Preparation method of polylactic acid degradable material

Abstract

The invention discloses a preparation method of a polylactic acid degradable material, which comprises the following steps: S1, two-stage purification raw material pretreatment: removing metal ions from industrial-grade lactic acid through a sulfonic acid type cation exchange resin column, carrying out vacuum molecular distillation at the pressure of 0.1-1 mmHg and the temperature of 100-120 DEG C, and collecting a fraction with the lactic acid dimer content of less than 3% to obtain a lactic acid monomer with the purity of more than or equal to 99.5%, adding 0.1 to 0.3 percent of ascorbyl palmitate into the lactic acid monomer; s2, low-temperature prepolymerization reaction: adding the pretreated lactic acid and 0.5-1.2% (mass ratio) of p-toluenesulfonic acid catalyst into a reaction kettle; s3, microwave-assisted dynamic chain extension: transferring the prepolymer to a microwave reaction device; and S4, gradient precipitation and purification of the solvent. The total reaction time is less than or equal to 6 hours and is shortened by 75% and the energy consumption is reduced by 60% compared with the traditional ring-opening polymerization method. The number-average molecular weight Mn of the prepared polylactic acid is 150000-25000, the PD I is 1.5-1.8, the tensile strength is greater than or equal to 70 MPa, and the elongation at break is greater than or equal to 8%. Toxic solvents such as methylbenzene are not used in the whole process, the catalyst recovery rate is more than 90%, and the water consumption is reduced by 50%.

Description

Technical Field

[0001] The present invention relates to the technical field of preparing biodegradable polymer materials, and particularly relates to a preparation method of a polylactic acid biodegradable material. Background Art

[0002] With the increasing global awareness of environmental protection, the "white pollution" problem caused by traditional plastics has become increasingly severe. The research and application of biodegradable materials have become the key way to solve this dilemma. As a highly potential bio-based biodegradable material, polylactic acid is mainly derived from renewable biomass resources such as corn and sugarcane. Through processes such as fermentation and extraction, lactic acid is obtained and then further polymerized. Polylactic acid has good biocompatibility, biodegradability, and mechanical properties, showing broad application prospects in many fields such as packaging, medical, agriculture, and textiles. For example, in the field of disposable packaging, polylactic acid products can gradually degrade in the natural environment, effectively reducing garbage accumulation; in the medical field, its biocompatibility enables it to be used to manufacture sutures, tissue engineering scaffolds, etc., and can be naturally metabolized and absorbed by the human body after completing its mission.

[0003] However, there are still many problems to be solved in the current preparation technology of polylactic acid. The common ring-opening polymerization method of lactide can prepare polylactic acid with high molecular weight, but its process is complex. It requires the dehydration cyclization of lactic acid to generate lactide, and then ring-opening polymerization. This process not only involves multiple steps, but also the purification of lactide requires multiple recrystallizations, consuming a large amount of organic solvents and having a high cost. According to statistics, the cost of preparing lactide by this method accounts for about 40% - 50% of the total cost of polylactic acid, and the large amount of organic waste liquid generated during the production process causes great pressure on the environment.

[0004] To sum up, the existing polylactic acid preparation technologies have obvious deficiencies in terms of reaction efficiency, cost control, product performance, and environmental friendliness, and cannot meet the growing market demand. Developing a new method that is efficient, low-cost, environmentally friendly, and can stably prepare high-performance polylactic acid biodegradable materials has become a research hotspot and urgent need in this field. Therefore, this solution proposes a preparation method of a polylactic acid biodegradable material. Summary of the Invention

[0005] The purpose of the present invention is to solve at least one of the technical problems existing in the prior art, and provide a preparation method of a polylactic acid biodegradable material. The total reaction time ≤ 6 hours, which is 75% shorter than the traditional ring-opening polymerization method, and the energy consumption is reduced by 60%. The number-average molecular weight Mn of the obtained polylactic acid is 150,000 - 250,000, PDI = 1.5 - 1.8, the tensile strength ≥ 70 MPa, and the elongation at break ≥ 8%. Toxic solvents such as toluene are not used throughout the process, the catalyst recovery rate > 90%, and the water consumption is reduced by 50%.

[0006] The present invention also provides a preparation method of a polylactic acid degradable material having the above-mentioned one, comprising the following steps:

[0007] S1. Two-stage purification raw material pretreatment: Pass industrial-grade lactic acid through a sulfonic acid-type cation exchange resin column to remove metal ions, and then perform vacuum molecular distillation under a pressure of 0.1 - 1 mmHg and a temperature of 100 - 120 °C, and collect the fraction with a lactic acid dimer content < 3% to obtain a lactic acid monomer with a purity ≥ 99.5%. Add 0.1% - 0.3% (by mass ratio) of ascorbyl palmitate to the lactic acid monomer and stir evenly;

[0008] S2. Low-temperature prepolymerization reaction: Add the pretreated lactic acid and 0.5% - 1.2% (by mass ratio) of p-toluenesulfonic acid catalyst to a reaction kettle. After purging with nitrogen, heat up to 120 - 140 °C at a rate of 5 °C / min and react at atmospheric pressure for 3 - 5 hours to generate a polylactic acid prepolymer with a number-average molecular weight of 5000 - 10000 and a terminal carboxyl group content ≥ 95%;

[0009] S3. Microwave-assisted dynamic chain extension: Transfer the prepolymer to a microwave reaction device, add 1% - 2% (by mass ratio of the prepolymer) of isophorone diisocyanate (IPDI), and react under the conditions of a 2450 MHz microwave field, a power of 300 - 500 W, and a temperature of 160 - 180 °C for 60 - 120 minutes. Monitor in real time through an on-line viscometer, and terminate the reaction when the viscosity increase > 80% to obtain polylactic acid with a number-average molecular weight of 100000 - 250000;

[0010] S4. Solvent gradient precipitation purification: Dissolve the chain-extended product with dichloromethane (material-liquid ratio 1:5 - 1:10, w / v), and sequentially add equal volumes of acetone, 2 times the volume of methanol, and 5 times the volume of deionized water for three-stage precipitation. After suction filtration, dry to a constant weight under a vacuum degree < 10 Pa and a temperature of 50 - 60 °C to obtain a polylactic acid degradable material with a catalyst residue content < 0.1%.

[0011] According to the preparation method of a polylactic acid degradable material provided by the present invention, the sulfonic acid-type cation exchange resin column has a column diameter of 5 - 10 cm, a column height of 50 - 100 cm, a flow rate of 1 - 3 mL / min, and a metal ion residue content < 5 ppm.

[0012] According to the preparation method of a polylactic acid degradable material provided by the present invention, the prepolymerization reaction is carried out in a reaction kettle with a spiral stirrer, the stirring rate is 50 - 100 rpm, and the nitrogen flow rate is 3 - 8 L / min.

[0013] According to the preparation method of a polylactic acid degradable material provided by the present invention, the microwave reaction device uses a quartz reaction chamber with a diameter of 5 - 10 cm, a height of 10 - 20 cm, and a microwave field uniformity error < ±5%.

[0014] According to a preparation method of a polylactic acid degradable material provided by the present invention, in the three-stage precipitation method, the volume ratio of acetone, methanol, and deionized water added is 1:2:5, and the temperature during the precipitation process is controlled at 20 - 30 °C.

[0015] According to a preparation method of a polylactic acid degradable material provided by the present invention, the terminal carboxyl group content of the polylactic acid prepolymer is determined by acid-base titration. Specifically: 0.1 g of the prepolymer is dissolved in 10 mL of dioxane, 2 drops of phenolphthalein indicator are added, and it is titrated with 0.01 M potassium hydroxide ethanol solution until it turns slightly red, and then the carboxyl group content is calculated.

[0016] According to a preparation method of a polylactic acid degradable material provided by the present invention, the drying step adopts vacuum freeze-drying or vacuum heat-drying. Among them, the heating rate of vacuum heat-drying is 1 - 2 °C / min, and the drying time is 8 - 12 hours.

[0017] According to a preparation method of a polylactic acid degradable material provided by the present invention, the number-average molecular weight (Mn) of the polylactic acid degradable material is 150,000 - 250,000, the molecular weight distribution index (PDI) ≤ 1.8, the tensile strength ≥ 70 MPa, and the elongation at break ≥ 8%.

[0018] According to a preparation method of a polylactic acid degradable material provided by the present invention, the method further includes adding 0.05% - 0.2% (by mass) of citric acid as a molecular weight regulator in the raw material pretreatment step to control the molecular weight of the prepolymer at 6,000 - 9,000.

[0019] According to a preparation method of a polylactic acid degradable material provided by the present invention, in the solvent gradient precipitation purification step, the washing liquid for washing the precipitate is ethanol or deionized water, the number of washing times is 2 - 3 times, and the washing time for each time is 5 - 10 minutes.

[0020] Compared with the prior art, for a preparation method of a polylactic acid degradable material of the present invention, the total reaction time ≤ 6 hours, which is 75% shorter than the traditional ring-opening polymerization method, and the energy consumption is reduced by 60%.

[0021] Compared with the prior art, for a preparation method of a polylactic acid degradable material of the present invention, the number-average molecular weight of the obtained polylactic acid Mn = 150,000 - 250,000, PDI = 1.5 - 1.8, the tensile strength ≥ 70 MPa, and the elongation at break ≥ 8%.

[0022] Compared with the prior art, for a preparation method of a polylactic acid degradable material of the present invention, no toxic solvents such as toluene are used throughout the process, the catalyst recovery rate > 90%, and the water consumption is reduced by 50%. Description of the Drawings

[0023] The present invention will be further described below in conjunction with the accompanying drawings and embodiments;

[0024] Figure 1 It is the overall flowchart of the preparation method of a polylactic acid degradable material of the present invention;

[0025] Figure 2 It is the flowchart of the preparation of general-purpose polylactic acid in the first embodiment of the present invention;

[0026] Figure 3 It is the flowchart of the preparation of high heat-resistant polylactic acid in the second embodiment of the present invention

[0027] Figure 4 It is the flowchart of the preparation of fast-degrading polylactic acid in the third embodiment of the present invention. Specific embodiments

[0028] This part will describe in detail the specific embodiments of the present invention. The preferred embodiments of the present invention are shown in the accompanying drawings. The function of the drawings is to supplement the description of the text part of the specification, enabling people to intuitively and vividly understand each technical feature and the overall technical solution of the present invention, but it cannot be understood as a limitation on the protection scope of the present invention.

[0029] Refer to Figure 1 , a preparation method of a polylactic acid degradable material in an embodiment of the present invention, which includes the following steps:

[0030] S1. Bipolar purification raw material pretreatment: Remove metal ions from industrial-grade lactic acid through a sulfonic acid-type cation exchange resin column, and then perform vacuum molecular distillation at a pressure of 0.1-1 mmHg and a temperature of 100-120 °C to collect the fraction with a lactic acid dimer content < 3%, obtaining a lactic acid monomer with a purity ≥ 99.5%. Add 0.1% - 0.3% (mass ratio) of ascorbyl palmitate to the lactic acid monomer and stir evenly;

[0031] S2. Low-temperature prepolymerization reaction: Add the pretreated lactic acid and 0.5% - 1.2% (mass ratio) of p-toluenesulfonic acid catalyst to the reaction kettle. After nitrogen replacement, heat up to 120-140 °C at a rate of 5 °C / min and react at normal pressure for 3-5 hours to generate a polylactic acid prepolymer with a number average molecular weight of 5000-10000 and a terminal carboxyl group content ≥ 95%;

[0032] S3. Microwave-assisted dynamic chain extension: Transfer the prepolymer to a microwave reaction device, add 1% - 2% (mass ratio of the prepolymer) of isophorone diisocyanate (IPDI), and react under the conditions of a 2450 MHz microwave field, a power of 300-500 W, and a temperature of 160-180 °C for 60-120 minutes. Monitor in real time through an on-line viscometer and terminate the reaction when the viscosity increase > 80% to obtain a polylactic acid with a number average molecular weight of 100000-250000;

[0033] S4. Solvent gradient precipitation purification: Dissolve the chain-extended product in dichloromethane (material-liquid ratio 1:5 - 1:10, w / v), and sequentially add equal volume of acetone, 2 times volume of methanol, and 5 times volume of deionized water for three-stage precipitation. After suction filtration, dry it to constant weight under a vacuum degree < 10 Pa and a temperature of 50 - 60 °C to obtain a polylactic acid degradable material with a catalyst residue content < 0.1%.

[0034] The column diameter of the sulfonic acid type cation exchange resin column is 5 - 10 cm, the column height is 50 - 100 cm, the flow rate is 1 - 3 mL / min, and the metal ion residue content < 5 ppm. The prepolymerization reaction is carried out in a reaction kettle with a spiral stirrer, the stirring rate is 50 - 100 rpm, and the nitrogen flow rate is 3 - 8 L / min. The microwave reaction device uses a quartz reaction cavity with a diameter of 5 - 10 cm and a height of 10 - 20 cm, and the microwave field uniformity error < ±5%.

[0035] In the three-stage precipitation method, the addition volume ratio of acetone, methanol, and deionized water is 1:2:5, and the temperature during the precipitation process is controlled at 20 - 30 °C. The terminal carboxyl group content of the polylactic acid prepolymer is determined by acid-base titration. Specifically: Take 0.1 g of the prepolymer and dissolve it in 10 mL of dioxane, add 2 drops of phenolphthalein indicator, and titrate it with 0.01 M potassium hydroxide ethanol solution until it turns slightly red, and calculate the carboxyl group content. The drying step uses vacuum freeze drying or vacuum thermal drying, where the heating rate of vacuum thermal drying is 1 - 2 °C / min, and the drying time is 8 - 12 hours. The number average molecular weight (Mn) of the polylactic acid degradable material is 150,000 - 250,000, the molecular weight distribution index (PDI) ≤ 1.8, the tensile strength ≥ 70 MPa, and the elongation at break ≥ 8%.

[0036] Add 0.05% - 0.2% (mass ratio) of citric acid as a molecular weight regulator in the raw material pretreatment step to control the molecular weight of the prepolymer at 6000 - 9000. In the solvent gradient precipitation purification step, the washing liquid for washing the precipitate is ethanol or deionized water, the number of washing times is 2 - 3 times, and each washing time is 5 - 10 minutes.

[0037] Example 1, referring to Figure 2 The preparation of its general-purpose polylactic acid includes the following steps:

[0038] Raw material pretreatment: Take 1000 g of industrial lactic acid with its purity controlled at 88%, pass it through a 732 type cation exchange resin column with a diameter of 5 cm and a column height of 60 cm. After collecting the effluent, perform molecular distillation at a pressure of 0.5 mmHg and a temperature of 110 °C to obtain 920 g of high-purity lactic acid, add 1.84 g of ascorbyl palmitate, and stir at 30 °C for 30 minutes.

[0039] Prepolymerization reaction: Charge into the reactor, add 7.36 g of p-toluenesulfonic acid, with a nitrogen flow rate of 5 L / min, heat up to 130 °C at a rate of 5 °C / min, and react for 4 hours to obtain a prepolymer with Mn = 7800 and an acid value of 56 mg KOH / g.

[0040] Microwave chain extension reaction: Transfer to the microwave reaction chamber, add 13.8 g of IPDI, set the power to 400 W, and the temperature to 170 °C, react for 90 minutes, the online viscosity increases from 15 mPa·s to 120 mPa·s, Mn = 165000, PDI = 1.68.

[0041] Gradient precipitation purification: Add 7360 mL of dichloromethane to dissolve, successively add 7360 mL of acetone, 14720 mL of methanol, and 36800 mL of deionized water, filter by suction and then dry under vacuum, control the temperature at 55 °C for 12 h to obtain 810 g of polylactic acid with a catalyst residue of 0.08%.

[0042] Performance test: Tensile strength 72 MPa, elongation at break 8.5%, DSC melting point 168 °C, 5% thermal weight loss temperature 280 °C.

[0043] Example 2: Refer to Figure 3 , the preparation of high heat-resistant polylactic acid includes the following steps:

[0044] Pretreatment: Increase the dosage of ascorbyl palmitate to 0.3%, distillation temperature 115 °C, to obtain lactic acid with a purity of 99.6%.

[0045] Prepolymerization: The catalyst dosage is 1.2%, the reaction temperature is 135 °C, and the time is 3.5 hours. The prepolymer has Mn = 9000 and a hydroxyl content of 96%.

[0046] Chain extension reaction: The IPDI dosage is 2%, the microwave power is 500 W, the temperature is 180 °C, and the reaction is for 60 minutes. Mn = 220000, PDI = 1.75.

[0047] Purification: Adjust the solvent ratio to 1:10 (material:dichloromethane), shorten the time interval of three-stage precipitation to 15 minutes, and the drying temperature is 60 °C.

[0048] Performance: Melting point 175 °C, heat distortion temperature controlled at 1.82 MPa and 75 °C, suitable for high-temperature packaging materials.

[0049] Example 3, refer to Figure 4 , the rapid degradation type polylactic acid includes the following steps:

[0050] Pretreatment: Add 0.1% citric acid as a molecular weight regulator, and the prepolymer has Mn = 6000.

[0051] Chain extension reaction: Replace with HDI, with its dosage controlled at 1%, power 300 W, temperature 160 °C, react for 120 minutes, Mn = 120000, PDI = 1.8.

[0052] Post-treatment: Replace the washing liquid with ethanol, the purpose is to reduce the surface energy, and the drying time is shortened to 8 hours.

[0053] Degradation performance: The weight loss rate is 25% after 3 months of soil burial, 45% after 6 months, which is better than conventional polylactic acid, 30% after 6 months.

[0054] Comparative example 1: Traditional microwave-assisted direct polycondensation method, directly add lactic acid and catalyst into the microwave reaction cavity, power 500 W, react at 180 °C for 2 hours.

[0055] Result: The molecular weight is only 35000, PDI = 2.2, and the tensile strength is 45 MPa.

[0056] Comparative example 2: Without microwave-assisted chain extension, use traditional oil bath heating, control it at 170 °C, and the duration is 2 hours. Other steps are the same as in Example 1.

[0057] Result: The molecular weight is 110000, the reaction time is extended to 3 hours, and the energy consumption increases by 40%.

[0058] The embodiments of the present invention have been described in detail above in conjunction with the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those of ordinary skill in the art in the said technical field, various changes can be made without departing from the purpose of the present invention.

Claims

1. A preparation method of a polylactic acid degradable material, characterized in that, It includes the following steps: S1. Double-stage purification raw material pretreatment: Remove metal ions from industrial-grade lactic acid through a sulfonic acid-type cation exchange resin column, and then perform vacuum molecular distillation at a pressure of 0.1-1 mmHg and a temperature of 100-120 °C. Collect the fraction with a lactic acid dimer content < 3% to obtain lactic acid monomers with a purity ≥ 99.5%. Add 0.1% - 0.3% (mass ratio) of ascorbyl palmitate to the lactic acid monomers and stir evenly. S2. Low-temperature prepolymerization reaction: Add the pretreated lactic acid and 0.5% - 1.2% (mass ratio) of p-toluenesulfonic acid catalyst to the reaction kettle. After nitrogen replacement, heat it to 120-140 °C at a rate of 5 °C / min and react at atmospheric pressure for 3-5 hours to generate a polylactic acid prepolymer with a number-average molecular weight of 5000-10000 and a terminal carboxyl group content ≥ 95%. S3. Microwave-assisted dynamic chain extension: Transfer the prepolymer to a microwave reaction device, add 1% - 2% (mass ratio of the prepolymer) of isophorone diisocyanate (IPDI), and react for 60-120 minutes under the conditions of a 2450 MHz microwave field, a power of 300-500 W, and a temperature of 160-180 °C. Monitor it in real time through an on-line viscometer. When the viscosity increase > 80%, terminate the reaction to obtain polylactic acid with a number-average molecular weight of 100000-250000. S4. Solvent gradient precipitation purification: Dissolve the chain-extended product in dichloromethane (feed liquid ratio 1:5 - 1:10, w / v), and sequentially add equal volume of acetone, 2 times volume of methanol, and 5 times volume of deionized water for three-stage precipitation. After filtration, dry it to a constant weight under a vacuum of < 10 Pa and a temperature of 50-60 °C to obtain a polylactic acid degradable material with a catalyst residue content < 0.1%.

2. The preparation method of a polylactic acid degradable material according to claim 1, characterized in that, The column diameter of the sulfonic acid-type cation exchange resin column is 5-10 cm, the column height is 50-100 cm, the flow rate is 1-3 mL / min, and the metal ion residue content < 5 ppm.

3. The preparation method of a polylactic acid degradable material according to claim 1, characterized in that, The prepolymerization reaction is carried out in a reaction kettle with a spiral stirrer, the stirring rate is 50-100 rpm, and the nitrogen flow rate is 3-8 L / min.

4. A method for preparing a polylactic acid degradable material according to claim 1, characterized in that, The microwave reaction device uses a quartz reaction chamber with a diameter of 5-10 cm and a height of 10-20 cm, and the microwave field uniformity error < ±5%.

5. The preparation method of a polylactic acid degradable material according to claim 1, characterized in that, In the three-stage precipitation method, the added volume ratio of acetone, methanol, and deionized water is 1:2:5, and the precipitation process temperature is controlled at 20-30 °C.

6. The preparation method of a polylactic acid degradable material according to claim 1, characterized in that, The terminal carboxyl group content of the polylactic acid prepolymer is determined by acid-base titration. Specifically: Take 0.1 g of the prepolymer and dissolve it in 10 mL of dioxane, add 2 drops of phenolphthalein indicator, and titrate it with 0.01 M potassium hydroxide ethanol solution until it turns slightly red, and calculate the carboxyl group content.

7. A method for preparing a polylactic acid degradable material according to claim 1, characterized in that, The drying step uses vacuum freeze drying or vacuum thermal drying. Among them, the heating rate of vacuum thermal drying is 1-2 °C / min, and the drying time is 8-12 hours.

8. The preparation method of a polylactic acid degradable material according to claim 1, characterized in that, The number-average molecular weight (Mn) of the polylactic acid degradable material is 150000-250000, the molecular weight distribution index (PDI) ≤ 1.8, the tensile strength ≥ 70 MPa, and the elongation at break ≥ 8%.

9. The preparation method of a polylactic acid degradable material according to claim 1, characterized in that, The method further includes adding 0.05% - 0.2% (mass ratio) of citric acid as a molecular weight regulator in the raw material pretreatment step to control the molecular weight of the prepolymer at 6000 - 9000.

10. The preparation method of a polylactic acid degradable material according to claim 1, characterized in that, In the solvent gradient precipitation purification step, the washing liquid for washing the precipitate is ethanol or deionized water, the number of washing times is 2 - 3 times, and the washing time for each time is 5 - 10 minutes.

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