to 6 mm.

[0113] (2) Second stage depolymerization: The crushed polymer II is transferred to a depolymerization kettle, 15 kg of methanol is added to the depolymerization kettle, and the polymer II is subjected to a second stage depolymerization under the second depolymerization conditions (temperature of 170° C., gauge pressure of 2.2 MPa, reaction time of 6 h). After the reaction is completed, the temperature is naturally lowered to room temperature to obtain a depolymerization liquid containing methyl glycolate.

[0114] (3) Separating the depolymerization liquid in step (2), removing solid impurities in the depolymerization liquid by centrifugal separation, and then distilling and concentrating the depolymerization liquid at 90° C. and normal pressure to obtain a concentrated liquid, wherein the evaporated methanol can be recovered and recycled for the next alcoholysis reaction; and then performing reduced pressure distillation on the remaining concentrated liquid at an absolute pressure of 80 kPa and 140° C., wherein the front fraction is methanol and 4.426 kg of methyl glycolate is collected in the rear stage, and the recovered methyl glycolate is subjected to polycondensation reaction to obtain a polymer I with a theoretical mass of 2.852 kg.

[0115] The total yield of glycolide in Example 10 is about 6.98 / (10-2.852)=97.63%.

[0116] Comparative Example 1:

[0117] The difference between this comparative example and Example 8 is that the reaction time in step (1) is 5h, the cracking conversion rate of polymer I is controlled to be 92%, and 9.11kg of glycolide and 0.8kg of polymer II are obtained; 1.2kg of methyl glycolate is added to the cyclization reactor according to the mass ratio of methyl glycolate to polymer II of 1.5:1. The amount of methanol added in step (2) is 2.4kg. In step (3), 1.72kg of methyl glycolate is collected in the latter stage (including 1.2kg of methyl glycolate added in step (1) and 0.52kg of methyl glycolate recovered in this step), and the recovered methyl glycolate is polycondensed to obtain a polymer I with a theoretical mass of 0.335kg.

[0118] The total yield of glycolide in Comparative Example 1 is about 9.11 / (10-0.335)=94.25%.

[0119] Comparative Example 2:

[0120] The prior art polyglycolide cleavage ring formation process is used to prepare glycolide, and the specific steps are as follows:

[0121] 8 kg of polymer I (PGA polymer) with a weight average molecular weight of about 3.5W was put into a ring-forming reactor, and about 40 g of stannous octoate catalyst and 2 kg of tridecane, 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. The reaction was stopped after no gas phase was generated in the ring-forming reactor. Finally, 7.55 kg of crude glycolide was obtained. 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.

[0122] The total yield of glycolide in Comparative Example 2 is about 7.58 / 8=94.37%.

[0123] The preparation method of glycolide proposed in the above-mentioned embodiments 1 to 10 of the present invention The preparation method of glycolide of the present invention does not require the addition of a high boiling point solvent, and overcomes the problem of increased viscosity of the material system at high temperature. By controlling the cracking conversion rate of polyglycolide, the coking and carbonization of the residue in the cyclization reactor is avoided. At the same time, the residue is further depolymerized into glycolic acid monomers in the later stage, and the glycolic acid monomers can be returned to the polycondensation system as raw materials to synthesize polyglycolide, and continue to be cracked to generate glycolide. There is almost no or only a small amount of coking and carbonization residues in the cyclization reactor of the present invention, and experiments have shown that the coking amount is less than 0.5%. The whole process further improves the yield of glycolide.

[0124] The preparation method of glycolide proposed in the above-mentioned embodiments 1 to 10 of the present invention does not require regular cleaning of the cyclization reactor, and the cyclization reactor can continue to perform the cracking cyclization reaction, and the entire production process can be continuously produced.

[0125] 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 secondary depolymerization method of polyglycolide, which specifically comprises the following steps: (1) First-stage depolymerization: performing first-stage depolymerization on polymer I under first depolymerization conditions, controlling the pyrolysis conversion rate of polymer I to be less than or equal to 90%, to obtain glycolide and polymer II, wherein polymer I is the polyglycolide; (2) second-stage depolymerization: performing a second-stage depolymerization on the polymer II under a second depolymerization condition to obtain a depolymerization solution containing glycolic acid monomers, wherein the second-stage depolymerization is hydrolysis or alcoholysis; (3) separating the depolymerization solution in step (2) to obtain glycolic acid monomers; in, The first depolymerization conditions include: temperature of 220-290°C, absolute pressure of 0.01 kPa-2 kPa, and reaction time of 0.5-12 hours; The second depolymerization conditions include: temperature of 100 to 250° C., gauge pressure of 1 to 7 MPa, and reaction time of 1 to 12 hours.

2. The method for preparing glycolide according to claim 1, characterized in that: After step (1), the polymer II is transferred from the cyclization reaction kettle of step (1) to a pulverizer, and the polymer II is pulverized into particles with a particle size of 2 to 8 mm; The pulverized polymer II is then transferred from the pulverizer to a depolymerization kettle to carry out step (2).

3. The method for preparing glycolide according to claim 1, characterized in that: The step (1) further comprises: adding methyl glycolate to the polymer II; the mass ratio of the methyl glycolate to the polymer II is 0.2:1 to 3:

1.

4. The method for preparing glycolide according to claim 3, characterized in that: The mass ratio of the methyl glycolate to the polymer II is: 0.5:1～1.75:1。 5. The method for preparing glycolide according to claim 3, characterized in that: After the step (1), the mixture of the polymer II and the methyl glycolate is transferred from the cyclization reactor of the step (1) to a depolymerization reactor to carry out the step (2).

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

7. The method for preparing glycolide according to any one of claims 1 to 6, characterized in that: The preparation method further comprises: subjecting the glycolic acid monomers described in step (3) to a condensation polymerization reaction to obtain the polymer I, and recycling the polymer to step (1) for use.

8. 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%.

9. The method for preparing glycolide according to claim 1, characterized in that: The second stage depolymerization in step (2) is alcoholysis, in which an alcohol substance is added to the polymer II, and the alcohol substance 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 II is 1:1 to 10:1.

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