Preparation process for preparing high-molecular-weight and high-color-phase polyglycolic acid by direct esterification method

Abstract

The invention discloses a preparation process for preparing high-molecular-weight high-color-phase polyglycolic acid by a direct esterification method, which comprises the following steps: (1) adding a mixture of glycolic acid and / or methyl glycolate and a catalyst into a reactor, heating to 170 DEG C, and reacting until no distillate is distilled out of a system; (2) reducing the reaction pressure to coarse vacuum, raising the temperature of a reaction system to 190 DEG C, and reacting until the theoretical water yield of the system reaches 98% or above to obtain a PGA oligomer; (3) reducing the temperature of the reaction system to 170 DEG C, and reacting until a reactant is cured; (4) adding an antioxidant and a stabilizer into the reaction system, rapidly heating the reaction system to 220-225 DEG C, controlling the heating rate at 2-15 DEG C / min, and increasing the vacuum degree of the reaction system to react to obtain a PGA crystalline polymer; and (5) cutting the PGA crystalline polymer into granules, drying to remove water, and carrying out solid-phase polymerization at 195-210 DEG C to improve the molecular weight of the PGA. Glycollic acid is directly subjected to polycondensation to synthesize polyglycolic acid, and good color and luster can be kept under high molecular weight.

Description

Technical Field

[0001] The invention belongs to the field of polyglycolic acid (PGA) preparation, and specifically relates to a preparation process and an auxiliary agent composition for preparing high-molecular-weight and high-color polyglycolic acid (PGA) by direct esterification. Background Art

[0002] At present, biodegradable plastic products are mainly polylactic acid PLA, PBAT, PHA, PBS and modified starch, among which PLA is the largest in industry consumption, with a global output of more than 450,000 tons, and is widely used in packaging, film materials, construction and other fields. Develop engineering biodegradable polymer materials with better comprehensive performance than PLA and gradually replace PLA. The global market capacity of biodegradable polymer materials has reached one million tons, and the market demand is strong, which will bring significant economic benefits in the future.

[0003] Polyglycolic acid (PGA) has the advantages of excellent biocompatibility, mechanical properties, high barrier properties, high load deformation temperature, etc. It is a rare degradable engineering polymer material. Its biodegradability is similar to that of natural cellulose. At the same time, it meets the requirements of industrial composting, household composting and seawater degradation. It has been certified as a safe biodegradable plastic material in the United States, the European Union, Japan and other countries. It is widely used in high-end biomedicine, food packaging, shale and oil extraction, agricultural production and other fields. It has broad market space. It is expected that the global PGA market will reach 10 billion yuan in 2030.

[0004] At present, the synthesis methods of polyglycolic acid with industrial value include direct polycondensation and glycolide ring-opening polymerization. At present, the synthesis of PGA at home and abroad is mainly based on glycolide ring-opening polymerization, which has high requirements for the purity of glycolide and high product cost. It is mainly produced on a large scale in Japan and the United States.

[0005] In recent years, with the development of coal chemical technology and the breakthrough of key technology for preparing glycolic acid (methyl ester) from dimethyl oxalate at low cost, my country's PGA industry has brought important turning points and hope. The rapid growth of domestic glycolic acid monomer production scale and the reduction of costs make direct polymerization more likely to become the preferred solution for large-scale production of PGA in the future.

[0006] The method of synthesizing PGA by direct polycondensation of glycolic acid is shorter and cheaper than the ring-opening polymerization of glycolide, and is more suitable for the production of bulk products. However, it is generally reported that this method is difficult to synthesize products with higher molecular weight and lighter color, and there is a problem of mutual exclusion between molecular weight and color. In the research on the direct synthesis of PGA, there is currently no report on the synthesis of high molecular weight (Mn>50000Da) light-colored PGA. Because the thermal stability of polyglycolic acid deteriorates due to the increase in the required temperature and the decrease in pressure in the later stage of the polycondensation reaction, the long-chain molecules will degrade into short-chain molecules, and color changes will occur due to complex mechanisms.

[0007] To sum up, the high technical difficulty of the direct esterification polymerization route brings challenges and opportunities to the development of the entire industry. Summary of the invention

[0008] The object of the present invention is to provide a preparation process of high molecular weight and high hue polyglycolic acid by direct esterification, so as to prepare light-colored high molecular weight polyglycolic acid.

[0009] In order to achieve the above-mentioned purpose of the invention, the preparation process provided by the present invention adopts the following technical scheme:

[0010] A preparation process for high molecular weight and high hue polyglycolic acid by direct esterification comprises the following steps:

[0011] (1) adding glycolic acid and / or methyl glycolate as raw materials and a catalyst mixture into a reactor, heating the reactor to 165-175° C. and reacting the reactor until no distillate is evaporated from the reaction system;

[0012] (2) reducing the pressure of the reaction system to a rough vacuum, and raising the temperature of the reaction system to 185-195° C., and reacting until the theoretical water yield of the system reaches more than 98%, thereby obtaining PGA oligomers;

[0013] (3) lowering the temperature of the reaction system to 165-175° C. and reacting until the reactants solidify;

[0014] (4) adding an antioxidant and a stabilizer to the reaction system, and heating the reaction system to 220-225° C. so as to be evenly mixed with the added antioxidant and stabilizer after melting, the heating rate is controlled at 2° C. / min-15° C. / min, and the vacuum degree of the reaction system is increased to high vacuum to obtain a PGA crystalline polymer by reaction;

[0015] (5) The PGA crystalline polymer prepared in step (4) is cut into pellets, dried to remove water, and the temperature is controlled to 195-210° C. to perform solid phase polymerization to further increase the molecular weight of PGA.

[0016] In step (1) of the present invention, a mixture of raw materials and a catalyst (such as stannous octoate) is added to a reactor, and the temperature is raised to 165-175° C., such as 170° C., and the reaction is carried out until no distillate is evaporated from the reaction system; wherein the reaction pressure can be normal pressure, and the reaction atmosphere can be a protective atmosphere such as a nitrogen atmosphere; it is understood in the art that esterification reaction mainly occurs in this process, and water will be evaporated as distillate during the reaction.

[0017] In step (2) of the present invention, the pressure of the reaction system is reduced to a rough vacuum, preferably to 2000-3000Pa, such as 2500Pa, and the temperature of the reaction system is raised to 185-195°C, such as 190°C, and the reaction is carried out until the theoretical water yield of the reaction system reaches more than 98% to obtain PGA oligomers; by reacting under rough vacuum conditions, the esterification reaction of the reactants can be made more complete, which is a conventional processing process in the field and will not be described in detail here.

[0018] In step (3) of the present invention, the temperature of the reaction system is lowered to 165-175°C, such as 170°C, and the reaction is carried out until the reactant solidifies. Studies have found that by lowering the reaction temperature, the melting point of PGA in the subsequent vacuum polycondensation process can be lowered, so that the PGA polycondensation temperature is controlled within a relatively low range that is not prone to yellowing (while directly raising the temperature from 190°C to the polycondensation temperature is likely to increase the melting point of PGA), thereby inhibiting the yellowing of the product. Reacting until the product solidifies can maximize the degree of polymerization of the PGA oligomer (for example, the degree of polymerization of polyglycolic acid after the reaction is solidified can be about 10, such as 8-12, such as 9-11), and reduce the subsequent high vacuum polycondensation time. If the high vacuum polycondensation time is too long, it is easy to cause PGA yellowing.

[0019] In a preferred embodiment, the molecular weight of the PGA reactant obtained in step (3) is 5000-10000 Da, such as 6000 or 8000 Da.

[0020] In step (4) of the present invention, an antioxidant and a stabilizer are added to the reaction system to control side reactions such as oxidation and decomposition during the reaction to achieve the purpose of inhibiting the yellowing of the product. In a preferred embodiment, the antioxidant is one or more of pentaerythritol tetrakis(di-tert-butylhydroxyhydrocinnamate) ester (Antioxidant 1010), α-tocopherol, and 5-tert-butyl-4-hydroxy-2-methylphenylene sulfide (Antioxidant 300).

[0021] In a preferred embodiment, the stabilizer is one or more of bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite (THP-24), diphenyl phosphite, triphenyl phosphite (TPP), and triethyl phosphate (TEP).

[0022] In a preferred embodiment, in step (4), based on the mass of the raw materials, the total amount of the antioxidant and the stabilizer is 1 / 10,000 to 1 / 1,000, that is, the total amount is 1 / 10,000 to 1 / 1,000 of the mass of the raw materials, preferably 4 / 10,000 to 6 / 10,000. Preferably, the amount ratio of the antioxidant to the stabilizer is (2-4):1, such as 3:1.

[0023] In step (4) of the present invention, after adding the antioxidant and the stabilizer, the reaction system is quickly heated to 220-225°C, the heating rate is controlled at 2°C / min-15°C / min, such as 4°C / min-10°C / min, such as 5°C / min or 8°C / min, and the vacuum degree of the reaction system is increased to obtain a PGA crystalline polymer. Studies have found that according to the characteristics of multiple melting of PGA, the rapid heating of the crystallized PGA is conducive to lowering its melting point. If the heating process is too slow, it will cause the melting point of PGA to increase in the later stage. The polymerization system may crystallize and solidify in advance, resulting in polymerization termination, and heating too fast may cause violent reactions and yellowing problems. In addition, it is understood in the art that in order to ensure the rapid heating effect of the system, when the temperature of the reaction system is raised to the final polymerization temperature (such as 220-225°C), the gas phase space temperature in the reactor should also be heated to 220-225°C by external heating or the like, and the agitator should be heated to 220-225°C.

[0024] In some embodiments, in step (4), after the temperature is raised to 220-225° C., the vacuum degree of the reaction system is gradually increased to not less than 400 Pa, and after the reaction is carried out until the viscosity of the reaction system reaches 0.3-0.5 cp, the vacuum degree is increased so that the final vacuum degree is below 70 Pa, such as 60, 40 or 20 Pa, and the polymerization is continued for 1-3 hours to obtain a PGA crystalline polymer.

[0025] Preferably, when the reaction vacuum is increased step by step, the reaction vacuum is first increased to 1200-800Pa, such as 1000Pa, and reacted for 0.5-1 hour, then increased to 600-500Pa, such as 550Pa, and reacted for 0.5-1 hour, and finally increased to 450-400Pa, and reacted for 1-2 hours.

[0026] In some embodiments, in step (4), after the temperature is raised to 220-225°C, the vacuum degree of the system is directly increased to 70Pa, such as 60, 40 or 20Pa or less, and the reaction is carried out for 2-4 hours to obtain a PGA crystalline polymer; and during the reaction, a mixture of glycolide and PGA oligomers evaporated from the reaction system is collected, and the mixture is reused as a part of the raw material for PGA synthesis, thereby improving the PGA polymerization yield.

[0027] In a preferred embodiment, the molecular weight of the PGA crystalline polymer obtained in step (4) is 30,000-50,000 Da, such as 40,000 Da.

[0028] In step (5) of the present invention, the PGA crystalline polymer prepared in step (4) is cut into pellets and dried to remove water, for example, dried to remove water under vacuum or blowing conditions at 100-120°C, and the temperature is controlled to 195-210°C for solid phase polymerization to further increase the molecular weight of PGA. The specific details are well known in the art and will not be repeated here.

[0029] In a preferred embodiment, in step (5), the molecular weight of PGA obtained by solid phase polymerization is 60,000-80,000 Da, such as 70,000 Da.

[0030] In the present invention, unless otherwise specified, all ratios or percentages involved are mass ratios or mass percentages.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] The auxiliary agent composition and process for inhibiting yellowing of polyglycolic acid (PGA) and improving product quality of the present invention have the characteristics of inhibiting degradation of long-chain polyglycolic acid and inhibiting yellowing under high temperature and high vacuum conditions, so that the polyglycolic acid product can be directly condensed and synthesized from glycolic acid, and can maintain good color at high molecular weight;

[0033] The direct esterification method for synthesizing PGA has low cost and is easy to promote to the bulk product market. In addition, the PGA prepared by the present invention has high quality and high profit margin, and has significant economic benefits. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only embodiments of a part of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0035] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values, such as values ​​of ±10% of the endpoint values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article. In the absence of conflict, the embodiments in this application and the features in the embodiments can be combined with each other.

[0036] It should be noted that the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or apparatus.

[0037] The present invention is further described below in conjunction with the following embodiments / comparative examples. In the following embodiments / comparative examples, unless otherwise specified, all reagents used are analytically pure.

[0038] Example 1

[0039] 100 mol of glycolic acid and 5 mmol of stannous octoate as a catalyst were added to a 10L polymerization reactor. Under N2 protection conditions and normal pressure, the reaction temperature was raised to 170°C for 1 hour without distillation, and then the temperature was raised to 190°C for 0.5 hour under a pressure of 3000Pa until the theoretical water yield of the system reached more than 98%, the system temperature was lowered to 170°C, and the reaction was solidified for 1.5 hours to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Antioxidants and stabilizing agents (see Table 1 below) were added, and the system was quickly heated to 220°C at 5°C / min, polycondensed at 1000Pa vacuum for 0.5 hour, polycondensed at 500Pa vacuum for 1 hour, and polycondensed at 400Pa vacuum for 1 hour until the viscosity was 4CP, to obtain PGA with a number average molecular weight of 23000Da. The vacuum degree of the reaction system was adjusted to 65Pa, and the polymerization was carried out for 1.5 hours. The material was cooled and granulated under vacuum conditions to obtain white PGA with a number average molecular weight of 42000Da. The PGA pellets were put into a vacuum drum reactor for solid phase polycondensation at a polycondensation temperature of 198°C and a polycondensation time of 20 hours to obtain a white PGA solid material with a yield of 89%, a polymer B value of 2, a number average molecular weight of 57000Da, a weight average molecular weight of 76800, and a molecular weight distribution of 1.35.

[0040] Example 2

[0041] Add 100mol of glycolic acid and 5mmol of stannous octoate to a 10L polymerization reactor. Under N2 protection and normal pressure, raise the reaction temperature to 170°C for 1 hour, then raise the temperature to 190°C for 0.5 hour, lower the system temperature to 170°C, and react for 1.5 hours under 3000Pa vacuum to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Add antioxidants and stabilizing agents (see Table 1 below), and quickly heat the system to 220°C at 8°C / min, polycondense at 1000Pa vacuum for 0.5 hour, polycondense at 500Pa vacuum for 1 hour, and polycondense at 400Pa vacuum for 1 hour to obtain PGA with a number average molecular weight of 22800Da. Regulate the vacuum degree of the reaction system to 65Pa, polymerize for 1.5 hours, cool the material under vacuum conditions and granulate to obtain white PGA with a number average molecular weight of 42100Da. The PGA pellets were put into a vacuum drum reactor for solid phase polycondensation at a temperature of 198°C for 20 hours to obtain a white PGA solid material with a yield of 89%. The polymer B value was 1.9, the number average molecular weight was 57500Da, the weight average molecular weight was 77600, and the molecular weight distribution was 1.38.

[0042] Example 3

[0043] Add 100mol of glycolic acid and 5mmol of stannous octoate to a 10L polymerization reactor. Under N2 protection and normal pressure, raise the reaction temperature to 170°C for 1 hour, then raise the temperature to 190°C for 0.5 hour, lower the system temperature to 170°C, and react for 1.5 hours under 3000Pa vacuum conditions to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Add antioxidants and stabilizing agents (see Table 1 below), and quickly heat the system to 220°C at 4°C / min, control the system vacuum at 65Pa, polycondense for 4 hours, cool the material under vacuum conditions and granulate to obtain white PGA with a number average molecular weight of 46000Da. A total of 2 kg of glycolide and PGA oligomers are collected during the polycondensation process. The PGA pellets were put into a vacuum drum reactor for solid phase polycondensation at a temperature of 198°C for 20 hours to obtain a white PGA solid material with a yield of 61%, a polymer B value of 2.2, a number average molecular weight of 60,000 Da, a weight average molecular weight of 87,600, and a molecular weight distribution of 1.46.

[0044] The PGA oligomers collected during the polymerization were mixed with 2 kg of glycolide and 5 kg of glycolic acid, and the above experiment was repeated to obtain a white PGA solid material with a yield of 61%, a polymer B value of 2.2, a number average molecular weight of 60,000 Da, a weight average molecular weight of 87,600, and a molecular weight distribution of 1.46.

[0045] The above experimental operation was repeated to form a semi-continuous preparation of PGA. The cycle was repeated 5 times, and the total yield of PGA was 95%.

[0046] Example 4

[0047] 100 mol of glycolic acid and 5 mmol of stannous octoate were added to a 10L polymerization reactor. Under N2 protection and normal pressure, the reaction temperature was raised to 170°C for 1 hour, then raised to 190°C for 0.5 hour, the system temperature was lowered to 170°C, and reacted under 3000Pa vacuum for 1.5 hours to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Antioxidants and stabilizing agents (see Table 1 below) were added, and the system was quickly heated to 220°C at 10°C / min, the system vacuum was controlled at 65Pa, polycondensed for 4 hours, cooled under vacuum conditions and granulated to obtain white PGA with a number average molecular weight of 47000Da. A total of 2 kg of glycolide and PGA oligomers were collected during the polycondensation process. The PGA pellets were put into a vacuum drum reactor for solid phase polycondensation at a temperature of 198°C for 20 hours to obtain a white PGA solid material with a yield of 61%, a polymer B value of 2.0, a number average molecular weight of 66,000 Da, a weight average molecular weight of 89,700, and a molecular weight distribution of 1.42.

[0048] The PGA oligomers collected during the polymerization were mixed with 2 kg of glycolide and 5 kg of glycolic acid, and the above experiment was repeated to obtain a white PGA solid material with a yield of 61%, a polymer B value of 2.1, a number average molecular weight of 62000 Da, a weight average molecular weight of 84500, and a molecular weight distribution of 1.49.

[0049] The above experimental operation was repeated to form a semi-continuous preparation of PGA. The cycle was repeated 5 times, and the total yield of PGA was 95%.

[0050] Example 5

[0051] 100 mol of methyl glycolate and 8 mmol of stannous octoate as a catalyst were added to a 10L polymerization reactor. Under N2 protection conditions and normal pressure, the reaction temperature was raised to 170°C for reaction for 1 hour, and then raised to 190°C for reaction for 0.5 hour, the system temperature was lowered to 170°C, and reacted under 3000Pa vacuum conditions for 1.5 hours to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Antioxidants and stabilizing agents (see Table 1 below) were added, and the system was quickly heated to 220°C at 5°C / min, polycondensed at 1000Pa vacuum for 0.5 hour, polycondensed at 500Pa vacuum for 1 hour, and polycondensed at 400Pa vacuum for 1 hour to obtain PGA with a number average molecular weight of 22000Da. The vacuum degree of the reaction system was adjusted to 65Pa, polymerized for 1.5 hours, cooled and discharged under vacuum conditions and granulated to obtain white PGA with a number average molecular weight of 41000Da. The PGA pellets were put into a vacuum drum reactor for solid phase polycondensation at a temperature of 198°C for 20 hours to obtain a white PGA solid material with a yield of 89%, a polymer B value of 2, a number average molecular weight of 57,000 Da, a weight average molecular weight of 75,900, and a molecular weight distribution of 1.4.

[0052] Comparative Example 1

[0053] 100 mol of glycolic acid and 5 mmol of stannous octoate were added to a 10L polymerization reactor. Under N2 protection and normal pressure, the reaction temperature was raised to 170°C for 1 hour, then raised to 190°C for 0.5 hour, and then continued to react for 1.5 hours under 3000Pa vacuum to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Antioxidants and stabilizing agents (see Table 1 below) were added, and the system was quickly heated to 220°C at 5°C / min, polycondensed at 1000Pa vacuum for 0.5 hour, polycondensed at 500Pa vacuum for 1 hour, and polycondensed at 400Pa vacuum for 1 hour to obtain PGA with a number average molecular weight of 23200Da. The vacuum degree of the reaction system was adjusted to 65Pa. After 0.5 hours of polymerization, the reaction system was prematurely crystallized and solidified due to PGA, and the polymerization was terminated to obtain PGA solid with a number average molecular weight of 27000Da.

[0054] Comparative Example 2

[0055] Add 100 mol of glycolic acid and 5 mmol of stannous octoate to a 10L polymerization reactor. Under N2 protection and normal pressure, raise the reaction temperature to 170°C for 1 hour, then raise the temperature to 190°C for 0.5 hour, and then react under 3000Pa vacuum for 1.5 hours to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Add antioxidants and stabilizers (see Table 1 below), and quickly heat the system to 220°C at 10°C / min. The vacuum degree of the system is controlled at 65Pa. After 1.5 hours of polycondensation, the reaction system is prematurely crystallized and solidified due to PGA, and the polymerization is terminated to obtain PGA solid with a number average molecular weight of 25000Da.

[0056] Comparative Example 3

[0057] Add 100mol of glycolic acid and 5mmol of stannous octoate to a 10L polymerization reactor. Under N2 protection and normal pressure, raise the reaction temperature to 170°C for 1 hour, then raise the temperature to 190°C for 0.5 hour, lower the system temperature to 170°C, and react for 1.5 hours under 3000Pa vacuum to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Add antioxidants and stabilizing agents (see Table 1 below), and raise the system temperature to 225°C at 1°C / min, polycondense at 1000Pa vacuum for 0.5 hour, polycondense at 500Pa vacuum for 1 hour, and polycondense at 400Pa vacuum for 1 hour to obtain PGA with a number average molecular weight of 22000Da. Regulate the vacuum degree of the reaction system to 65Pa, and after 0.6 hours of polymerization, the reaction system is prematurely crystallized and solidified due to PGA, and the polymerization is terminated to obtain PGA solid with a number average molecular weight of 27600Da.

[0058] Comparative Example 3

[0059] Add 100mol of glycolic acid and 5mmol of stannous octoate to a 10L polymerization reactor. Under N2 protection and normal pressure, raise the reaction temperature to 170°C for 1 hour, then raise the temperature to 190°C for 0.5 hour, lower the system temperature to 170°C, and react for 1.5 hours under 3000Pa vacuum to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Add antioxidants and stabilizing agents (see Table 1 below), and raise the system temperature to 220°C at 1°C / min, polycondense at 1000Pa vacuum for 0.5 hour, polycondense at 500Pa vacuum for 1 hour, and polycondense at 400Pa vacuum for 1 hour to obtain PGA with a number average molecular weight of 23500Da. Regulate the vacuum degree of the reaction system to 65Pa, polymerize for 1.5 hours, cool the material under vacuum conditions and granulate to obtain yellow PGA with a number average molecular weight of 44000Da. The PGA pellets were put into a vacuum drum reactor for solid phase polycondensation at a temperature of 198°C for 20 hours to obtain a yellow PGA solid material with a yield of 89%. The polymer B value was 4.6, the number average molecular weight was 59,000 Da, the weight average molecular weight was 86,800, and the molecular weight distribution was 1.47.

[0060] Comparative Example 5

[0061] Add 100mol of glycolic acid and 5mmol of stannous octoate to a 10L polymerization reactor. Under N2 protection and normal pressure, raise the reaction temperature to 170°C for 1 hour, then raise the temperature to 190°C for 0.5 hour, lower the system temperature to 170°C, and react for 1.5 hours under 3000Pa vacuum to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Add antioxidants and stabilizing agents (see Table 1 below), and raise the system temperature to 225°C at 1°C / min. The system vacuum is controlled at 65Pa. After 2 hours of polycondensation, the reaction system is prematurely crystallized and solidified due to PGA, and the polymerization is terminated to obtain PGA solids with a number average molecular weight of 29800Da.

[0062] Comparative Example 6

[0063] Add 100mol of glycolic acid and 5mmol of stannous octoate to a 10L polymerization reactor. Under N2 protection and normal pressure, raise the reaction temperature to 170°C for 1 hour, then raise the temperature to 190°C for 0.5 hour, lower the system temperature to 170°C, and react for 1.5 hours under 3000Pa vacuum to obtain PGA oligomers with a number average molecular weight of 5000-10000Da. Add antioxidants and stabilizing agents (see Table 1 below), and raise the system temperature to 220°C at 1°C / min, control the system vacuum at 65Pa, polycondense for 4 hours, cool the material under vacuum and granulate to obtain PGA with a number average molecular weight of 48000Da. A total of 2 kg of glycolide and PGA oligomers are collected during the polycondensation process. The PGA pellets were put into a vacuum drum reactor for solid phase polycondensation at a temperature of 198°C for 20 hours to obtain a yellow PGA solid material with a yield of 61%, a polymer B value of 4.9, a number average molecular weight of 65,000 Da, a weight average molecular weight of 95,600, and a molecular weight distribution of 1.36.

[0064] The PGA oligomer collected during the polymerization process was mixed with 2 kg of glycolide and 5 kg of glycolic acid, and the above experiment was repeated to obtain a yellow PGA solid material with a yield of 61%, a polymer B value of 5.1, a number average molecular weight of 61000 Da, a weight average molecular weight of 85600, and a molecular weight distribution of 1.38.

[0065] The above experimental operation was repeated to form a semi-continuous preparation of PGA. The cycle was repeated 5 times, and the total yield of PGA was 95%.

[0066] Comparative Example 7

[0067] The difference compared with Example 1 is that when starting the reaction, the reaction temperature is directly raised to 190° C. and reacted until the theoretical water output rate of the system reaches more than 98%, and then the system temperature is lowered to 170° C. for subsequent reactions.

[0068] Reaction results: number average molecular weight is 41000Da. PGA pellets were put into a vacuum drum reactor for solid phase polycondensation, the polycondensation temperature was 198°C, the polycondensation time was 20 hours, and a light yellow PGA solid material was obtained with a yield of 88%, a polymer B value of 3, a number average molecular weight of 58000Da, a weight average molecular weight of 78000, and a molecular weight distribution of 1.55.

[0069] Comparative Example 8

[0070] The difference compared with Example 3 is that the reaction temperature is raised to 190°C for reaction until solidification, and then the temperature needs to be raised to 235°C to melt the solidified product and then cooled to 220°C, and the vacuum degree is directly controlled to 65Pa to continue the subsequent reaction for 3 hours.

[0071] Reaction results: number average molecular weight is 51000Da. PGA pellets were put into a vacuum drum reactor for solid phase polycondensation, the polycondensation temperature was 198°C, the polycondensation time was 20 hours, and yellow PGA solid material was obtained with a yield of 68%, a polymer B value of 6, a number average molecular weight of 70000Da, a weight average molecular weight of 86000, and a molecular weight distribution of 1.75.

[0072] Comparative Example 9

[0073] The difference compared with Example 3 is that no additional antioxidant and stabilizer are added.

[0074] Reaction results: number average molecular weight is 43000Da. PGA pellets were put into a vacuum drum reactor for solid phase polycondensation, the polycondensation temperature was 198°C, the polycondensation time was 20 hours, and a light yellow PGA solid material was obtained with a yield of 89%, a polymer B value of 8, a number average molecular weight of 59000Da, a weight average molecular weight of 89000, and a molecular weight distribution of 1.45.

[0075] Comparative Example 10

[0076] Cited from the embodiment of CN 202110265690.3:

[0077] 3000 g of methyl glycolate and 0.3 g of antimony trioxide were added to the first pressure reactor (volume 5 L), and a pre-polycondensation reaction was carried out at 500 kPa and 200 ° C. The reaction lasted for about 3 hours, and about 403 g of methanol was removed. The pressure was then reduced to normal pressure, and then the temperature was raised to 220 ° C. at normal pressure. The reaction was carried out at normal pressure for about 1 hour to further remove methanol. About 130 g of methanol was removed. Sampling was performed, and the Mw of the material in the reactor was measured to be about 148 by end group analysis. The sample was subjected to gas chromatography analysis and testing, and it was measured that the unreacted methyl glycolate monomer in the material in the first pressure reactor was about 16% of its initial feed amount. 300 g of polyglycolic acid with a Mww' of about 15,000 was added to the first pressure reactor, and the reaction was continued at 500 kPa and 190 ° C. After about 2 hours, sampling was taken, and the Mw1 of the material in the reactor was measured by end group analysis to be about 170. The sample was subjected to gas chromatography analysis and testing, and it was measured that the unreacted methyl glycolate monomer in the material in the first pressure reactor was about 0.8% of its initial charge amount. The material was then introduced into the second pressure reactor for final polymerization, and the reaction temperature was controlled to 220°C. The absolute pressure in the second pressure reactor was reduced to <2 kPa at a rate of 2 kPa per minute by a vacuum pump, and the unreacted methyl glycolate monomer was removed by vacuum to further promote the polymerization reaction. The reaction was maintained for about 70 minutes, and about 934 g of material was removed by negative pressure and vacuum cumulatively, and about 1769 g of polyglycolic acid was obtained. The Mww of polyglycolic acid was about 39,800 as measured by GPC. The theoretical yield of polyglycolic acid calculated based on the amount of methyl glycolate fed is 1933 g. Therefore, the yield of polyglycolic acid prepared in this embodiment is: actual yield after excluding PGA as a reaction raw material / theoretical yield = (1769-300) / 1933X100% = 75.99%.

[0078] Table 1

[0079]

[0080] Comparing the above embodiments with comparative examples 1-6, it can be seen that when the temperature is directly raised from 190°C to the polycondensation temperature in the later stage of the first step, the melting point of PGA is likely to increase, and the polymerization system may crystallize and solidify in advance, resulting in polymerization termination, low polymer molecular weight and yellowing. The heating rate of the second step should be controlled at 2°C / min-15°C / min. According to the characteristics of multiple melting of PGA, the fast heating of crystallized PGA is conducive to lowering its melting point. If the heating process is too slow, the melting point of PGA will increase in the later stage, and the polymerization system may crystallize and solidify in advance, resulting in polymerization termination, low polymer molecular weight and yellowing.

[0081] By comparing Example 1 with Comparative Example 7, it can be seen that the present invention is beneficial to the preparation of high-hue polymers through step-by-step control during the initial polymerization process; by comparing Example 3 with Comparative Example 8, it can be seen that the present invention is also beneficial to lowering the subsequent melting temperature through step-by-step control during the initial polymerization process, which is further beneficial to improving the polymer hue; from Example 3 and Comparative Example 9, it can be seen that by adding the antioxidant and stabilizer, it is beneficial to the preparation of high-hue polymers.

[0082] It can be seen from Comparative Example 10 that a key process control described in the present invention using a direct esterification method to prepare high molecular weight and high hue polyglycolic acid (PGA) is easier to obtain a PGA product with higher molecular weight and lighter color than the conventional method, and the comprehensive yield of the product is higher.

[0083] Obviously, the present invention is not limited to the above embodiments, and many variations are possible. All variations that can be directly derived or associated with the contents disclosed by a person skilled in the art should be considered as falling within the protection scope of the present invention.

Claims

1. A process for preparing high molecular weight and high hue polyglycolic acid by direct esterification, comprising the following steps: (1) adding glycolic acid and / or methyl glycolate as raw materials and a catalyst mixture into a reactor, heating the reactor to 165-175° C. and reacting the reactor until no distillate is evaporated from the reaction system; (2) reducing the pressure of the reaction system to a rough vacuum, and raising the temperature of the reaction system to 185-195° C., and reacting until the theoretical water yield of the system reaches more than 98%, thereby obtaining PGA oligomers; (3) lowering the temperature of the reaction system to 165-175° C. and reacting until the reactants solidify; (4) adding an antioxidant and a stabilizer to the reaction system, and heating the reaction system to 220-225° C. so as to be evenly mixed with the added antioxidant and stabilizer after melting, the heating rate is controlled at 2° C. / min-15° C. / min, and the vacuum degree of the reaction system is increased to high vacuum to obtain a PGA crystalline polymer by reaction; (5) The PGA crystalline polymer prepared in step (4) is cut into pellets, dried to remove water, and the temperature is controlled to 195-210° C. to perform solid phase polymerization to further increase the molecular weight of PGA.

2. The preparation process according to claim 1, characterized in that: In step (4), after the temperature is raised to 220-225° C., the vacuum degree of the reaction system is gradually increased to not less than 400 Pa, and after the reaction is carried out until the viscosity of the system reaches 0.3-0.5 cp, the vacuum degree of the reaction system is increased to below 70 Pa, and the polymerization is continued for 1-3 hours to obtain a PGA crystalline polymer.

3. The preparation process according to claim 2, characterized in that: In step (4), when the vacuum degree of the reaction system is gradually increased, the reaction vacuum degree is first increased to 1200-800 Pa and reacted for 0.5-1 hour, then increased to 600-500 Pa and reacted for 0.5-1 hour, and finally increased to 450-400 Pa and reacted for 1-2 hours.

4. The preparation process according to claim 1, characterized in that: In step (4), after the temperature is raised to 220-225°C, the vacuum degree of the reaction system is directly increased to below 70 Pa and the reaction is carried out for 2-4 hours to obtain a PGA crystalline polymer; and during the reaction, a mixture of glycolide and PGA oligomers evaporated from the reaction system is collected and reused as a raw material for PGA synthesis, thereby improving the PGA polymerization yield.

5. The preparation process according to claim 1, characterized in that: The antioxidant is one or more of pentaerythritol tetrakis (di-tert-butyl hydroxyhydrocinnamate) (Antioxidant 1010), α-tocopherol, and 5-tert-butyl-4-hydroxy-2-methylphenyl sulfide (Antioxidant 300).

6. The preparation process according to claim 1, characterized in that: The stabilizer is one or more of bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite (THP-24), diphenyl phosphite, triphenyl phosphite (TPP), and triethyl phosphate (TEP).

7. The preparation process according to claim 1, characterized in that: The dosage ratio of the antioxidant to the stabilizer is (2-4):

1.

8. The preparation process according to claim 1, characterized in that: In step (2), the pressure of the reaction system is reduced to 2000-3000 Pa; In step (4), the heating rate is 4°C / min-10°C / min, and the vacuum degree of the reaction system is increased to 10-100 Pa to obtain a PGA crystalline polymer.

9. The preparation process according to claim 1, characterized in that: The number average molecular weight of the PGA reactant obtained in step (3) is 5000-10000Da; the number average molecular weight of the PGA crystalline polymer obtained in step (4) is 30000-50000Da; in step (5), the number average molecular weight of the PGA obtained by solid phase polymerization is 60000-80000Da.

10. The preparation process according to claim 1, characterized in that: In step (4), based on the mass of the raw materials, the total amount of the antioxidant and the stabilizing agent used is 1 / 10,000 to 1 / 1,000, preferably 4 / 10,000 to 6 / 10,000.

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