Method for preparing polylactic acid

Abstract

The invention relates to the field of polylactic acid, and discloses a polylactic acid preparation method, which comprises: in the presence of a catalyst, mixing lactic acid and succinic anhydride, and carrying out polycondensation, the catalyst is selected from p-toluenesulfonic acid and / or tin salt. According to the method, polylactic acid with higher yield and higher molecular weight can be obtained. The catalyst disclosed by the invention is high in cost performance, high in activity and low in cost, so that the catalyst has important significance in social and economic benefits and three-dimensional regularity and final performance of the polymer.

Description

Technical Field

[0001] The invention relates to the field of polylactic acid, and in particular to a method for preparing polylactic acid. Background Art

[0002] In recent years, plastics or commodities produced from fossil fuels have been used in many different fields, including packaging, encapsulation, electrical equipment, construction, medical applications, etc. However, the non-degradable nature of these materials has inevitably brought severe environmental problems to the world. In this context, it is very necessary to develop a new, environmentally friendly biodegradable material to reduce the impact on the environment. Due to the low production cost of polylactic acid, good biocompatibility, high rigidity and strength of the material itself, the study of the performance of degradable polylactic acid has always been a hot topic for researchers. It is precisely because of the good performance and biodegradability of polylactic acid that it can replace fossil fuel-derived polymers in different application fields. Therefore, it is very necessary to study the synthesis mechanism of polylactic acid and develop the most favorable conditions for the synthesis of polylactic acid.

[0003] The properties of polylactic acid-derived polymers make this material highly attractive for different applications, among which the biodegradability of the polymer plays a crucial role. Therefore, in order to obtain biodegradable materials with better properties (mechanical properties, thermal properties or optical properties), the synthesis of improved polylactic acid needs to be studied in depth. Polylactic acid can be synthesized by different polymerization methods, mainly including polycondensation of lactic acid monomers and ring-opening polymerization of lactic acid and azeotropic dehydration condensation reaction of lactic acid. From the above three methods, the two most commonly used in industry are direct polycondensation of lactic acid and ring-opening polymerization of lactic acid. However, in all the synthetic methods used, different factors can determine the final properties of the obtained polylactic acid (catalyst, temperature, solvent, etc.), so in order to control the stereochemical properties of polylactic acid in the polymerization reaction and its final properties, it is very necessary to explore the optimal synthesis conditions of polylactic acid.

[0004] In addition, one of the parameters that affects the growth mechanism of polymer chains is the catalyst. The choice of catalyst has an important influence on the production of polylactic acid in terms of the stereoregularity and final properties of the polymer. The use of different catalysts will yield polylactic acid with different properties. Due to the potential application of polylactic acid in the field of biodegradation, it is a necessary topic to develop different organic catalysts with the same reaction activity as organic metal complexes. This type of catalyst is mainly divided into two categories: metal-based catalysts and organic catalysts. These organic catalysts are relatively cheap and highly active. In order to improve the performance of polylactic acid and expand its scope of application, it is very meaningful to study and develop catalysts for the synthesis of polylactic acid and explore the optimal conditions for the synthesis of polylactic acid under the action of different catalysts. Summary of the invention

[0005] The purpose of the present invention is to overcome the problems of low yield and low molecular weight in the prior art and to provide a method for preparing polylactic acid.

[0006] In order to achieve the above object, the present invention provides a method for preparing polylactic acid, the method comprising: mixing lactic acid and succinic anhydride for polycondensation in the presence of a catalyst;

[0007] The catalyst is selected from p-toluenesulfonic acid and / or tin salts.

[0008] Through the above technical scheme, the method of the present invention can obtain polylactic acid with higher yield and higher molecular weight. The catalyst studied in the present invention has high cost performance, high activity and low cost, so it is of great significance for social and economic benefits and the stereoregularity and final performance of the polymer. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is the result diagram of Example 4;

[0010] Figure 2 This is a graph showing the results of Example 5. DETAILED DESCRIPTION

[0011] 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. 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.

[0012] The invention provides a method for preparing polylactic acid, which comprises: mixing lactic acid and succinic anhydride for polycondensation in the presence of a catalyst; the catalyst is selected from p-toluenesulfonic acid and / or tin salt.

[0013] In the present invention, in order to obtain polylactic acid with higher yield and higher molecular weight, in the mixed system, the content of succinic anhydride is 0.001-0.4wt%, which can be 0.001wt%, 0.003wt%, 0.005wt%, 0.01wt%, 0.03wt%, 0.05wt%, 0.07wt%, 0.1wt%, 0.12wt%, 0.14wt%, 0.16wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.35wt%, 0.4wt% or the range formed by any two of the above values ​​and the value within the range.

[0014] In the present invention, in order to obtain polylactic acid with higher yield and higher molecular weight, in the mixed system, the content of the tin salt is 0.01-0.7wt%, which can be 0.01wt%, 0.04wt%, 0.07wt%, 0.1wt%, 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.35wt%, 0.4wt%, 0.45wt%, 0.5wt%, 0.55wt%, 0.6wt%, 0.65wt%, 0.7wt% or the range formed by any two of the above values ​​and the value within the range.

[0015] The inventors of the present invention have found that when tin salt catalyzes polycondensation at a relatively high temperature, the catalytic reaction proceeds in the direction of synthesizing a high polymer while also promoting the thermal degradation of the polymer. When the rate of thermal degradation is greater than the rate of polymerization, the molecular weight of the product is reduced. At this time, too much or too little catalyst dosage is not conducive to increasing the molecular weight of the product. The inventors of the present invention have found that tin salt can reduce the activation energy of the lactic acid polymerization reaction and promote the system to proceed in the direction of the polycondensation reaction. When the tin salt is insufficient, the reaction is not sufficient within a certain period of time and a high molecular weight product cannot be obtained; when the amount of tin salt is too much, short chain molecules are easily formed and a high molecular weight product cannot be obtained.

[0016] In the present invention, in order to obtain polylactic acid with higher yield and higher molecular weight, in the mixed system, the content of lactic acid is 70-95wt%, which can be 70, 75, 80, 85, 90, 95wt% or the range formed by any two of the above values ​​and the value within the range.

[0017] In the present invention, in order to obtain polylactic acid with higher yield and higher molecular weight, in the mixed system, the content of the catalyst is 0.3-1wt%, which can be 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt% or the range formed by any two of the above values ​​and the value within the range.

[0018] In the present invention, in order to obtain polylactic acid with higher yield and higher molecular weight, the catalyst is p-toluenesulfonic acid and tin salt, and the molar ratio of p-toluenesulfonic acid and tin salt is 0.1-2.5:1, which can be 0.1:1, 0.3:1, 0.5:1, 0.8:1, 1:1, 1.5:1, 2:1, 2.5:1 or the range formed by any two of the above values ​​and the value within the range.

[0019] In the present invention, in order to obtain polylactic acid with higher yield and higher molecular weight, water is also included in the mixed system. In the mixed system, when the content range of each reagent mentioned above is met, the balance is supplemented with water.

[0020] In the present invention, in order to obtain polylactic acid with higher yield and higher molecular weight, the tin salt is selected from at least one of SnCl2, stannous octoate and dibutyltin dilaurate.

[0021] In the present invention, in order to obtain polylactic acid with higher yield and higher molecular weight, the polycondensation conditions include: temperature of 170-190°C, which can be 170°C, 175°C, 180°C, 185°C, 190°C or any two of the above values ​​and values ​​within the range; time of 8-12h, which can be 8h, 9h, 10h, 11h, 12h or any two of the above values ​​and values ​​within the range; pressure of 0.05-0.1MPa, which can be 0.05MPa, 0.06MPa, 0.07MPa, 0.08MPa, 0.09MPa, 0.1MPa or any two of the above values ​​and values ​​within the range.

[0022] In the present invention, in order to obtain purer polylactic acid, the method further comprises: purifying the product obtained by the polycondensation reaction. The purification method comprises: first dissolving with a ketone solvent and then precipitating with an alcohol solvent and / or water; preferably, the ketone solvent is a ketone solvent with a carbon number of less than 4, more preferably acetone; preferably, the alcohol solvent is an alcohol solvent with a carbon number of less than 4, preferably ethanol.

[0023] According to a particularly preferred embodiment of the present invention, lactic acid and succinic anhydride are mixed for polycondensation in the presence of p-toluenesulfonic acid and SnCl2; wherein the molar ratio of p-toluenesulfonic acid to SnCl2 is 0.5-1.5:1, and in the mixed system, the contents of p-toluenesulfonic acid and SnCl2 are 0.4-0.7wt%, and the content of succinic anhydride is 0.05-0.2wt%, and the conditions of the polycondensation reaction include: temperature of 175-185°C, time of 9-11h, and pressure of 0.07-0.09MPa.

[0024] The present invention will be described in detail below through examples. In the following examples, the yield test and calculation method is: (mass of polylactic acid produced / total mass of raw materials used in the reaction process)×100%;

[0025] Weight average molecular weight (Mv) test method: measured by GPC method;

[0026] Intrinsic viscosity test method (η): measured using an Ubbelohde viscometer;

[0027] Viscosity average molecular weight (Mη) test method: Mη=ηγ / ρ The viscosity average molecular weight (Mη) of the polymer is calculated, where η is the intrinsic viscosity, γ is the flow time, and ρ is the density of polylactic acid.

[0028] Example 1

[0029] Add 20 ml of 90 wt% lactic acid solution to a 50 ml three-necked flask, then add different amounts of succinic anhydride and SnCl2·2H2O to the reaction vessel and continue to reduce pressure. Stir magnetically in an oil bath at 180°C for about 10 hours. After cooling, dissolve with acetone, then precipitate with ethanol, and finally reprecipitate with distilled water to obtain a white powdery solid polymer. The above experiment was carried out at 180°C, 0.085 MPa and 10 h, and 0.12 g of SnCl2·2H2O was added. The output and weight average molecular weight under different amounts of succinic anhydride are shown in Table 1.

[0030] Table 1

[0031] Experimental sequence Succinic anhydride content (wt) % Yield (%) <![CDATA[Weight-average molecular weight Mv (×10 -3 )]]> 1 0 47 0.6 2 0.005 58 2.0 3 0.01 60.5 2.5 4 0.035 63 2.5 5 0.07 78 4.0 6 0.14 76.2 2.0 7 0.28 61.9 1.8 8 0.56 50 1.5

[0032] Example 2

[0033] Add 20 ml of 90 wt% lactic acid solution to a 50 ml three-necked flask, then add different amounts of succinic anhydride and SnCl2·2H2O to the reaction vessel and continue to reduce pressure. Stir magnetically in an oil bath at 180°C for about 10 hours. After cooling, dissolve with acetone, then precipitate with ethanol, and finally reprecipitate with distilled water to obtain a white powdery solid polymer. The above experiment was carried out at 180°C, 0.085 MPa and 10 h, and 0.07 g of succinic anhydride was added. The output and weight average molecular weight under different amounts of SnCl2·2H2O are shown in Table 2.

[0034] Table 2

[0035] sequence <![CDATA[Content of SnCl2·2H2O (wt%)]]> Yield (%) <![CDATA[Weight-average molecular weight Mv (×10 -3 )]]> 1 0 40 1.0 2 0.07 68 1.5 3 0.14 72 1.8 4 0.28 74 2.5 5 0.56 78 4.0 6 1.12 55 2.8

[0036] Example 3

[0037] The method of Example 2 was followed, with a catalyst content of 0.56 wt %, except that the type of catalyst was changed. The results are shown in Table 3.

[0038] Table 3

[0039]

[0040]

[0041] Example 4

[0042] According to the method of Example 3, SnCl2 and p-toluenesulfonic acid were used as catalysts, except that p-toluenesulfonic acid and SnCl2 were used in different molar ratios. The results under different p-toluenesulfonic acid and SnCl2 molar ratios are as follows: Figure 1 shown.

[0043] Example 5

[0044] According to the method of Example 4, the molar ratio of p-toluenesulfonic acid to SnCl2 is 1:1, except that different catalyst contents are used. The results under different catalyst contents are as follows: Figure 2 shown.

[0045] It can be seen from the above results that the method of the present invention can obtain polylactic acid with higher yield and higher molecular weight.

[0046] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for preparing polylactic acid, characterized in that: The method comprises: mixing lactic acid and succinic anhydride for polycondensation in the presence of a catalyst; The catalyst is selected from p-toluenesulfonic acid and / or tin salts.

2. The method according to claim 1, wherein: In the mixed system, the content of succinic anhydride is 0.001-0.4 wt %.

3. The method according to claim 1 or 2, wherein: In the mixed system, the content of the tin salt is 0.01-0.7 wt %.

4. The method according to claim 1 or 2, wherein: In the mixed system, the content of lactic acid is 70-95wt%.

5. The method according to claim 1 or 2, wherein: In the mixed system, the content of the catalyst is 0.3-1wt%; And / or, the catalyst is p-toluenesulfonic acid and tin salt, and the molar ratio of p-toluenesulfonic acid to tin salt is 0.1-2.5:

1.

6. The method according to claim 1 or 2, wherein: The mixing system also includes water.

7. The method according to claim 1 or 2, wherein: The tin salt is selected from at least one of SnCl2, stannous octoate and dibutyltin dilaurate.

8. The method according to claim 1 or 2, wherein: The conditions of the polycondensation reaction include: temperature of 170-190° C., time of 8-12 h, and pressure of 0.05-0.1 MPa.

9. The method according to claim 1 or 2, wherein: The method further comprises: purifying the product obtained from the polycondensation reaction; Preferably, the purification method comprises: first dissolving with a ketone solvent and then precipitating with an alcohol solvent and / or water; Preferably, the ketone solvent is a ketone solvent having less than 4 carbon atoms, more preferably acetone; Preferably, the alcohol solvent is an alcohol solvent having less than 4 carbon atoms, preferably ethanol.

10. The method according to claim 1 or 2, wherein: In the presence of p-toluenesulfonic acid and SnCl2, lactic acid and succinic anhydride are mixed for condensation polymerization; wherein the molar ratio of p-toluenesulfonic acid to SnCl2 is 0.5-1.5:1, and in the mixed system, the contents of p-toluenesulfonic acid and SnCl2 are 0.4-0.7wt%, and the content of succinic anhydride is 0.05-0.2wt%, and the conditions of the condensation polymerization reaction include: temperature of 175-185°C, time of 9-11h, and pressure of 0.07-0.09MPa.

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