Process for the preparation of stereocomplex polylactic acid

Abstract

The application discloses a preparation method of stereocomplex polylactic acid and belongs to the technical field of polylactic acid. The preparation method specifically comprises the following steps: mixing, preparing a composite nucleating agent, and block copolymerization. In the mixing step, zinc chloride, polyethylene glycol and anhydrous ethanol are added into a reaction device, stirring is conducted at room temperature, the reaction device is sealed and vacuumized, the temperature is increased to 60-65 DEG C, and all the anhydrous ethanol is removed to obtain a mixture. In the preparation of the composite nucleating agent, microcrystalline cellulose and a zinc chloride aqueous solution are added into the reaction device, stirring is conducted at room temperature, filtration is conducted, the filter residue is taken out, the filter residue is added into the reaction device together with a polyethylene glycol aqueous solution, stirring is conducted at room temperature, filtration is conducted, the filter residue is taken out, and drying is conducted to obtain the composite nucleating agent. The preparation method can greatly increase the melting temperature of the prepared stereocomplex polylactic acid, and the prepared stereocomplex polylactic acid does not have the problem of yellowing.

Description

Technical Field

[0001] This invention relates to the field of polylactic acid technology, and more specifically to a method for preparing stereocomposite polylactic acid. Background Technology

[0002] Polylactic acid (PLA), also known as polylactide, is a thermoplastic aliphatic polyester polymer obtained by polymerizing lactic acid as the main raw material. It has three isomers: poly-L-lactic acid, poly-D-lactic acid, and poly-DL-lactic acid. There are two main methods for preparing PLA: direct polycondensation and a two-step method. In the direct polycondensation method, lactic acid monomers are directly condensed. In the presence of a dehydrating agent, the hydroxyl and carboxyl groups in the lactic acid monomers are dehydrated upon heating, directly condensing into low-molecular-weight PLA oligomers. Then, a catalyst is added, and the temperature is further increased, causing the oligomers to polymerize into high-molecular-weight PLA. The advantages of the direct polycondensation method are high monomer conversion rate, simple process, and low cost. The disadvantage is that during polymerization, the reaction system is in a dynamic equilibrium between polycondensation and depolymerization. As the reaction proceeds, the system viscosity gradually increases, making it more difficult to remove the byproduct water. The presence of water strengthens the depolymerization direction, making it difficult to control the relative molecular mass and distribution of PLA, and thus difficult to obtain PLA with a high relative molecular mass. The two-step method, also known as the lactide ring-opening polymerization method, specifically includes oligomer preparation, lactide synthesis, and ring-opening polymerization. The two-step method can obtain polylactic acid with adjustable relative molecular mass and microstructure. Currently, commercial polylactic acid is generally prepared using the two-step method.

[0003] Polylactic acid (PLA) has good biocompatibility and can be completely degraded into carbon dioxide and water in the environment after use, causing no pollution and demonstrating its environmental friendliness. In recent years, it has been used as a material to replace petroleum-based polymers and is widely used in packaging films, textile fibers, agricultural products, and medical supplies. High molecular weight PLA, in particular, has great potential to replace petroleum-based polymers due to its mechanical strength being similar to that of commonly used petroleum-based polymers. However, PLA suffers from low elongation at break, low softening point, and easy degradation under high temperature and humidity conditions, which limits its applications, especially in high temperature and humidity environments.

[0004] To address the aforementioned problems, common solutions include blending modification, nanocompositing, plasticizing, and stereocompositing. Among these solutions, stereocompositing is considered the simplest and most economical. Stereocomposites are a class of polymers with unique properties formed by mixing components with the same composition but different stereochemical structures. Due to the existence of two optical isomers, left-handed and right-handed, certain monomers result in synthesized polymers with different forms, such as left-handed, right-handed, and racemic. Left-handed polymers and their enantiomers, right-handed polymers, can achieve alternating layer-by-layer molecular chain arrangement and close packing through strong hydrogen bonding, forming crystals different from homogeneous crystals, thus exhibiting characteristics such as rapid crystallization, high melting point, and heat resistance. Studies have shown that poly(L-lactic acid) and poly(D-lactic acid) can be stereocomposited to form stereocomposite polylactic acid, which possesses excellent thermal stability, with a melting temperature reaching 230℃, approximately 50℃ higher than homogeneous poly(L-lactic acid) and poly(D-lactic acid). It also exhibits good mechanical properties and hydrolysis resistance, further expanding the application range of polylactic acid.

[0005] For stereocomposite polylactic acid (PLA), the traditional preparation method involves blending equal amounts of poly(L-lactic acid) and poly(D-lactic acid). However, according to research progress on the regulation of PLA-based material properties and applications through stereocomposite blending (Jing Zhanxin, Kuang Qian, Li Guangrui, Guo Shirui, Lü Ruixue. Engineering Plastics Applications. December 2023), when the molecular weights of PLA and D-lactic acid are small, their molecular chain segments have good mobility, which is conducive to stereocomposite blending. However, when the molecular weights of PLA and D-lactic acid are large, their molecular chain mobility and regularity decrease, which is not conducive to stereocomposite blending, resulting in homogeneous crystallization of PLA and D-lactic acid, leading to a decrease in the stereocomposite blending rate. Moreover, in equal-amount blending, it is difficult to achieve uniform mixing of PLA and D-lactic acid.

[0006] To address the aforementioned issues, research progress on the control of polylactic acid (PLA)-based material properties and applications using stereocomposite methods was published in the December 2023 issue of *Engineering Plastics Applications*. Common solutions included adding heterogeneous nucleating agents and molecular structure design. Specifically, molecular structure design involved preparing PLA stereocomposite block copolymers, adding topological structures, and adding flexible polymer blocks. The preparation of PLA stereocomposite block copolymers involved preparing block copolymers of poly(L-lactic acid) and poly(D-lactic acid). These block copolymers could form single crystals, which tended to aggregate to form spherical stereocomposite crystals. Simultaneously, the two polymeric segments of poly(L-lactic acid) and poly(D-lactic acid) in the block copolymer exhibited strong stereocomposite effects, thus ensuring high stereocomposite crystallinity and, further, a high melting temperature. Currently, in order to improve the stereocomposite crystallinity of stereocomposite polylactic acid, the applicant also adopts the solution of preparing stereoblock copolymers of polylactic acid, and obtains block copolymers of poly-L-lactic acid and poly-D-lactic acid through copolymerization.

[0007] However, the applicant encountered the following problems when preparing the block copolymer of poly-L-lactic acid and poly-D-lactic acid:

[0008] First, the research and structural performance characterization of stereoblock polylactic acid prepared by ring-opening polymerization. Wang Rui. Master's thesis of Beijing Institute of Fashion Technology. Published in December 2015. The two chiral enantiomers of lactic acid, L-lactic acid and D-lactic acid, are copolymerized and formed stereocomposite polylactic acid through hydrogen bonding in the macromolecular chain. In this structure, when the isomer content is not in equimolar ratio, such as when the poly-L-lactic acid chain segment is longer than the poly-D-lactic acid chain segment, microphase separation is very likely to occur in the prepared stereocomposite polylactic acid. That is, poly-L-lactic acid aggregate phase and stereocomposite polylactic acid phase appear simultaneously. The excess poly-L-lactic acid not only makes it difficult to form homopolymer crystals, but also interferes with the process of ordered arrangement of stereocomposite crystals. Furthermore, it leads to a decrease in the melting temperature of the prepared stereocomposite polylactic acid. Therefore, it is necessary to strictly control the isomer content in the preparation process, which increases the difficulty of preparation.

[0009] Second, as mentioned in the research and structural performance characterization of the preparation of stereoblock polylactic acid by ring-opening polymerization, published in December 2015 by Wang Rui, Master's Thesis of Beijing Institute of Fashion Technology, the preparation process first uses lauryl alcohol and stannous octoate as catalysts to prepare poly-L-lactic acid. Then, poly-L-lactic acid is used as a macromolecular initiator to initiate subsequent reactions. Because the end groups with chain growth activity are more likely to be wrapped in the coil of the macromolecular chain, the relative content of active hydroxyl functional groups is reduced. This results in a widening of the molecular weight distribution of the prepared stereocomposite polylactic acid. The widening of the molecular weight distribution leads to a decrease in the stereocomposite crystallinity of the prepared stereocomposite polylactic acid, which further limits the effect of increasing the melting temperature of the prepared stereocomposite polylactic acid.

[0010] Third, as mentioned in the research progress on polylactic acid stereocomplexes published in Guangzhou Chemical Industry in March 2017 by Xiao Wei, Chen Weixing, Yang Minrui, Luo Chunyan, and Yang Jingjing, when preparing block copolymers of poly-L-lactic acid and poly-D-lactic acid using a two-step method, a small amount of unreacted L-lactide remains after the monomer polymerization reaction in the first step, hindering the polymerization reaction between D-lactide and polymer segments in the second step. This affects the molecular weight of the block copolymer and can also cause homogeneous crystallization, resulting in a decrease in the melting temperature of the prepared stereocomplex polylactic acid.

[0011] To address the aforementioned issues, the applicant employed a lactide melt polymerization method, promptly removing unreacted L-lactide after obtaining poly-L-lactic acid. Furthermore, during the crystallization process following the preparation of the block copolymer of poly-L-lactic acid and poly-D-lactic acid, the applicant accelerated the ordered arrangement of the stereocomposite crystals by controlling the crystallization temperature, adding a crosslinking agent, and adding microcrystalline cellulose as a nucleating agent. However, the following problems still exist:

[0012] First, as mentioned in the research on the molecular structure design and nucleation mechanism of polylactic acid nucleating agents (Xiao Wenhao, Doctoral Dissertation, Guangdong University of Technology, May 2025), cellulose is the most abundant renewable compound in nature. Cellulose nanofibers have good nucleation-promoting ability in polylactic acid. However, due to its polarity, the compatibility and dispersibility between natural cellulose nanofibers and polylactic acid are poor, further affecting the nucleation effect. Therefore, when the applicant uses microcrystalline cellulose as a nucleating agent, the poor compatibility and dispersibility between microcrystalline cellulose and polylactic acid also limit the effect on increasing the melting temperature of the prepared stereocomposite polylactic acid.

[0013] Second, when poly-L-lactic acid is used as a macromolecular initiator to initiate subsequent reactions, there is still a problem that the relative content of active hydroxyl functional groups is reduced. Therefore, the effect of increasing the melting temperature of the prepared stereocomposite polylactic acid is still limited.

[0014] Third, as mentioned in the research and structural performance characterization of the preparation of stereoblock polylactic acid by ring-opening polymerization, published in December 2015 by Wang Rui, Master's Thesis of Beijing Institute of Fashion Technology, it is found that when the reaction temperature is too high, polylactic acid may degrade, which can cause the prepared stereocomposite polylactic acid to turn yellow. Summary of the Invention

[0015] To address the shortcomings of existing technologies, this invention provides a method for preparing stereocomposite polylactic acid, which can significantly increase the melting temperature of the prepared stereocomposite polylactic acid and prevent the prepared stereocomposite polylactic acid from yellowing.

[0016] To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0017] A method for preparing stereocomposite polylactic acid specifically includes the following steps: mixing, preparing a composite nucleating agent, and block copolymerization;

[0018] The mixing process involves adding zinc chloride, polyethylene glycol, and anhydrous ethanol into a reaction apparatus, controlling the stirring speed of the reaction apparatus to 40-60 r / min, stirring at room temperature for 60-90 min, sealing the reaction apparatus and evacuating it, then raising the temperature inside the reaction apparatus to 60-65°C, and removing all anhydrous ethanol by vacuum rotary evaporation to obtain the mixture.

[0019] In the mixture, the number-average molecular weight of the polyethylene glycol is 1000;

[0020] The weight ratio of zinc chloride, polyethylene glycol, and anhydrous ethanol is 0.54-0.56:2-2.1:100-120;

[0021] To prepare the composite nucleating agent, microcrystalline cellulose and zinc chloride aqueous solution are added to a reaction apparatus. The stirring speed of the reaction apparatus is controlled at 40-60 r / min, and the mixture is stirred at room temperature for 15-20 min. After filtration, the filter residue is collected and added to the reaction apparatus along with polyethylene glycol aqueous solution. The stirring speed of the reaction apparatus is controlled at 40-60 r / min, and the mixture is stirred at room temperature for 15-20 min. After filtration, the filter residue is collected and dried to obtain the composite nucleating agent.

[0022] In the preparation of the composite nucleating agent, the particle size of the microcrystalline cellulose is 20 μm;

[0023] The zinc chloride aqueous solution has a weight concentration of 5%;

[0024] The weight concentration of the polyethylene glycol aqueous solution is 5%;

[0025] The number-average molecular weight of the polyethylene glycol in the aqueous solution is 1000.

[0026] The weight ratio of microcrystalline cellulose, zinc chloride aqueous solution, and polyethylene glycol aqueous solution is 10-12:90-110:90-110;

[0027] The block copolymerization process involves adding polymer-grade L-lactide to a reaction apparatus, along with stannous octoate, a monohydric alcohol, and a mixture. The reaction apparatus is then sealed and evacuated, and nitrogen gas is introduced to atmospheric pressure. The stirring speed of the reaction apparatus is controlled at 40-60 r / min. The temperature inside the reaction apparatus is then raised to 120-125°C. After the materials are completely melted, the temperature is raised to 140-145°C and stirred for 2-2.5 hours. The temperature is then raised to 160-165°C and stirred for 1-1.5 hours. The temperature inside the reaction apparatus is maintained constant, and a vacuum is applied to remove all unreacted polymer-grade L-lactide. D-lactide was added, and nitrogen gas was introduced to atmospheric pressure while maintaining the temperature inside the reaction apparatus. Polymer-grade D-lactide was added and stirred for 1-1.5 hours. The temperature inside the reaction apparatus was then raised to 190-200°C and stirred for 1-1.5 hours. The temperature inside the reaction apparatus was maintained, and a vacuum was drawn to remove all unreacted polymer-grade D-lactide. TAIC and DCP were added and stirred for 0.5-1 hours. A composite nucleating agent was added and stirred for 0.5-1 hours. The material was removed from the three-necked flask and the material temperature was lowered to 120-125°C. The mixture was allowed to stand and crystallize. The material temperature was then lowered to room temperature to obtain stereocomposite polylactic acid.

[0028] In the block copolymer, the optical purity of the polymer-grade L-lactide is 99.5%;

[0029] The weight of the stannous octoate is 0.02-0.03% of the weight of the polymer-grade L-lactide;

[0030] The monohydric alcohol is 1-dodecyl alcohol or n-octanol;

[0031] The molar amount of the monohydric alcohol is 0.31-0.33% of the molar amount of polymer-grade L-lactide;

[0032] The weight of the mixture is 0.15-0.16% of the weight of polymer-grade L-lactide;

[0033] The optical purity of the polymer-grade D-lactide is 99.6%.

[0034] The weight ratio of polymer-grade L-lactide to polymer-grade D-lactide is 100:105-120;

[0035] Preferably, the weight ratio of polymer-grade L-lactide to polymer-grade D-lactide is 100:110-115;

[0036] The chemical name of the TAIC is 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione;

[0037] The weight of TAIC is 0.1-1.0% of the weight of polymer-grade L-lactide;

[0038] Preferably, the weight of TAIC is 0.3-0.5% of the weight of polymer-grade L-lactide;

[0039] The chemical name of the DCP is dicumyl peroxide;

[0040] The weight of the DCP used is 0.1-1.0% of the weight of polymer-grade L-lactide;

[0041] Preferably, the weight of the DCP is 0.3-0.5% of the weight of the polymer-grade L-lactide;

[0042] The weight of the composite nucleating agent is 2-2.2% of the weight of polymer-grade L-lactide;

[0043] The static incubation and crystallization time is 0.5-3 hours;

[0044] Preferably, the static heat preservation and crystallization time is 1-1.5 hours.

[0045] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0046] (1) The method for preparing stereocomposite polylactic acid of the present invention uses a mixture in block copolymerization. The mixture is prepared by mixing zinc chloride and polyethylene glycol. In the mixture, zinc chloride and polyethylene glycol are combined through complexation. The effect of the mixture is as follows: First, polyethylene glycol in the mixture can act as an initiator, and zinc chloride can act as a synergistic catalyst with stannous octoate, thereby increasing the active center of the prepared poly-L-lactic acid. Polyethylene glycol improves the flexibility of poly-L-lactic acid, thereby improving the activity of poly-L-lactic acid and promoting the exposure of active hydroxyl functional groups. Second, polyethylene glycol can improve the dispersibility of zinc chloride in the reaction system. Third, after the polyethylene glycol and zinc chloride in the mixture are combined, they can act as active centers, reducing the binding time of polyethylene glycol and catalyst. Through the above-mentioned effects of the mixture, the molecular weight is controlled, ensuring that the prepared stereocomposite polylactic acid has a narrow molecular weight distribution, and further ensuring that the prepared stereocomposite polylactic acid has a high melting temperature.

[0047] (2) The method for preparing stereocomposite polylactic acid of the present invention uses a composite nucleating agent in crystallization. The composite nucleating agent is composed of microcrystalline cellulose, zinc ions, and polyethylene glycol. In the preparation of the composite nucleating agent, zinc ions are first bound to the surface of microcrystalline cellulose, and then polyethylene glycol is bound to the surface of microcrystalline cellulose through the interaction between zinc ions and polyethylene glycol. As disclosed in Wang Zan and Liu Hewen, Chinese Journal of Chemical Physics, April 2021, zinc ions can promote the crystallization of polylactic acid. In the present invention, microcrystalline cellulose and zinc ions can play a synergistic crystallization role, thereby accelerating the process of ordered arrangement of stereocomposite crystals and reducing homogeneous crystallization. Polyethylene glycol can play a role in improving the compatibility between the composite nucleating agent and polylactic acid. At the same time, zinc ions can also play a connecting role, thereby improving the bonding force between polyethylene glycol and microcrystalline cellulose.

[0048] (3) The method for preparing stereocomposite polylactic acid of the present invention can significantly increase the melting temperature of the prepared stereocomposite polylactic acid, raising the maximum melting temperature of polylactic acid to above 240°C, and the prepared stereocomposite polylactic acid does not exhibit yellowing. Attached Figure Description

[0049] Figure 1 The DSC spectrum of the stereocomposite polylactic acid prepared in Example 1;

[0050] Figure 2 The DSC spectrum of the stereocomposite polylactic acid prepared in Example 2;

[0051] Figure 3 The DSC spectrum of the stereocomposite polylactic acid prepared in Comparative Example 1;

[0052] Figure 4 The DSC spectrum of the stereocomposite polylactic acid prepared in Comparative Example 2;

[0053] Figure 5 The DSC spectrum of the stereocomposite polylactic acid prepared in Comparative Example 3;

[0054] Figure 6 The DSC spectrum of the stereocomposite polylactic acid prepared in Comparative Example 4 is shown. Detailed Implementation

[0055] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention are now described.

[0056] Example 1

[0057] This embodiment provides a method for preparing stereocomposite polylactic acid, specifically including the following steps:

[0058] 1. Mixing: Add zinc chloride, polyethylene glycol, and anhydrous ethanol to a three-necked flask equipped with a stirrer. Control the stirring speed of the stirrer to 40 r / min and stir for 60 min at room temperature. Seal the three-necked flask and evacuate it. Then raise the temperature inside the three-necked flask to 60°C and remove all anhydrous ethanol by vacuum rotary evaporation to obtain the mixture.

[0059] The number-average molecular weight of the polyethylene glycol is 1000;

[0060] The weight ratio of zinc chloride, polyethylene glycol, and anhydrous ethanol is 0.54:2:100;

[0061] 2. Preparation of composite nucleating agent: Microcrystalline cellulose and zinc chloride aqueous solution were added to a three-necked flask equipped with a stirrer. The stirring speed of the stirrer was controlled at 40 r / min, and the mixture was stirred at room temperature for 15 min. The mixture was filtered, and the filter residue was collected and added to a three-necked flask equipped with a stirrer along with polyethylene glycol aqueous solution. The stirring speed of the stirrer was controlled at 40 r / min, and the mixture was stirred at room temperature for 15 min. The mixture was filtered, and the filter residue was collected and dried to obtain the composite nucleating agent.

[0062] The microcrystalline cellulose has a particle size of 20 μm;

[0063] The zinc chloride aqueous solution has a weight concentration of 5%;

[0064] The weight concentration of the polyethylene glycol aqueous solution is 5%;

[0065] The number-average molecular weight of the polyethylene glycol in the aqueous solution is 1000.

[0066] The weight ratio of microcrystalline cellulose, zinc chloride aqueous solution, and polyethylene glycol aqueous solution is 10:90:90;

[0067] 3. Block copolymerization: Add polymer-grade L-lactide to a three-necked flask equipped with a stirrer, along with stannous octoate, monohydric alcohol, and other mixtures. Seal the flask and evacuate it. Purge with nitrogen to atmospheric pressure. Control the stirring speed of the stirrer to 40 rpm. Then, raise the temperature inside the flask to 120°C. After the materials in the flask have completely melted, raise the temperature to 140°C and stir for 2 hours. Then, raise the temperature to 160°C and stir for 1 hour. Maintain a constant temperature inside the flask. Evacuate the flask to remove all unreacted substances. Polymer-grade L-lactide was added, and the temperature inside the three-necked flask was kept constant. Nitrogen gas was introduced to atmospheric pressure, and polymer-grade D-lactide was added. The mixture was stirred for 1 hour, and then the temperature inside the three-necked flask was raised to 190°C and stirred for 1 hour. The temperature inside the three-necked flask was kept constant, and the three-necked flask was evacuated to remove all unreacted polymer-grade D-lactide. TAIC and DCP were added, and the mixture was stirred for 0.5 hours. A composite nucleating agent was added, and the mixture was stirred for 0.5 hours. The material was removed from the three-necked flask, and the temperature of the material was lowered to 120°C. The mixture was allowed to stand for 1 hour, and then the temperature of the material was lowered to room temperature to obtain stereocomposite polylactic acid.

[0068] The optical purity of the polymer-grade L-lactide is 99.5%.

[0069] The weight of the stannous octoate is 0.02% of the weight of the polymer-grade L-lactide;

[0070] The monohydric alcohol is 1-dodecyl alcohol;

[0071] The molar amount of the monohydric alcohol is 0.31% of the molar amount of polymer-grade L-lactide;

[0072] The weight of the mixture is 0.15% of the weight of polymer-grade L-lactide;

[0073] The optical purity of the polymer-grade D-lactide is 99.6%.

[0074] The weight ratio of polymer-grade L-lactide to polymer-grade D-lactide is 100:110;

[0075] The chemical name of the TAIC is 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione;

[0076] The weight of TAIC is 0.5% of the weight of polymer-grade L-lactide;

[0077] The chemical name of the DCP is dicumyl peroxide;

[0078] The weight of the DCP used is 0.5% of the weight of polymer-grade L-lactide;

[0079] The weight of the composite nucleating agent is 2% of the weight of polymer-grade L-lactide.

[0080] Example 2

[0081] This embodiment provides a method for preparing stereocomposite polylactic acid, specifically including the following steps:

[0082] 1. Mixing: Add zinc chloride, polyethylene glycol, and anhydrous ethanol to a three-necked flask equipped with a stirrer. Control the stirring speed of the stirrer to 60 r / min and stir for 90 min at room temperature. Seal the three-necked flask and evacuate it. Then raise the temperature inside the three-necked flask to 65°C and remove all anhydrous ethanol by vacuum rotary evaporation to obtain the mixture.

[0083] The number-average molecular weight of the polyethylene glycol is 1000;

[0084] The weight ratio of zinc chloride, polyethylene glycol, and anhydrous ethanol is 0.56:2.1:120;

[0085] 2. Preparation of composite nucleating agent: Microcrystalline cellulose and zinc chloride aqueous solution were added to a three-necked flask equipped with a stirrer. The stirring speed of the stirrer was controlled at 60 r / min, and the mixture was stirred at room temperature for 20 min. The mixture was filtered, and the filter residue was collected and added to a three-necked flask equipped with a stirrer along with polyethylene glycol aqueous solution. The stirring speed of the stirrer was controlled at 60 r / min, and the mixture was stirred at room temperature for 20 min. The mixture was filtered, and the filter residue was collected and dried to obtain the composite nucleating agent.

[0086] The microcrystalline cellulose has a particle size of 20 μm;

[0087] The zinc chloride aqueous solution has a weight concentration of 5%;

[0088] The weight concentration of the polyethylene glycol aqueous solution is 5%;

[0089] The number-average molecular weight of the polyethylene glycol in the aqueous solution is 1000.

[0090] The weight ratio of microcrystalline cellulose, zinc chloride aqueous solution, and polyethylene glycol aqueous solution is 12:110:110;

[0091] 3. Block copolymerization: Add polymer-grade L-lactide to a three-necked flask equipped with a stirrer, along with stannous octoate, monohydric alcohol, and other components. Seal the flask and evacuate it. Purge with nitrogen to atmospheric pressure. Control the stirring speed of the stirrer to 60 rpm. Then, raise the temperature inside the flask to 125°C. After the materials in the flask are completely melted, raise the temperature to 145°C and stir for 2.5 hours. Then, raise the temperature to 165°C and stir for 1.5 hours. Maintain a constant temperature inside the flask. Evacuate the flask to remove any unreacted components. The corresponding polymer-grade L-lactide was added, and the temperature inside the three-necked flask was kept constant. Nitrogen gas was introduced to atmospheric pressure, and polymer-grade D-lactide was added. The mixture was stirred for 1.5 hours. The temperature inside the three-necked flask was then raised to 200°C and stirred for 1.5 hours. The temperature inside the three-necked flask was kept constant, and the three-necked flask was evacuated to remove all unreacted polymer-grade D-lactide. TAIC and DCP were added and stirred for 1 hour. A composite nucleating agent was added and stirred for 1 hour. The material was removed from the three-necked flask and the temperature of the material was lowered to 125°C. The mixture was allowed to stand for 1.5 hours and then the temperature of the material was lowered to room temperature to obtain stereocomposite polylactic acid.

[0092] The optical purity of the polymer-grade L-lactide is 99.5%.

[0093] The weight of the stannous octoate is 0.03% of the weight of the polymer-grade L-lactide;

[0094] The monohydric alcohol is n-octanol;

[0095] The molar amount of the monohydric alcohol is 0.33% of the molar amount of polymer-grade L-lactide;

[0096] The weight of the mixture is 0.16% of the weight of polymer-grade L-lactide;

[0097] The optical purity of the polymer-grade D-lactide is 99.6%.

[0098] The weight ratio of polymer-grade L-lactide to polymer-grade D-lactide is 100:115;

[0099] The chemical name of the TAIC is 1,3,5-triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione;

[0100] The weight of TAIC is 0.3% of the weight of polymer-grade L-lactide;

[0101] The chemical name of the DCP is dicumyl peroxide;

[0102] The weight of the DCP used is 0.3% of the weight of polymer-grade L-lactide;

[0103] The weight of the composite nucleating agent is 2.2% of the weight of polymer-grade L-lactide.

[0104] Comparative Example 1

[0105] This comparative example provides a method for preparing stereocomposite polylactic acid, specifically based on the method for preparing stereocomposite polylactic acid in Example 1, by omitting the first mixing step and using an equal weight of polyethylene glycol instead of the mixture in the third block copolymerization step; the number average molecular weight of the polyethylene glycol is 1000.

[0106] The remaining technical solutions are the same as in Example 1.

[0107] Comparative Example 2

[0108] This comparative example provides a method for preparing stereocomposite polylactic acid, specifically based on the method for preparing stereocomposite polylactic acid in Example 1, by omitting the first mixing step and replacing the addition of the mixture with the addition of polyethylene glycol and zinc chloride respectively in the third block copolymerization step; the number average molecular weight of the polyethylene glycol is 1000; the total weight of polyethylene glycol and zinc chloride is 0.15% of the weight of polymer-grade L-lactide; the weight ratio of zinc chloride to polyethylene glycol is 0.54:2.

[0109] The remaining technical solutions are the same as in Example 1.

[0110] Comparative Example 3

[0111] This comparative example provides a method for preparing stereocomposite polylactic acid, specifically, based on the method for preparing stereocomposite polylactic acid in Example 1, the second step of preparing the composite nucleating agent is omitted, and an equal weight of microcrystalline cellulose is used instead of the composite nucleating agent in the third step of block copolymerization; the particle size of the microcrystalline cellulose is 20 μm.

[0112] The remaining technical solutions are the same as in Example 1.

[0113] Comparative Example 4

[0114] This comparative example provides a method for preparing stereocomposite polylactic acid. Specifically, based on the method for preparing stereocomposite polylactic acid in Example 1, the second step of preparing the composite nucleating agent is omitted, and the third step of block copolymerization is performed by adding microcrystalline cellulose and polyethylene glycol respectively instead of the composite nucleating agent. The particle size of the microcrystalline cellulose is 20 μm; the number average molecular weight of the polyethylene glycol is 1000; the total weight of microcrystalline cellulose and polyethylene glycol is 2% of the weight of polymer-grade L-lactide; and the weight ratio of microcrystalline cellulose to polyethylene glycol is 10:4.5.

[0115] The remaining technical solutions are the same as in Example 1.

[0116] Test Example 1

[0117] DSC scanning experiments were performed on the stereocomposite polylactic acid prepared in Examples 1-2 and Comparative Examples 1-4, and the obtained DSC spectra are shown in the figures below. Figure 1-6 ,Depend on Figure 1-6 It can be seen that the stereocomposite polylactic acid prepared in Examples 1 and 2 has a higher maximum melting temperature and an extremely low homogeneous crystallinity. The stereocomposite polylactic acid prepared in Comparative Examples 1 and 2 has a lower maximum melting temperature than that prepared in Example 1, but there is no problem of homogeneous crystallinity. The stereocomposite polylactic acid prepared in Comparative Examples 3 and 4 has a maximum melting temperature that is not much different from that prepared in Example 1, but all of them have a high degree of homogeneous crystallinity.

[0118] Test Example 2

[0119] The color of the stereocomposite polylactic acid prepared in Examples 1-2 and Comparative Examples 1-4 was observed to determine if there was any yellowing. The results are shown in Table 1.

[0120] Table 1

[0121]

[0122] The results above show that both the stereocomposite polylactic acid prepared in Comparative Example 1 and Comparative Example 2 exhibit a slight yellowing problem.

Claims

1. A method for preparing stereocomposite polylactic acid, characterized in that, Specifically, the following steps are included: Mixing, preparing composite nucleating agents, and block copolymerization; The mixing process involves adding zinc chloride, polyethylene glycol, and anhydrous ethanol into a reaction apparatus, stirring at room temperature, sealing the reaction apparatus and evacuating it, heating it to 60-65°C to remove all the anhydrous ethanol, and obtaining a mixture. In the mixture, the weight ratio of zinc chloride, polyethylene glycol, and anhydrous ethanol is 0.54-0.56:2-2.1:100-120; The preparation of the composite nucleating agent involves mixing microcrystalline cellulose and zinc chloride aqueous solution, stirring at room temperature, filtering, taking the filter residue, mixing it with polyethylene glycol aqueous solution, stirring at room temperature, filtering, taking the filter residue, and drying to obtain the composite nucleating agent. In the preparation of the composite nucleating agent, the weight ratio of microcrystalline cellulose, zinc chloride aqueous solution, and polyethylene glycol aqueous solution is 10-12:90-110:90-110; The block copolymerization process involves adding polymer-grade L-lactide to a reaction apparatus, along with stannous octoate, a monohydric alcohol, and a mixture. The reaction apparatus is sealed and evacuated. Nitrogen gas is introduced to atmospheric pressure, and the mixture is stirred. The temperature is raised to 120-125°C until the material is completely melted. The temperature is then raised to 140-145°C and stirred for 2-2.5 hours. The temperature is then raised to 160-165°C and stirred for 1-1.5 hours. The temperature is maintained constant, and a vacuum is applied to remove all unreacted polymer-grade L-lactide. While maintaining a constant temperature, nitrogen gas is introduced to atmospheric pressure, and polymer-grade D-lactide is added. The mixture is stirred for 1-1.5 hours, then heated to 190-200℃ and stirred for 1-1.5 hours. The temperature is maintained constant, and a vacuum is drawn to remove all unreacted polymer-grade D-lactide. TAIC and DCP are added and stirred until homogeneous. A composite nucleating agent is added and stirred until homogeneous. The material is cooled to 120-125℃ and allowed to stand for crystallization. The material temperature is then lowered to room temperature to obtain stereocomposite polylactic acid.

2. The method for preparing stereocomposite polylactic acid according to claim 1, characterized in that, In the mixture, the number-average molecular weight of the polyethylene glycol is 1000.

3. The method for preparing stereocomposite polylactic acid according to claim 1, characterized in that, In the preparation of the composite nucleating agent, the particle size of the microcrystalline cellulose is 20 μm; The zinc chloride aqueous solution has a weight concentration of 5%; The weight concentration of the polyethylene glycol aqueous solution is 5%; The number-average molecular weight of the polyethylene glycol in the aqueous solution is 1000.

4. The method for preparing stereocomposite polylactic acid according to claim 1, characterized in that, In the block copolymer, the optical purity of the polymer-grade L-lactide is 99.5%; The optical purity of the polymer-grade D-lactide is 99.6%. The monohydric alcohol is 1-dodecyl alcohol or n-octanol.

5. The method for preparing stereocomposite polylactic acid according to claim 1, characterized in that, In the block copolymerization, the weight of stannous octoate is 0.02-0.03% of the weight of polymer-grade L-lactide; The molar amount of the monohydric alcohol is 0.31-0.33% of the molar amount of polymer-grade L-lactide; The weight of the mixture is 0.15-0.16% of the weight of polymer-grade L-lactide; The weight ratio of polymer-grade L-lactide to polymer-grade D-lactide is 100:105-120.

6. The method for preparing stereocomposite polylactic acid according to claim 1, characterized in that, In the block copolymerization, the weight of TAIC is 0.1-1.0% of the weight of polymer-grade L-lactide; The weight of the DCP used is 0.1-1.0% of the weight of polymer-grade L-lactide.

7. The method for preparing stereocomposite polylactic acid according to claim 1, characterized in that, In the block copolymerization, the weight of the composite nucleating agent is 2-2.2% of the weight of the polymer-grade L-lactide.

8. The method for preparing stereocomposite polylactic acid according to claim 1, characterized in that, In the block copolymerization, the static incubation and crystallization time is 0.5-3 hours.

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