Method for efficient catalytic synthesis of polylactic acid

Abstract

The invention belongs to the field of high polymer materials, and discloses a method for efficiently catalytically synthesizing polylactic acid, which is characterized by comprising the following steps: adding lactide, a tin catalyst and an initiator into a reactor, quickly heating to 180 + / -20 DEG C under the protective atmosphere and stirring, and carrying out polymerization reaction for more than 10 minutes to obtain high-molecular-weight polylactic acid; the initiator is hydroxy acid and is monobasic acid. The lactide polymerization rate is greatly improved, the polymerization reaction can be completed within more than ten minutes, the lactide conversion rate is 94% or above, the molecular weight is 2.17 * 10 < 5 > Da or above, and the lactide reaches the level of commercial polylactic acid. Compared with a tin catalyst and a tin catalyst / alcohol composite catalytic system, the catalytic system has the advantages that the polymerization rate at the initial reaction stage is increased by 20-30%, and the reaction time is shortened by more than 80%. The catalytic system is applied to a process for synthesizing polylactic acid through reactive extrusion, and the production efficiency of an extruder can be remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials, specifically relating to a method for the efficient catalytic synthesis of polylactic acid. Background Technology

[0002] Polylactic acid (PLA), a biodegradable material derived from bio-based materials, has been widely used in packaging, textiles, agricultural films, and drug delivery systems due to its excellent mechanical properties, processability, and bioabsorption, making it one of the most promising alternative materials. Currently, industrially, high-molecular-weight, high-quality PLA is mainly synthesized through the ring-opening polymerization of lactide.

[0003] Reactive extrusion is a highly efficient process for synthesizing PLA. The high shear strength and modular design of the screw not only improve the synthesis efficiency of polylactic acid (PLA) but also enable integrated polymerization-modification-processing of PLA products. Due to the low chamber volume and limited material residence time of the extruder, its production efficiency significantly depends on a highly efficient catalytic system. Stannous octoate, due to its high catalytic activity, is often used as a catalytic system in reactive extrusion synthesis of PLA. Many patents have reported stannous octoate-based catalytic systems. For example, patent US5378801 uses stannous octoate to catalyze the reactive extrusion of lactide to prepare PLA; patent US6166169 uses a composite catalytic system of stannous octoate and triphenylphosphine to synthesize PLA with a weight-average molecular weight of 1.6 × 10⁻⁶ after a residence time of 7 minutes in the extruder. 5 Da's polylactic acid has a production efficiency of only 0.7 kg / h, which has room for improvement.

[0004] Polylactic acid (PLA) can be prepared from lactide through ring-opening polymerization under the action of a catalyst, but this reaction has high requirements for monomer purity and anhydrous and oxygen-free conditions. In actual polymerization processes, failure to meet these factors significantly affects the increase of polymer molecular weight, making it difficult to obtain ultra-high molecular weight PLA in a short time. Furthermore, existing ultra-high molecular weight PLA preparation processes often involve multiple steps, are complex, have long reaction times, high energy consumption, and high production costs, and the required reagents are not readily available, further limiting its large-scale preparation. Summary of the Invention

[0005] To address the above problems, this invention provides a method for the efficient catalytic synthesis of polylactic acid.

[0006] The technical solution adopted in this invention is as follows: A method for the efficient catalytic synthesis of polylactic acid includes the following steps: A composite catalytic system composed of different hydroxy acids and stannous octoate was used for the ring-opening polymerization of lactide to prepare polylactic acid (PLA). A torque rheometer was used in conjunction with a high-temperature reactor as the reactor, with the rotor set to rotation mode, to monitor the viscosity increase rate of the system in situ during polymerization. Specifically, a certain amount of lactide, stannous octoate, and hydroxy acids were added to the high-temperature reactor. The molar ratio of lactide to stannous octoate was 2000:1, and the molar ratio of hydroxy acids to stannous octoate was 0:1 to 2:1. Nitrogen gas was purged three times, and then the temperature was rapidly raised to 180°C to start the polymerization reaction. When the system viscosity reached 100,000 mPa·s, the rotor rotation was stopped, and the reaction continued for 20 min. The reaction was then terminated to obtain PLA, and the molecular weight and lactide conversion rate were measured.

[0007] Compared with the prior art, the present invention has the following beneficial effects: This invention employs a composite catalytic system of hydroxy acid (monocarboxylic acid) and tin-based catalysts, which significantly enhances the polymerization rate of lactide. Experiments conducted in a reaction flask at 180°C resulted in polymerization completing in just over ten minutes, achieving a lactide conversion rate of over 94% and a molecular weight of 2.17 × 10⁻⁶. 5 The polymerization rate reaches the level of commercially available polylactic acid (PLA) with a polymerization density of over 100%. Compared with tin-based catalysts and tin-based catalyst / alcohol composite catalyst systems, this catalytic system increases the polymerization rate by 20-30% in the initial stage of the reaction and shortens the reaction time by more than 80%. When applied to the reactive extrusion synthesis of PLA, this catalytic system can significantly improve the production efficiency of the extruder. Attached Figure Description

[0008] Figure 1 This is a graph showing the viscosity of the reaction system in Examples 1-5 as a function of time.

[0009] Figure 2 This is a graph showing the viscosity of the reaction system in Examples 9-13 changing over time. Detailed Implementation

[0010] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto. For process parameters not specifically noted, conventional techniques can be referred to.

[0011] The torque rheometer was purchased from Anton Paar, model MCR302e.

[0012] Example 1 A torque rheometer was used in conjunction with a high-temperature reactor as the reactor, with the rotor set to rotation mode, to monitor the viscosity increase rate of the system during polymerization in situ. Specifically, a certain amount of lactide, stannous octoate, and glycolic acid were added to the high-temperature reactor, with a molar ratio of lactide to stannous octoate of 2000:1 and a molar ratio of glycolic acid to stannous octoate of 1:1. Nitrogen gas was purged three times, and then the temperature was rapidly increased to 180℃ to begin the polymerization reaction. When the system viscosity reached 10000 mPa·s, the reaction time was recorded (see Tables 1 and 2), and the rotor rotation was stopped. The reaction continued for 20 min, and the reaction was terminated to obtain PLA, with a weight-average molecular weight of 2.72 × 10⁻⁶. 5 The Da, PDI is 2.40, and the lactide conversion rate is 96.3%.

[0013] The difference between Examples 2-8 and Example 1 is that different initiators are used, as detailed in Table 1.

[0014] Table 1. Synergistic Synthesis of PLA with Different Initiators / Stamenium Octanoate

[0015] a Time when the viscosity of the reaction system reaches 10000 mPa·s As shown in Table 1, when the initiator is a polybasic acid such as malic acid, tartaric acid, and citric acid, the polymerization rate decreases significantly, the conversion rate is less than 10%, and the molecular weight of the synthesized PLA also decreases significantly. Therefore, the monobasic acid among hydroxy acids is selected as the initiator for synergistic catalysis in this invention.

[0016] The difference between Examples 9-13 and Example 1 is that the molar ratio of glycolic acid to stannous octoate is different, as detailed in Table 2.

[0017] Table 2. Effect of different molar ratios of glycolic acid to stannous octoate on the polymerization of lactide.

[0018] a Reaction time when the viscosity of the reaction system reaches 10000 mPa·s As shown in Table 2, the reaction time begins to increase when the molar ratio of glycolic acid to stannous octoate is 0.15:1 and 1.5:1. Therefore, the preferred molar ratio of glycolic acid to stannous octoate is 0.15:1-1.5:1, and more preferably 0.25:1-1.0:1.

[0019] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for efficient catalytic synthesis of polylactic acid, characterized by, The method comprises the following steps: adding lactide, tin catalyst and initiator into a reactor, heating to 180±20℃ under protection of nitrogen and stirring, and polymerizing for 10 minutes or more to obtain high molecular weight polylactic acid; the initiator is hydroxy acid, and is monobasic acid.

2. The method of claim 1, wherein, the catalyst is one or more of stannous octoate, tin tetrachloride, dibutyltin dilaurate and dibutyltin diacetate.

3. The method of claim 1, wherein, the initiator comprises one or more of glycolic acid, lactic acid, 2-hydroxybutyric acid, 2-hydroxyoctanoic acid and salicylic acid.

4. The method of claim 1, wherein, the polymerization time is 10-40 minutes.

5. The method of claim 3, wherein, the polymerization time is 15±5 minutes.

6. The method according to any one of claims 1 to 5, characterized in that, the molar ratio of lactide to catalyst is 2000±500:1, and the molar ratio of initiator to catalyst is 0.1:1-2.0:

1.

7. The method of claim 6, wherein, the molar ratio of initiator to catalyst is 0.15:1-1.5:

1.

8. The method of claim 7, wherein, the molar ratio of initiator to catalyst is 0.5:1-1.0:

1.

9. The method according to any one of claims 1 to 5, characterized in that, the reactor is a reaction kettle or a reaction extruder.

10. The method according to any one of claims 1 to 5, characterized in that, the protection gas is nitrogen.

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