Polylactic acid and preparation method thereof

Abstract

The invention provides polylactic acid and a preparation method thereof. The preparation method comprises the following steps: S1, mixing lactic acid with an esterification auxiliary agent, carrying out esterification reaction, and carrying out dehydration polycondensation to obtain medium-molecular-weight polylactic acid with the number-average molecular weight of 5000-30000; wherein the esterification auxiliary agent contains hydroxyl and / or amido, the amido is primary amido or secondary amido, and the sum of the hydroxyl and the amido in each esterification auxiliary agent molecule is not less than 2; s2, performing ring-opening polymerization reaction on the medium-molecular-weight polylactic acid and lactide to obtain the polylactic acid. The preparation method is low in cost, and the prepared polylactic acid is high in molecular weight.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer chemistry, and in particular to polylactic acid and a preparation method thereof. Background Art

[0002] Biodegradable materials refer to polymer materials that have excellent performance during use and can be rapidly degraded by enzymes or microorganisms after use. Among various biodegradable materials, polylactic acid has attracted extensive attention due to its unique good biocompatibility, degradability and excellent processing properties.

[0003] There are two main methods for synthesizing polylactic acid; one method (one-step method) is direct esterification and dehydration condensation of lactic acid. The process of this method is simple, but due to the presence of impurities in the system and the fact that the lactic acid condensation reaction is a reversible reaction, the resulting polylactic acid has a small relative molecular mass, poor strength and no practical value; the other method (two-step method) is that lactic acid is first oligomerized and depolymerized to obtain a cyclic dimer lactide, and then a ring-opening polymerization reaction is carried out with lactide as a monomer to obtain polylactic acid. This method can obtain a high relative molecular mass product of millions, and has become the main method for synthesizing polylactic acid.

[0004] Lactic acid is a chiral molecule, and the commonly used lactic acid is L-lactic acid. Therefore, the resulting lactide and polylactic acid also have chirality problems. At present, for common applications involving L-lactide, L-lactide is required to have high optical purity; the existing technology for obtaining such purity requires a step of separating meso-lactide (m-lactide). The first aspect of m-lactide is that it appears due to a racemization reaction during the oligomerization-depolymerization process of the synthesis of lactide.

[0005] Lactide with different optical purity can be used to prepare polylactic acid with different optical purity. The crystallinity and heat resistance of polylactic acid with different optical purity are different. The market and application of polylactic acid with different optical purity are different. Generally speaking, polylactic acid with high optical purity and high heat resistance is more popular in the market. Polylactic acid with high optical purity must be used in the fields of heat-resistant straws, heat-resistant lunch boxes, etc. In order to obtain lactide with high optical purity, m-lactide and L-lactide need to be separated from the crude lactide product. M-lactide and L-lactide are usually separated by distillation or crystallization. Since m-lactide and L-lactide have the same molecular weight and only one chirality is different, the separation of the two is extremely difficult and the energy consumption is very high.

[0006] In order to prepare high-quality PLA, it is very important to use high-purity lactide. Therefore, before the ring-opening polymerization reaction can occur to prepare high-quality PLA, lactide needs to be purified to a high degree to remove lactic acid, water, and oligomers. The purification process usually uses a combination of distillation and melt crystallization, which has high energy consumption and a certain loss in yield.

[0007] The above reasons have caused the production cost and price of lactide to remain high. Therefore, the advantages of two-step polylactic acid are high molecular weight and good mechanical properties of the product, but the disadvantage is high cost, especially the high optical purity polylactic acid is more expensive. On the other hand, the polylactic acid obtained by the one-step method (direct esterification and dehydration polycondensation of lactic acid) has the advantages of low cost and high optical purity, but the disadvantage is low molecular weight and poor product performance.

[0008] Therefore, it is necessary to develop a new polylactic acid synthesis process to fully utilize the advantages of the one-step method and the two-step method while avoiding their disadvantages.

[0009] CN1332995C discloses a method, which uses lactic acid as a raw material, firstly dehydrates and polymerizes to obtain lactic acid oligomers, then adds anhydrous lactide or methyl lactate and other esterified derivatives to react to obtain polylactic acid. However, the molecular weight of the lactic acid oligomer in this method is only 5000, and even if lactide is added at a ratio of 1:1 for ring-opening polymerization, the theoretical molecular weight of the product can only reach 10000, which is far lower than the molecular weight of polylactic acid that can actually be used as a material.

[0010] Therefore, it is necessary to overcome the shortcomings of the prior art and conduct further process optimization to obtain high molecular weight polylactic acid products with high number average molecular weight to meet the demand for high molecular weight polylactic acid under processing conditions such as industrial production injection molding and extrusion. Summary of the invention

[0011] In order to solve the above problems in the prior art, the object of the present invention is to provide a polylactic acid and a preparation method thereof. The preparation method of the present invention has low cost and the prepared polylactic acid has a high molecular weight.

[0012] In order to achieve the above object, according to a first aspect of the present invention, a method for preparing polylactic acid is provided, which comprises: S1, mixing lactic acid with an esterification auxiliary agent, causing an esterification reaction, and dehydration polycondensation to obtain a medium molecular weight polylactic acid with a number average molecular weight of 5000 to 30000; wherein the esterification auxiliary agent contains hydroxyl groups and / or amine groups, the amine groups are primary amine groups or secondary amine groups, and the sum of hydroxyl groups and amine groups in the molecule is not less than 2; S2, subjecting the medium molecular weight polylactic acid to a ring-opening polymerization reaction with lactide to obtain the polylactic acid.

[0013] The preparation method provided by the present invention is a preparation method combining a one-step polylactic acid synthesis process and a two-step polylactic acid synthesis process, wherein in the first step, a medium molecular weight polylactic acid is obtained, the molecular weight of which is in the range of 5000 to 30000, the degree of polymerization is not high, and the terminal contains a hydroxyl group, which can be used as a starting reactant for the ring-opening polymerization in S2. On this basis, the present invention performs ring-opening polymerization on the medium molecular weight polylactic acid and lactide to obtain a high molecular weight polylactic acid.

[0014] It should be noted that in the preparation method of the present invention, the molecular weight of the medium molecular weight polylactic acid is not the larger the better. When the medium molecular weight polylactic acid with a number average molecular weight of 35,000 is used as a raw material for ring-opening polymerization, in most cases, the molecular weight of the polylactic acid product is lower than that of the medium molecular weight polylactic acid with a molecular weight of less than 30,000 as a raw material for ring-opening polymerization. This may be because the preparation of the medium molecular weight polylactic acid with a molecular weight of 35,000 requires a longer reaction time at high temperature and high vacuum, so more side reactions occur, and the products of these side reactions affect the subsequent ring-opening polymerization. At the same time, the molecular weight of the medium molecular weight polylactic acid is not the lower the better, because the process of obtaining the medium molecular weight polylactic acid by lactic acid esterification dehydration polycondensation is a reversible equilibrium reaction: on the one hand, the hydroxyl and carboxyl groups react to generate water, which increases the molecular weight, and on the other hand, water causes ester hydrolysis, which reduces the molecular weight; when there is a large amount of water in the system, due to the equilibrium reaction, the molecular weight of the polylactic acid is small. Therefore, when the molecular weight of the medium molecular weight polylactic acid obtained by simply lactic acid esterification dehydration polycondensation is lower than 5000, in fact, there is still a considerable amount of water in the system. Water can also be used as an initiator for ring-opening polymerization, thus affecting the molecular weight of the final polylactic acid. Therefore, the preparation method of the present invention needs to be carried out under the conditions of controlling the appropriate medium molecular weight polylactic acid to obtain a high molecular weight polylactic acid product.

[0015] In addition, the present invention also designs for the first time a preparation method in which a small amount of the esterification aid is added in the lactic acid esterification and dehydration step. This method has at least three advantages. First, the total number of hydroxyl groups and amine groups in the system is greater than the carboxyl group, which can make the esterification reaction move in the direction of the positive reaction, and the reaction tends to be complete. Correspondingly, there is very little water in the system at the end of the reaction, thereby reducing the negative impact of water on the ring-opening polymerization. Second, the terminal of the medium molecular weight polylactic acid obtained by the present invention is more than one hydroxyl group, and each terminal can start the lactide ring-opening polymerization. Therefore, the molecular weight of the obtained product is higher than that of the polylactic acid obtained by simply using the medium molecular weight polylactic acid (containing only one terminal hydroxyl group) obtained by lactic acid esterification and dehydration polycondensation as the initiator for ring-opening polymerization. Third, by accurately controlling the ratio of polyols or polyamines to lactic acid, the molecular weight of the medium molecular weight polylactic acid can be accurately controlled, and then the molecular weight of the high molecular weight polylactic acid obtained can be relatively accurately controlled. The third point is particularly important for industrial stable production.

[0016] Another unexpected result is that the racemization reaction occurs very little during the lactic acid esterification process of the present invention, so a lactic acid raw material with high optical purity (>99%) can be used to obtain a medium molecular weight polylactic acid with high optical purity (optical purity>99%), and then a lactide with relatively low optical purity, such as lactide with an optical purity of about 96%, can be used in the ring-opening polymerization to finally obtain a high molecular weight polylactic acid product with an average optical purity of about 98%. Surprisingly, the crystalline melting temperature of the high molecular weight polylactic acid product with an average optical purity of 98% is around 172°C, which is higher than the crystalline melting temperature (about 165°C) of the polylactic acid with an optical purity of 98% obtained by directly polymerizing lactide with an optical purity of 98%, and is even close to the melting point (175°C) of the polylactic acid obtained by polymerizing lactide with an optical purity of 99%.

[0017] In the present invention, lactic acid includes L-lactic acid and / or D-lactic acid. Lactide includes one or a combination of two or more of L-lactide, D-lactide and m-lactide.

[0018] In some preferred embodiments of the present invention, in S1, the esterification auxiliary agent includes one or a combination of two or more of ethylene glycol, propylene glycol, butylene glycol, pentanediol, hexylene glycol, cyclohexane dimethanol, decanediol, ethylenediamine, propylenediamine, butylene diamine, pentanediamine, hexanediamine, isophoronediamine, ethanolamine, diethylene glycol, glycerol, trimethylolpropane, pentaerythritol, glucose, fructose, sorbitol, isosorbide, diethylenetriamine, triethylenetetramine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, 4,4-diaminodiphenylmethane, polytetramethylene glycol, polyester polyol, polycarbonate polyol, and polyether polyol.

[0019] In some preferred embodiments of the present invention, in S1, the molar ratio of the lactic acid to the esterification aid is 69:1 to 420:1.

[0020] In some preferred embodiments of the present invention, the number average molecular weight of the medium molecular weight polylactic acid is 10000 to 20000. The above range is preferred, which is more conducive to obtaining high molecular weight polylactic acid.

[0021] In some preferred embodiments of the present invention, the acid value of the medium molecular weight polylactic acid is ≤20 mol / t. It is preferred to control the acid value of the medium molecular weight polylactic acid within the above range, which is more conducive to promoting the ring-opening polymerization reaction and to obtaining high molecular weight polylactic acid subsequently.

[0022] Further preferably, the acid value of the medium molecular weight polylactic acid is ≤10 mol / t.

[0023] In some preferred embodiments of the present invention, the mass ratio of the medium molecular weight polylactic acid to the lactide is 1:1 to 9. When the ratio is higher than 1:1, it is difficult to obtain a high molecular weight product; when the ratio is lower than 1:9, although a high molecular weight polylactic acid can be obtained, the amount of medium molecular weight polylactic acid used is small, and the effect of reducing costs is not significant.

[0024] In some preferred embodiments of the present invention, in S2, the temperature of the ring-opening polymerization reaction is 170-240° C. Regarding the temperature, time, catalyst dosage and other conditions of the ring-opening polymerization, reference may be made to existing literature, and the present invention will not repeat them in detail.

[0025] In some preferred embodiments of the present invention, S1 further comprises: adding an esterification catalyst to the esterification reaction. Preferably, the esterification catalyst is selected from one or a combination of two or more of titanium, tin, antimony, zirconium and bismuth catalysts; preferably, the esterification catalyst is selected from one or a combination of two or more of isopropyl titanate, tetrabutyl titanate, dibutyltin dilaurate, stannous octoate and bismuth laurate. The amount of catalyst added is easy to grasp by those skilled in the art, and the present invention does not make further limitations on this.

[0026] In some preferred embodiments of the present invention, after the ring-opening polymerization reaction is completed, a deactivator is added to reduce the activity of the catalyst. The type and amount of the deactivator can be conventional in the art, and the present invention does not make further limitations on this.

[0027] In some preferred embodiments of the present invention, an antioxidant is also added during the preparation process, and the antioxidant can be added at any time during the esterification reaction, the ring-opening polymerization reaction, and after the ring-opening polymerization reaction. The type and amount of the antioxidant can be conventional in the art, and the present invention does not make further limitations on this.

[0028] In some preferred embodiments of the present invention, the S2 further comprises: after the ring-opening polymerization reaction, performing a vacuum stripping treatment to remove unreacted lactide.

[0029] In some preferred embodiments of the present invention, the optical purity of the lactic acid is not lower than the optical purity of the lactide.

[0030] According to another aspect of the present invention, there is provided a polylactic acid obtained according to the above preparation method.

[0031] In some preferred embodiments of the present invention, the polylactic acid satisfies one or a combination of two of the following conditions: The number average molecular weight is not less than 60,000; PDI is 1.6~2.1.

[0032] Compared with the prior art, the beneficial technical effects of the present invention include: 1. In the preparation method of the present invention, part of the polylactic acid is derived from the direct polymerization of lactic acid, and does not require the participation of lactide, thereby reducing costs; 2. The polylactic acid product obtained by the present invention has high molecular weight, high melting point, good heat resistance and optical purity. The polylactic acid has huge potential application prospects and economic value. DETAILED DESCRIPTION

[0033] In order to have a clearer understanding of the technical features, purposes and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be construed as limiting the applicable scope of the present invention.

[0034] In the following examples and comparative examples, unless otherwise specified, other raw materials used in the examples or comparative examples are commercially available.

[0035] Among them, some of the raw materials and reagents used are as follows: L-lactic acid with an optical purity of >99% and a chemical purity of >99% was purchased from Anhui Glatt Biochemical Co., Ltd.

[0036] The optical purity of D-lactic acid was >99%, and the chemical purity was >99%, which was purchased from Sinopharm Reagent.

[0037] The esterification catalyst was stannous octoate, purchased from Aladdin, reagent grade.

[0038] L-lactide, D-lactide and m-lactide were purchased from Aladdin and used after two recrystallizations. The moisture content was <100 ppm and the acid value was <5 mol / t before use.

[0039] Test methods include: The acid value test is conducted by potentiometric acid-base titration.

[0040] The molecular weights of the polymers are all number average molecular weights, which were determined by GPC using dichloromethane as the mobile phase and polystyrene as the calibration. Example 1

[0041] This embodiment provides a method for preparing polylactic acid, which comprises: S1, in a 5L glass reactor equipped with a distillation column, a condenser and a fraction collection bottle, 3500g of L-lactic acid (optical purity>99%), 3.5g of stannous octoate, 15.1g of 1,6-hexanediol were added, and nitrogen was purged for 3 minutes, then the pressure was reduced to 20kPa, and the temperature was gradually increased; liquid condensation was observed in the condenser from 95°C, and the temperature was gradually slowly increased to 190°C, and then the pressure was reduced from 20kPa to 10kPa within 5 minutes, and maintained at 10kPa for about 30 minutes, and then the pressure was further reduced to 0.2kPa, and the reaction was continued until the temperature at the top of the distillation column dropped below 30°C and no fraction was produced, and a total of 685g of fractions were collected; medium molecular weight polylactic acid 1 was obtained in the glass reactor, and the acid value was 11.6mol / t.

[0042] S2, 0.4 kg of medium molecular weight polylactic acid 1 and 1.2 kg of L-lactide (optical purity>99%) are placed in a 2L glass bottle, and the glass bottle is connected to a distillation head, a condenser and a collecting bottle; the condenser is temperature-controlled using 70°C circulating heat transfer oil, the tail end of the collecting bottle is connected to a vacuum pump, and nitrogen is purged for 3 minutes; then, the temperature is first raised to 190°C, and the reaction is stirred for 4 hours, and then the air pressure is quickly reduced to 0.5 kPa, and the vacuum monomer removal treatment step is started, which takes about 5 minutes, until no obvious monomer is removed, and then the heating, stirring and vacuum are stopped; nitrogen is added to normal pressure, the glass bottle is removed, the glass bottle is broken, and the product is scraped into cold water with a nickel spoon and drawn to obtain polylactic acid 1.

[0043] According to GPC, the molecular weight of medium molecular weight polylactic acid 1 is 21828, and the PDI is 2.12. The molecular weight of polylactic acid 1 is 79820, and the PDI is 1.94. The melting point of polylactic acid is 175°C according to DSC. Example 2

[0044] This embodiment provides a method for preparing polylactic acid, which comprises: S1, in a 5L glass reactor equipped with a distillation column, a condenser and a fraction collection bottle, add 3500g of D-lactic acid (optical purity>97%), 3.5g of stannous octoate, 12.66g of 1,4-butanediol, and purge with nitrogen for 3 minutes, then reduce the pressure to 20kPa and gradually increase the temperature; from 95°C, liquid condensation is observed in the condenser, and the temperature is gradually and slowly increased to 190°C, and then the pressure is reduced from 20kPa to 10kPa within 5 minutes, and maintained at 10kPa for about 30 minutes, and then the pressure is further reduced to 0.2kPa, and the reaction is continued until the temperature at the top of the distillation column drops below 30°C and no fraction is produced, and a total of 688g of fractions are collected; medium molecular weight polylactic acid 2 is obtained in the glass reactor, and the acid value is 8.6mol / t.

[0045] S2, 0.25kg of medium molecular weight polylactic acid 2 and 1.25kg of D-lactide (optical purity>97%) are placed in a 2L glass bottle, and the glass bottle is connected to a distillation head, a condenser and a collecting bottle; the condenser is temperature-controlled using 70℃ circulating heat transfer oil, the tail end of the collecting bottle is connected to a vacuum pump, and nitrogen is purged for 3 minutes; then, the temperature is first raised to 190℃, and the reaction is stirred for 4 hours, and then the air pressure is quickly reduced to 0.4kPa, and the vacuum monomer removal treatment step is started, which takes about 5 minutes, until no obvious monomer is removed, and then the heating, stirring and vacuum are stopped; nitrogen is added to normal pressure, the glass bottle is removed, the glass bottle is broken, and the product is scraped into cold water with a nickel spoon and drawn to obtain polylactic acid 2.

[0046] According to GPC, the molecular weight of medium molecular weight polylactic acid 2 is 18129 and the PDI is 2.09. The molecular weight of polylactic acid 2 is 102600 and the PDI is 1.86. The melting point of polylactic acid is 162°C according to DSC. Example 3

[0047] This embodiment provides a method for preparing polylactic acid, which comprises: S1, in a 5L glass reactor equipped with a distillation column, a condenser and a fraction collection bottle, 3500g of L-lactic acid (optical purity>99%), 3.5g of stannous octoate, and 7.83g of glycerol were added, and nitrogen was purged for 3 minutes, and then the pressure was reduced to 20kPa and the temperature was gradually increased; liquid condensation was observed in the condenser from 95°C, and the temperature was gradually and slowly increased to 190°C, and then the pressure was reduced from 20kPa to 10kPa within 5 minutes, and maintained at 10kPa for about 30 minutes, and then the pressure was further reduced to below 0.2kPa, and the temperature was increased to 200°C, and the reaction was continued until the temperature at the top of the distillation column was reduced to below 30°C and no fraction was produced, and a total of 689g of fractions were collected; medium molecular weight polylactic acid 3 was obtained in the glass reactor, and the acid value was 8.4mol / t.

[0048] S2, 0.7 kg of medium molecular weight polylactic acid 3, 0.828 kg of L-lactide, and 72 g of m-lactide (optical purity of the mixture is about 96%) are placed in a 2L glass bottle, the glass bottle is connected to a distillation head, a condenser and a collecting bottle, the condenser is temperature-controlled using 70°C circulating heat transfer oil, the tail end of the collecting bottle is connected to a vacuum pump, and nitrogen is purged for 3 minutes; then, the temperature is first raised to 190°C, the reaction is stirred for 4 hours, and then the air pressure is quickly reduced to 0.5 kPa, and the vacuum monomer removal treatment step is started, which takes about 5 minutes, until no obvious monomer is removed, and then the heating, stirring, and vacuum are stopped; nitrogen is added to normal pressure, the glass bottle is removed, the glass bottle is broken, the product is scraped into cold water with a nickel spoon and drawn to obtain polylactic acid 3.

[0049] The molecular weight of the medium molecular weight polylactic acid 3 was 28864 and the PDI was 2.2 according to GPC. The molecular weight of polylactic acid 3 was 60771 and the PDI was 2.06. The melting point of polylactic acid was 172°C according to DSC. Example 4

[0050] This embodiment provides a method for preparing polylactic acid, which comprises: S1, in a 5L glass reactor equipped with a distillation column, a condenser and a fraction collection bottle, 3500g of L-lactic acid (optical purity>99%), 3.5g of stannous octoate, 32.86g of hexamethylenediamine were added, and nitrogen was purged for 3 minutes, and then the pressure was reduced to 20kPa, and the temperature was gradually increased; liquid condensation was observed in the condenser from 95°C, and the temperature was gradually and slowly increased to 190°C, and then the pressure was reduced from 20kPa to 10kPa within 5 minutes, and maintained at 10kPa for about 30 minutes, and then the pressure was further reduced to 0.2kPa, and the reaction was continued until the temperature at the top of the distillation column dropped below 30°C and no fraction was produced, and a total of 692g of fractions were collected; medium molecular weight polylactic acid 4 was obtained in the glass reactor, and the acid value was 4.3mol / t.

[0051] S2, 0.15kg of medium molecular weight polylactic acid 4 and 1.35kg of L-lactide (optical purity>99%) are placed in a 2L glass bottle, the glass bottle is connected to a distillation head, a condenser and a collecting bottle, the condenser is temperature-controlled using 70°C circulating heat transfer oil, the tail end of the collecting bottle is connected to a vacuum pump, and nitrogen is purged for 3 minutes; then, the temperature is first raised to 190°C, the reaction is stirred for 4 hours, and then the air pressure is quickly reduced to 0.4kPa, and the vacuum monomer removal treatment step is started, which takes about 5 minutes, until no obvious monomer is removed, and then the heating, stirring and vacuum are stopped; nitrogen is added to normal pressure, the glass bottle is removed, the glass bottle is broken, the product is scraped into cold water with a nickel spoon and drawn to obtain polylactic acid 4.

[0052] According to GPC, the molecular weight of medium molecular weight polylactic acid 4 is 9364, and the PDI is 1.93. The molecular weight of polylactic acid 4 is 88958, and the PDI is 1.77. The melting point of polylactic acid is 175°C according to DSC.

[0053] Comparative Example 1 This comparative example provides a method for preparing polylactic acid, which comprises: S1, in a 5L glass reactor equipped with a distillation column, a condenser and a fraction collection bottle, 3500g of L-lactic acid (optical purity>99%), 3.5g of stannous octoate, and 7.38g of glycerol were added, and nitrogen was purged for 3 minutes, and then the pressure was reduced to 20kPa and the temperature was gradually increased; liquid condensation was observed in the condenser from 95°C, and the temperature was gradually and slowly increased to 190°C, and then the pressure was reduced from 20kPa to 10kPa within 5 minutes, and maintained at 10kPa for about 30 minutes, and then the pressure was further reduced to below 0.2kPa, and the temperature was increased to 200°C, and the reaction was continued until the temperature at the top of the distillation column was reduced to below 30°C and no fraction was produced, and a total of 690g of fractions were collected; medium molecular weight polylactic acid D1 was obtained in the glass reactor, and the acid value was 10.8 mol / t.

[0054] S2, 0.7 kg of medium molecular weight polylactic acid D1, 0.828 kg of L-lactide and 72 g of m-lactide (optical purity of the mixture is about 96%) are placed in a 2L glass bottle, the glass bottle is connected to a distillation head, a condenser and a collecting bottle, the condenser is temperature-controlled using 70°C circulating heat transfer oil, the tail end of the collecting bottle is connected to a vacuum pump, and nitrogen is purged for 3 minutes; then, the temperature is first raised to 190°C, the reaction is stirred for 4 hours, and then the air pressure is quickly reduced to 0.5 kPa, and the vacuum monomer removal treatment step is started, which takes about 5 minutes, until no obvious monomer is removed, and then the heating, stirring and vacuum are stopped; nitrogen is added to normal pressure, the glass bottle is removed, the glass bottle is broken, the product is scraped into cold water with a nickel spoon and drawn to obtain polylactic acid D1.

[0055] According to GPC, the molecular weight of medium molecular weight polylactic acid D1 is 30573, and the PDI is 2.26. The molecular weight of polylactic acid D1 is 61353, and the PDI is 2.16. The melting point of polylactic acid is 172°C as determined by DSC. Compared with Example 3, although Comparative Example 1 attempts to reduce the amount of glycerol to obtain medium molecular weight polylactic acid with a molecular weight of >30000, the molecular weight of the final product is not much different, and the PDI becomes wider.

[0056] It can be seen that increasing the molecular weight of medium molecular weight polylactic acid to above 30,000 is not very significant; medium molecular weight polylactic acid D1 requires longer esterification reaction time, higher energy consumption and lower production efficiency than medium molecular weight polylactic acid 3.

[0057] Comparative Example 2 This comparative example provides a method for preparing polylactic acid, which comprises: S1, in a 5L glass reactor equipped with a distillation column, a condenser and a fraction collection bottle, 3500g of L-lactic acid (optical purity>99%) and 3.5g of stannous octoate were added, and nitrogen was purged for 3 minutes, and then the pressure was reduced to 20kPa and the temperature was gradually increased; liquid condensation was observed in the condenser from 95°C, and the temperature was gradually and slowly increased to 190°C, and then the pressure was reduced from 20kPa to 10kPa within 5 minutes, and maintained at 10kPa for about 30 minutes, and then the pressure was further reduced to 0.2kPa, and the reaction was continued until the temperature at the top of the distillation column dropped below 30°C and no fraction was produced, and a total of 678g of fractions were collected; medium molecular weight polylactic acid D2 was obtained in the glass reactor, and the acid value was 204.3mol / t.

[0058] S2, 0.15kg of medium molecular weight polylactic acid D2 and 1.35kg of L-lactide (optical purity>99%) are placed in a 2L glass bottle, the glass bottle is connected to a distillation head, a condenser and a collecting bottle, the condenser is temperature-controlled using 70°C circulating heat transfer oil, the tail end of the collecting bottle is connected to a vacuum pump, and nitrogen is purged for 3 minutes; then, the temperature is first raised to 190°C, the reaction is stirred for 4 hours, and then the air pressure is quickly reduced to 0.4kPa, and the vacuum monomer removal treatment step is started, which takes about 5 minutes, until no obvious monomer is removed, and then the heating, stirring and vacuum are stopped; nitrogen is added to normal pressure, the glass bottle is removed, the glass bottle is broken, the product is scraped into cold water with a nickel spoon and drawn to obtain polylactic acid D2.

[0059] According to GPC, the molecular weight of medium molecular weight polylactic acid D2 is 4872, and the PDI is 1.94. The molecular weight of polylactic acid D2 is 39337, and the PDI is 2.23. DSC is not measured.

[0060] Comparative Example 2 shows that it is difficult to reduce the acid value of medium molecular weight polylactic acid without adding an esterification aid. Therefore, even though a high ratio of medium molecular weight polylactic acid: lactide = 1:9 is used, it is still difficult to obtain a high molecular weight polylactic acid final product.

[0061] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A method for preparing polylactic acid, characterized in that: include: S1, mixing lactic acid with an esterification auxiliary agent, causing an esterification reaction, and dehydration polycondensation to obtain a medium molecular weight polylactic acid with a number average molecular weight of 5000 to 30000; wherein the esterification auxiliary agent contains hydroxyl groups and / or amine groups, the amine groups are primary amine groups and / or secondary amine groups, and the sum of hydroxyl groups and amine groups in each esterification auxiliary agent molecule is not less than 2; S2, subjecting the medium molecular weight polylactic acid to a ring-opening polymerization reaction with lactide to obtain the polylactic acid.

2. The method for preparing polylactic acid according to claim 1, characterized in that: In S1, the esterification auxiliary agent includes one or a combination of two or more of ethylene glycol, propylene glycol, butylene glycol, pentanediol, hexylene glycol, cyclohexane dimethanol, decanediol, ethylenediamine, propylenediamine, butylenediamine, pentanediamine, hexanediamine, isophoronediamine, ethanolamine, diethylene glycol, glycerol, trimethylolpropane, pentaerythritol, glucose, fructose, sorbitol, isosorbide, diethylenetriamine, triethylenetetramine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, 4,4-diaminodiphenylmethane, polytetramethylene glycol, polyester polyol, polycarbonate polyol, and polyether polyol.

3. The method for preparing polylactic acid according to claim 1, characterized in that: In the S1, the molar ratio of the lactic acid to the esterification aid is 69:1 to 420:

1.

4. The method for preparing polylactic acid according to claim 1, characterized in that: The number average molecular weight of the medium molecular weight polylactic acid is 10,000-20,000.

5. The method for preparing polylactic acid according to claim 1, characterized in that: The acid value of the medium molecular weight polylactic acid is ≤20 mol / t.

6. The method for preparing polylactic acid according to claim 1, characterized in that: The mass ratio of the medium molecular weight polylactic acid to the lactide is 1:1-9.

7. The method for preparing polylactic acid according to claim 1, characterized in that: In S2, the temperature of the ring-opening polymerization reaction is 170-240°C.

8. The method for preparing polylactic acid according to claim 1, characterized in that: The step S2 further comprises: after the ring-opening polymerization reaction, performing vacuum stripping treatment to remove unreacted lactide.

9. The method for preparing polylactic acid according to claim 1, characterized in that: The optical purity of the lactic acid is not lower than the optical purity of the lactide.

10. A polylactic acid, characterized in that: The polylactic acid is prepared by the preparation method according to any one of claims 1 to 9.

11. The polylactic acid according to claim 10, characterized in that The polylactic acid satisfies one or a combination of two of the following conditions: The number average molecular weight is not less than 60,000; PDI is 1.6~2.1.