A low tin content polylactic acid and a method of making the same

Abstract

The application discloses a low-tin-content polylactic acid and a preparation method thereof, and belongs to the technical field of polymer synthesis. A solvent I is added to crude polylactic acid for dissolution, a substance with active hydrogen is added for fully stirring, then a flocculating agent is added for fully stirring, filtration is performed, a solvent II is added to the filtrate for fully stirring, a product is precipitated, the precipitated product is filtered and dried, and the low-tin-content polylactic acid is obtained. The method has a wide application range, can ensure a low tin residue in a one-time refining process within a wide range of weight-average molecular weight, does not need multiple repeated refining processes, has a good removal effect on multiple types of tin catalysts, produces less solid waste and waste liquid, and can stabilize the molecular weight of the polylactic acid and guarantee the rate of a catalytic reaction in an early stage.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polymer synthesis, and particularly relates to a low-tin-content polylactic acid and a preparation method thereof. BACKGROUND

[0002] Polylactic acid is an aliphatic polyester prepared from renewable resources, has good degradability and biocompatibility, and belongs to a chemically synthesized degradable material. Compared with microbial synthesis and natural polymers, polylactic acid has obvious advantages in yield and controllability. Polylactic acid has excellent mechanical properties, and the tensile strength of partially crystalline polylactic acid is 50-70 MPa, and the elastic modulus is 3-4 kMPa, so polylactic acid can replace general-purpose plastics in specific application scenarios. Its products are widely used in the fields of textiles, drug carriers, packaging materials and the like.

[0003] There are two main ways for polylactic acid synthesis: one is to directly condense lactic acid to generate polylactic acid; and the other is to prepare polylactic acid by ring-opening of lactide. The direct method has low product molecular weight, wide molecular weight distribution, and the reaction process needs to control the vacuum degree and remove low molecular byproducts, so the reaction conditions are harsh. Currently, products with commercial value are generally prepared by ring-opening of lactide, and single-carboxylic acid iron complex, alkali metal alcoholate, tin and the like can be used for catalyzing the reaction, among which the tin catalyst has high catalytic efficiency and is not easy to undergo racemization, so it is widely used. However, when the tin catalyst is used for catalyzing the reaction, it is difficult to remove the tin in the product. If the content is too high, it will limit the application of polylactic acid in some scenarios, such as Jamshidi K, who confirmed that high tin residues reduce the pyrolysis temperature of polylactic acid.

[0004] Currently, there are few methods for reducing the tin content in polylactic acid. One method is to repeatedly wash the polylactic acid dichloromethane solution with hydrochloric acid to remove the tin residues in the product. With the increase of the number of washing times, the tin residues are continuously reduced, but one-time hydrochloric acid washing cannot significantly reduce the tin residues, and multiple repeated operations are required. After hydrochloric acid washing is completed, distilled water needs to be used for multiple washing until neutralization. The operation is complex, a large amount of waste liquid is generated, the cost is high, and polylactic acid will decompose under acidic conditions, resulting in a decrease in molecular weight. Another method is to reduce the amount of tin-containing catalyst used in the synthesis of polylactic acid to reduce the tin content in the final product. Although reducing the amount of catalyst can reduce the tin residues in the product to some extent, the reduction effect is very limited, and polylactic acid with low tin residues cannot be obtained. Moreover, if the amount of catalyst is too small, the reaction time is prolonged, and due to the competition between chain growth and chain degradation, the molecular weight of polylactic acid product is low. SUMMARY

[0005] In view of the problem that the molecular weight of polylactic acid is reduced when the high tin content in polylactic acid is solved in the prior art, the application provides a low-tin-content polylactic acid and a preparation method thereof, which can significantly reduce the tin content in polylactic acid, and the molecular weight of polylactic acid is stable during the preparation process.

[0006] The application is realized by the following technical solutions.

[0007] A preparation method of low-tin-content polylactic acid comprises the following steps:

[0008] (1) Dissolving polylactic acid crude product in solvent I;

[0009] (2) Adding a substance with active hydrogen to the solution in step (1) and fully stirring;

[0010] (3) Adding a flocculating agent to the solution in step (2) and fully stirring;

[0011] (4) Filtering the solution in step (3);

[0012] (5) Adding solvent II to the filtrate after filtering in step (4), fully stirring, and precipitating a product;

[0013] (6) Filtering and drying the precipitated product to obtain low-tin-content polylactic acid.

[0014] Further, the polylactic acid crude product in step (1) is obtained by ring-opening polymerization of lactide with the addition of an alcohol initiator and a tin catalyst.

[0015] Further, the solvent I in step (1) is one or more of dichloromethane, trichloromethane, DMF and acetone, and the concentration of polylactic acid crude product after being added into the solvent I is 25-500 g / L.

[0016] Further, the substance with active hydrogen in step (2) is one of alcohol, phenol, amine and mercaptan; the flocculating agent in step (3) is an organic flocculating agent or an inorganic flocculating agent; and the solvent II in step (5) is one or more of n-heptane, methanol, ethanol, diethyl ether and ethylene glycol.

[0017] Further, the substance with active hydrogen in step (2) is one of methanol, ethanol, dodecanol, 1,3-propanediol, 1,5-pentanediol, trimethylolpropane, phenol, cresol, diethylamine, ethanethiol and propanethiol; and the flocculating agent in step (3) is one of ferric chloride, aluminum sulfate, polyaluminum chloride, polyferric sulfate and polyacrylamide.

[0018] Further, the adding amount of the active hydrogen substance in step (2) is 0.1-1 times of the mass of the polylactic acid crude product; the adding amount of the flocculating agent in step (3) is 0.1-2‰ of the mass of the polylactic acid crude product; and the adding amount of the solvent II in step (5) is 1-2.5 times of the mass of the solvent I.

[0019] Further, the adding amount of the active hydrogen substance in step (2) is 0.3-0.5 times of the mass of the polylactic acid crude product; and the adding amount of the flocculating agent in step (3) is 0.3-0.7‰ of the mass of the polylactic acid crude product.

[0020] Further, the pore size of the filter membrane used in the filtering in step (4) is 0.01-10 μm.

[0021] Further, the system temperature is maintained at -5-15 ℃ before the active hydrogen substance is added in step (2), and the stirring is performed for 5-30 min after the active hydrogen substance is added.

[0022] In the present application, the low-tin-content polylactic acid is prepared by the preparation method.

[0023] In the present application, the adding sequence of the active hydrogen substance and the adding sequence of the flocculating agent can be reversed, and the adding of the solvent II must be after the filtering.

[0024] In the present application, the mechanism diagram of the preparation of the low-tin-content polylactic acid is shown in Figure 1 , wherein (a): the tin catalyst (stannous) and the alcohol initiator (R'-OH) rapidly react to generate a coordination compound containing a tin-alcohol bond, and the compound has stronger nucleophilicity than the stannous compound; (b): one ring external oxygen atom of the lactide is temporarily coordinated with the metal atom, and the lactide is ring-opening; (c): the remaining lactide continues to participate in the polymerization reaction, so that the polymerization chain is continuously grown to generate the polylactic acid crude product, and at this time, the polylactic acid crude product contains a higher content of tin; (d): the polylactic acid crude product is dissolved by adding the solvent I, the active hydrogen substance is added after the dissolution, the tin-containing substance Sn(OCOR)2 group is separated from the polylactic acid crude product under the action of the active hydrogen substance, and is converted into the stannous catalyst, and then the stannous catalyst is flocculated after the flocculating agent is added, and is filtered out, the filtrate is added with the solvent II, and then the low-tin-content polylactic acid solid is precipitated.

[0025] The present application has the following beneficial effects:

[0026] (1) The preparation method of the low-tin-content polylactic acid has a wide application range, can treat the crude product with tin residue ≤1500 ppm in a wide range of weight average molecular weight, can ensure a lower tin residue by one refining without repeated refining, has a good removal effect on various types of tin catalysts, and the product treated by the process has a tin content ≤15 ppm.

[0027] (2) The preparation method of the low-tin-content polylactic acid has less solid waste and waste liquid, stable polylactic acid molecular weight, and can guarantee the rate of the early catalytic reaction. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 A mechanism diagram of the preparation of the low-tin-content polylactic acid. DETAILED DESCRIPTION

[0029] The application will be further illustrated below in conjunction with specific examples. It should be understood that the examples are only used to illustrate the application and not used to limit the scope of the application. After reading the application, those skilled in the art can make various equivalent modifications of the application, which all fall within the scope defined by the appended claims.

[0030] In the following examples and comparative examples, polylactic acid crude is prepared by ring-opening polymerization using lactide as a raw material, adding an alcohol initiator and a tin catalyst.

[0031] Example 1

[0032] 100 g of polylactic acid crude (PLA-1: tin content is 338 ppm, weight average molecular weight is 63809, and molecular weight distribution is 1.67) is dissolved in 500 mL of dichloromethane, the system is cooled to 5 ℃ after complete dissolution, 40 g of ethanol is added, and stirring is performed for 30 min, then 0.05 g of ferric chloride is added, stirring is performed for 15 min, then 0.22 μm filter membrane is used for filtration, after the filtration is completed, 1000 ml of n-heptane is added dropwise to precipitate the product, after the dropwise addition is completed, stirring is performed for 30 min, 0.22 μm filter membrane is used for filtration, and vacuum drying is performed at 70 ℃ for 15 h to obtain low-tin-content polylactic acid; the tin content of the low-tin-content polylactic acid is 8 ppm, the weight average molecular weight is 64274, and the molecular weight distribution is 1.65.

[0033] Example 2

[0034] 100 g of polylactic acid crude (PLA-2: tin content is 865 ppm, weight average molecular weight is 82309, and molecular weight distribution is 1.48) is dissolved in 500 mL of dichloromethane, the system is cooled to 5 ℃ after complete dissolution, 40 g of ethanol is added, and stirring is performed for 30 min, then 0.05 g of ferric chloride is added, stirring is performed for 15 min, then 0.22 μm filter membrane is used for filtration, after the filtration is completed, 1000 ml of n-heptane is added dropwise to precipitate the product, after the dropwise addition is completed, stirring is performed for 30 min, 0.22 μm filter membrane is used for filtration, and vacuum drying is performed at 70 ℃ for 15 h to obtain low-tin-content polylactic acid; the tin content of the low-tin-content polylactic acid is 8 ppm, the weight average molecular weight is 64274, and the molecular weight distribution is 1.65.

[0035] Example 3

[0036] 100 g of a crude polylactic acid (PLA-3: tin content of 1430 ppm, weight average molecular weight of 103850, and molecular weight distribution of 1.54) was dissolved in 500 mL of dichloromethane, and after complete dissolution, the system was cooled to 5°C, 40 g of ethanethiol was added, and stirred for 20 min. Then, 0.05 g of aluminum sulfate was added, and stirred for 15 min. After filtration using a 0.22 μm filter, 1000 mL of methanol was added dropwise to precipitate the product. After completion of the dropwise addition, the mixture was stirred for 30 min, and then filtered using a 0.22 μm filter. The filtrate was dried at 70°C under vacuum for 15 h to obtain a low-tin-content polylactic acid. The low-tin-content polylactic acid had a tin content of 12 ppm, a weight average molecular weight of 102640, and a molecular weight distribution of 1.57.

[0037] Example 4

[0038] 100 g of a crude polylactic acid (PLA-4: tin content of 95 ppm, weight average molecular weight of 128909, and molecular weight distribution of 1.39) was dissolved in 500 mL of DMF, and after complete dissolution, the system was cooled to 5°C, 40 g of propanethiol was added, and stirred for 25 min. Then, 0.05 g of polyferric sulfate was added, and stirred for 15 min. After filtration using a 0.22 μm filter, 1000 mL of ethanol was added dropwise to precipitate the product. After completion of the dropwise addition, the mixture was stirred for 30 min, and then filtered using a 0.22 μm filter. The filtrate was dried at 70°C under vacuum for 15 h to obtain a low-tin-content polylactic acid. The low-tin-content polylactic acid had a tin content of 5 ppm, a weight average molecular weight of 129483, and a molecular weight distribution of 1.36.

[0039] Comparative Example 1

[0040] 100 g of a crude polylactic acid (PLA-1: tin content of 338 ppm, weight average molecular weight of 63809, and molecular weight distribution of 1.67) was dissolved in 500 mL of dichloromethane, and after complete dissolution, the system was cooled to 5°C. Then, 1000 mL of n-heptane was added dropwise to precipitate the product. After completion of the dropwise addition, the mixture was stirred for 30 min, and then filtered using a 0.22 μm filter. The filtrate was dried at 70°C under vacuum for 15 h to obtain a polylactic acid. The polylactic acid had a tin content of 331 ppm, a weight average molecular weight of 62209, and a molecular weight distribution of 1.64.

[0041] Comparative Example 2

[0042] 100 g of a crude polylactic acid (PLA-1: tin content of 338 ppm, weight average molecular weight of 63809, and molecular weight distribution of 1.67) was dissolved in 500 mL of dichloromethane, and after complete dissolution, the system was cooled to 5°C, 40 g of ethanol was added, and stirring was performed for 30 min. After filtration using a 0.22 μm filter, 1000 mL of n-heptane was added dropwise to precipitate the product. After the dropwise addition was completed, stirring was performed for 30 min, and then the product was filtered using a 0.22 μm filter. The product was dried at 70°C under vacuum for 15 h to obtain polylactic acid. The tin content of the polylactic acid was 108 ppm, the weight average molecular weight was 64089, and the molecular weight distribution was 1.66.

[0043] Comparative Example 3

[0044] 100 g of a crude polylactic acid (PLA-1: tin content of 338 ppm, weight average molecular weight of 63809, and molecular weight distribution of 1.67) was dissolved in 500 mL of dichloromethane, and after complete dissolution, the system was cooled to 5°C, 0.05 g of iron chloride was added, and stirring was performed for 15 min. After filtration using a 0.22 μm filter, 1000 mL of n-heptane was added dropwise to precipitate the product. After the dropwise addition was completed, stirring was performed for 30 min, and then the product was filtered using a 0.22 μm filter. The product was dried at 70°C under vacuum for 15 h to obtain polylactic acid. The tin content of the polylactic acid was 309 ppm, the weight average molecular weight was 64012, and the molecular weight distribution was 1.63.

[0045] Comparative Example 4

[0046] A crude polylactic acid (PLA-1: tin content of 338 ppm, weight average molecular weight of 63809, and molecular weight distribution of 1.67) was washed with 1 mol / L hydrochloric acid, and after washing 4 times, polylactic acid was obtained. The tin content of the polylactic acid was 32 ppm, the weight average molecular weight was 48953, and the molecular weight distribution was 1.81.

Claims

1. A method for preparing polylactic acid with low tin content, characterized in that, Includes the following steps: (1) Dissolve the crude polylactic acid in solvent I; (2) Add an active hydrogen substance to the solution from step (1) and stir thoroughly; (3) Add flocculant to the solution from step (2) and stir thoroughly; (4) Filter the solution from step (3); (5) Add solvent II to the filtrate after filtration in step (4), stir thoroughly, and precipitate the product; (6) The precipitated product was filtered and dried to obtain polylactic acid with low tin content; Among them, crude polylactic acid is obtained by ring-opening polymerization of lactide as raw material, with the addition of alcohol initiators and tin catalysts; Solvent I is one or more of dichloromethane, trichloromethane, DMF and acetone, and the concentration of crude polylactic acid after adding solvent I is 25-500 g / L; Solvent II is one of n-heptane, methanol, ethanol, diethyl ether, and ethylene glycol; The active hydrogen substance is one of methanol, ethanol, 1,3-propanediol, 1,5-pentanediol, trimethylolpropane, phenol, cresol, direfenac, ethylenediamine, ethanethiol, and propanethiol; The flocculant is one of ferric chloride, aluminum sulfate, polyaluminum chloride, or polyferric sulfate.

2. The method for preparing low-tin-content polylactic acid according to claim 1, characterized in that, In step (2), the amount of active hydrogen added is 0.1 to 1 times the mass of crude polylactic acid; in step (4), the amount of flocculant added is 0.1 to 2‰ of the mass of crude polylactic acid; in step (5), the amount of solvent II added is 1 to 2.5 times the mass of solvent I.

3. The method for preparing low-tin-content polylactic acid according to claim 1, characterized in that, In step (2), the amount of active hydrogen substance added is 0.3 to 0.5 times the mass of crude polylactic acid; in step (4), the amount of flocculant added is 0.3 to 0.7‰ of the mass of crude polylactic acid.

4. The method for preparing low-tin-content polylactic acid according to claim 1, characterized in that, The filter membrane used for filtration in step (4) has a pore size of 0.01-10 μm.

5. The method for preparing low-tin-content polylactic acid according to claim 1, characterized in that, In step (2), the system temperature is maintained at -5 to 15℃ before the active hydrogen substance is added, and the mixture is stirred for 5 to 30 minutes after the active hydrogen substance is added.

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