Polylactic acid spherulite and preparation method thereof

Abstract

The application belongs to the technical field of crystal form compounds, and particularly relates to a spherulite of polylactic acid and a preparation method thereof. The application provides a spherulite of polylactic acid, which is a new crystal form of polylactic acid, and has characteristic peaks at diffraction angles 2 θ 17.9+0.2°, 20.2+0.2°, 23.5+0.2° and 32.7+0.2° in Cu-K alpha radiation X-ray powder diffraction pattern, and does not contain crystal water or other crystallization solvents, has concentrated particle size distribution and good fluidity, and is more suitable for industrial processing. The application also provides a preparation process of the spherulite, which comprises fully dissolving polylactic acid by using chloromethane, dropping into an alcohol solvent, making polylactic acid precipitate in the alcohol system, and drying to obtain the spherulite. The preparation method is simple, efficient and cost-effective, and has important significance for the application of polylactic acid.

Description

Technical Field

[0001] This invention belongs to the field of crystalline compound technology, specifically relating to polylactic acid spherulites and their preparation method. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Polylactic acid (PLA, as shown in Formula I), also known as polylactide, belongs to the polyester family. Its structural characteristic is that a single lactic acid molecule contains one hydroxyl group and one carboxyl group. When multiple lactic acid molecules come together, the -OH group dehydrates and condenses with the -COOH group of other molecules, and vice versa, forming a polymer called polylactic acid. The structural formula of polylactic acid is as follows:

[0004]

[0005] Polylactic acid (PLA) is produced without pollution, and its products are biodegradable, allowing for natural recycling, making it an ideal green polymer material. It has excellent biocompatibility and can be used as packaging materials, fibers, and nonwovens, finding wide application in clothing (underwear, outerwear), industry (construction, agriculture, forestry, papermaking), and healthcare.

[0006] Polylactic acid (PLA) exists in various crystalline forms, each with significantly different properties. Currently known PLA crystalline forms include α, β, γ, neo-c, and an intermediate phase (strictly speaking, the intermediate phase is not considered a crystalline form). Crystalline form α is the most stable and common, capable of crystallizing from solution or by melting and cooling at temperatures above 120 °C. It belongs to the orthorhombic crystal system. Crystalline form α also gives rise to two different metastable crystalline forms, α' and α''. Crystalline form α' has a similar chain structure to α, but its molecular chains are more disordered, and it can transform into α crystals at temperatures above 120 °C. Crystalline form α'' yields nanorod-like crystals under high-pressure CO2 conditions and can transform into α crystals at even higher temperatures. Crystal form β is stretched into polylactic acid fibers at high stretch ratio and high temperature, and its thermal stability is worse than that of crystal form α; crystal form γ is obtained by epitaxial crystallization of polylactic acid on hexamethylbenzene, and has better order; the new crystal form c is formed by mixing poly(L-lactic acid) and poly(D-lactic acid) in an equimolar ratio, and has better heat resistance; the meso phase is formed under low temperature and low pressure CO2.

[0007] Patent CN112940296A discloses a method for preparing polylactic acid nanoparticles. According to the specification, the method involves uniformly spraying a dichloromethane solution of polylactic acid into a constant-temperature reaction vessel, followed by constant-temperature standing, nitrogen purging, two-gradient reduced-pressure cooling to recover the solvent, and washing with water to obtain polylactic acid nanoparticles. However, no further research was conducted on the crystal form of polylactic acid. Furthermore, the preparation process of nano-polylactic acid is quite complicated, and washing with water makes it difficult to dry, which may result in the presence of water of crystallization.

[0008] Patent CN117180501A reports the application of polylactic acid microspheres in the preparation of tissue regeneration products, and discloses a method for preparing polylactic acid microspheres. The method involves adding a polylactic acid solution dropwise to an emulsifier solution and stirring, followed by centrifugation to obtain polylactic acid microspheres. This preparation method requires the introduction of at least one emulsifier, which adds a subsequent removal step.

[0009] Polylactic acid (PLA), as a polycrystalline polymer, still faces many challenges in crystal form research that require further resolution. For example, crystallization under high temperature and pressure is energy-intensive and does not align with green and sustainable development principles. Melt crystallization and crystallization under high stretch ratios are cumbersome preparation methods, undoubtedly increasing the difficulty and cost of formulation processing. Different crystallization conditions significantly impact crystal form; variations in crystallinity greatly affect PLA's heat resistance, gloss, tensile strength, and other application properties. Summary of the Invention

[0010] To address the shortcomings of existing methods for preparing polylactic acid (PLA) crystalline compounds, this invention provides a more easily implemented preparation process. Therefore, this invention utilizes a solution-precipitation cooling crystallization method. PLA is dissolved in a volatile solvent, chloromethane, and then added dropwise to an alcohol-based solvent to precipitate PLA. The sample is then filtered and vacuum-dried to obtain a new crystalline form, specifically PLA spherulites. These spherulites solve the problems of PLA's filamentous morphology and difficulty in preservation, while also addressing the yield issue in PLA preparation. This provides technical support for the widespread application of PLA, significantly reduces the difficulty of R&D and production in the PLA formulation processing field, and undoubtedly greatly improves industrial production efficiency while reducing production costs.

[0011] Based on the above-mentioned technical achievements, the present invention provides the following technical solution:

[0012] In a first aspect, a polylactic acid spherulite is provided, characterized in that, in the X-ray powder diffraction pattern of the polylactic acid spherulite under Cu-Kα radiation, at a diffraction angle of 2... θ Characteristic peaks are observed at 17.9±0.2°, 20.2±0.2°, 23.5±0.2° and 32.7±0.2°.

[0013] Secondly, a method for preparing the polylactic acid spherulites described in the first aspect is provided, comprising the following steps:

[0014] S1: Prepare a polylactic acid chloromethane solution; keep the crystallizer at a low constant temperature, add alcohol reagents and stir continuously in the subsequent reaction, add part of the polylactic acid chloromethane solution dropwise to the alcohol solvent, and induce crystallization by ultrasonic treatment;

[0015] S2: Continue to add the remaining polylactic acid chloromethane solution dropwise. After the addition is complete, allow crystals to grow. Filter, retain the solid portion, and dry to obtain the final product.

[0016] The above step S1 also has the following preferred implementation:

[0017] Furthermore, in the above polylactic acid chloromethane solution, the dosage ratio of polylactic acid to chloromethane is 2g:15~50mL; the chloromethane is selected from one of dichloromethane and trichloromethane or a combination of both in any proportion; the preparation method is as follows: add polylactic acid to chloromethane solvent, stir continuously until the solid is completely dissolved, and obtain a clear solution, which is the polylactic acid chloromethane solution.

[0018] In some embodiments verified by the present invention, the chloromethane is dichloromethane, and in this embodiment, the dosage ratio of polylactic acid to dichloromethane is 2g:30~50mL.

[0019] In other embodiments, the chloromethane is trichloromethane, and in this embodiment, the dosage ratio of polylactic acid to dichloromethane is 2g:15~25mL.

[0020] Furthermore, in the above preparation method, the reactions from the addition of the alcohol reagent in S1 to the crystallization in S2 all occur in the crystallizer, and the reaction process is maintained at a low temperature, which is 5~25 °C. This low-temperature reaction environment can be achieved by a low-temperature thermostat. After the alcohol reagent is added to the crystallizer, continuous stirring is started until all the polylactic acid in the chloromethane solution is added dropwise and dissolved to equilibrium. Taking magnetic stirring as an example, an appropriate stirring speed is 300~700 rpm.

[0021] Furthermore, the alcohol solvent is a low-carbon alcohol solvent, and even more specifically, it is one or a combination of methanol, ethanol, propanol, or isopropanol. The volume of the alcohol solvent is 1.4 to 1.8 times the volume of the polylactic acid chloromethane solution prepared in S1.

[0022] Furthermore, the frequency of the ultrasound is 20~40 kHz, and the ultrasound treatment lasts for 15~30 min.

[0023] Furthermore, in the step of adding a portion of polylactic acid chloromethane solution to the alcohol solvent, the amount of polylactic acid chloromethane solution added at this time accounts for 30% to 60%; during the addition, the dropping rate is controlled at 4 to 6 mL / min.

[0024] The above step S2 also has the following preferred implementation:

[0025] Furthermore, the dripping rate of the remaining polylactic acid in chloromethane solution is 1~3 mL / min.

[0026] Furthermore, the crystal growth time is 30-60 minutes.

[0027] Furthermore, the drying methods can include vacuum drying, freeze drying, desiccant drying, thermal radiation drying, etc. In a specific example of the present invention, a vacuum drying oven is used for drying at a temperature of 35~45℃.

[0028] Compared with the prior art, the beneficial effects of the present invention are:

[0029] 1. This invention provides a method for preparing a new crystalline form of polylactic acid (PLA), solving the problems of PLA's filamentous morphology and difficulty in preservation. The use of a volatile solvent reduces solvent residue on the PLA crystals, eliminates the presence of water of crystallization or other crystallization solvents, and reduces drying time. Furthermore, it significantly improves the yield and purity of PLA, providing technical support for large-scale PLA production, significantly reducing the difficulty of R&D and production in the PLA formulation processing field, facilitating industrialization, and saving production costs.

[0030] 2. In this invention, the liquid-to-solid ratio of chloromethane solvent to polylactic acid and the dropping rate are key process parameters that have an important impact on the control of crystallization kinetics. A low liquid-to-solid ratio and a fast dropping rate will lead to a deterioration in crystal morphology. Attached Figure Description

[0031] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0032] Figure 1 The X-ray powder diffraction pattern of the polylactic acid spherulites obtained in Example 1;

[0033] Figure 2 The results of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of the polylactic acid spherulites obtained in Example 1 are shown.

[0034] Figure 3 X-ray powder diffraction patterns of the polylactic acid raw materials used in Examples 1-7;

[0035] Figure 4 This is a photograph of the polylactic acid spherulites obtained in Example 1;

[0036] Figure 5The images show actual photos of the polylactic acid raw materials used in Examples 1-7;

[0037] Figure 6 This is a microscopic image of the polylactic acid spherulites obtained in Example 1.

[0038] Figure 7 This is a particle size distribution diagram of the polylactic acid spherulites obtained in Example 1. Detailed Implementation

[0039] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0040] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0041] In the context of this specification, the word "including" is considered to mean "particularly including". It should not be interpreted as "consisting of only".

[0042] To enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments. The polylactic acid raw material used in the following embodiments was obtained through commercial channels, and its XRD diffraction pattern is shown below. Figure 3 As shown, the polylactic acid raw material is in the form of filaments, and its appearance is as follows: Figure 5 As shown.

[0043] Example 1

[0044] In this embodiment, a polylactic acid spherulite is provided, such as... Figure 1 As shown, in the X-ray powder diffraction pattern of this crystal form under Cu-Kα radiation, at a diffraction angle of 2... θ Characteristic peaks are found at 17.9°, 20.2°, and 23.5°.

[0045] The preparation method of the above-mentioned polylactic acid spherulites is as follows: 2 g of polylactic acid was dissolved in 50 mL of dichloromethane. 70 mL of methanol was added to a crystallizer, and the temperature was controlled at 20 ℃ with continuous stirring using a magnetic stirrer. After the system stabilized, the dichloromethane polylactic acid solution was added dropwise to the methanol at a rate of 6 mL / min. After 5 min of dropwise addition, ultrasonic induction crystallization was initiated at a frequency of 30 kHz for 30 min. Then, the remaining dichloromethane solution was added dropwise at a rate of 1 mL / min. After the addition was complete, the crystals were kept at a constant temperature for 30 min. The crystals were then filtered and dried under vacuum at 40 ℃ to obtain the final product.

[0046] The polylactic acid (PLA) spherulites obtained in this embodiment had a yield of 96.5% and a purity of 99.94% as determined by gas chromatography. Differential scanning calorimetry (DSC) showed a sharp endothermic peak at 181°C. Combined with TGA analysis results, this indicates that the product is not amorphous but rather a crystal with a fixed melting point. In the above preparation method, the PLA used as a raw material is in filamentous form at room temperature (e.g., ...). Figure 5 The intertwined and interwoven filamentous structure of polylactic acid (PLA) results in aggregates with numerous voids, leading to a significantly low bulk density. This low bulk density obviously increases logistical and warehousing pressures, directly increasing costs. Furthermore, to meet the process requirements of subsequent formulation manufacturing, these filamentous PLA fibers must be pulverized. However, mechanical pulverization itself has several unavoidable drawbacks: firstly, the pulverization process releases heat, and PLA is highly sensitive to temperature, easily triggering degradation reactions in high-temperature environments; secondly, achieving completely consistent processing results during pulverization leads to significant differences in particle size distribution and physicochemical properties between different batches, indicating poor batch-to-batch stability. Such fluctuations in stability pose significant challenges to process control in subsequent formulation production, potentially causing product quality fluctuations and further increasing quality control costs and risks during production. This filamentous structure also slows down the crystallization rate of PLA during melt processing, resulting in uneven degradation rates, which affects its controllable degradation requirements in the medical or packaging fields.

[0047] The polylactic acid spherulite powder prepared by the method in this embodiment is in the form of uniform particles at room temperature (e.g., Figure 4 Under a microscope, the above-mentioned crystalline powder appears to be nearly spherical. Figure 6 ), and the size distribution of spherical particles is relatively concentrated ( Figure 7 ), less than 90% of the particles have a diameter of less than 319.746 μm.

[0048] Example 2

[0049] In this embodiment, another method for preparing the polylactic acid spherulites described in Example 1 is provided, comprising the following steps:

[0050] 2 g of polylactic acid (PLA) was dissolved in 40 mL of dichloromethane. 68 mL of ethanol was added to a crystallizer, and the mixture was stirred continuously with a magnetic stirrer at 25 °C. After the system stabilized, the dichloromethane PLA solution was added dropwise to the ethanol at a rate of 4 mL / min. After 6 min of addition, ultrasonic crystallization was induced at a frequency of 40 kHz for 15 min. The remaining dichloromethane solution was then added dropwise at a rate of 2 mL / min. After the addition was complete, the crystals were kept at a constant temperature for 45 min. The crystals were filtered and dried under vacuum at 40 °C. X-ray powder diffraction confirmed that the obtained PLA spherulites were consistent with the crystal form of Example 1, with a yield of 93.8% and a purity of 99.95% as determined by gas chromatography.

[0051] Example 3

[0052] In this embodiment, another method for preparing the polylactic acid spherulites described in Example 1 is provided, comprising the following steps:

[0053] 2 g of polylactic acid (PLA) was dissolved in 30 mL of dichloromethane. 42 mL of isopropanol was added to a crystallizer, and the mixture was stirred continuously with a magnetic stirrer at 15°C. After the system stabilized, the dichloromethane PLA solution was added dropwise to the isopropanol at a rate of 5 mL / min. After 2 min of addition, ultrasonic induction crystallization was initiated at a frequency of 35 kHz for 20 min. The remaining dichloromethane solution was then added dropwise at a rate of 3 mL / min. After the addition was complete, the crystals were kept at a constant temperature for 60 min. The crystals were filtered and dried under vacuum at 40°C. X-ray powder diffraction confirmed that the obtained PLA spherulites were consistent with the crystal form of Example 1, with a yield of 95.7% and a purity of 99.97% as determined by gas chromatography.

[0054] Example 4

[0055] In this embodiment, another method for preparing the polylactic acid spherulites described in Example 1 is provided, comprising the following steps:

[0056] 2 g of polylactic acid (PLA) was dissolved in 40 mL of dichloromethane. 60 mL of methanol was added to a crystallizer, and the mixture was stirred continuously with a magnetic stirrer at 5 °C. After the system stabilized, the dichloromethane PLA solution was added dropwise to the methanol at a rate of 6 mL / min. After 4 min of addition, ultrasonic crystallization was induced at a frequency of 20 kHz for 25 min. The remaining dichloromethane solution was then added dropwise at a rate of 2 mL / min. After the addition was complete, the crystals were kept at a constant temperature for 45 min. The crystals were filtered and dried under vacuum at 40 °C. X-ray powder diffraction confirmed that the obtained PLA spherulites were consistent with the crystal form of Example 1, with a yield of 96.5% and a purity of 99.94% as determined by gas chromatography.

[0057] Example 5

[0058] In this embodiment, another method for preparing the polylactic acid spherulites described in Example 1 is provided, comprising the following steps:

[0059] 2 g of polylactic acid (PLA) was dissolved in 15 mL of chloroform. 21 mL of methanol was added to a crystallizer, and the mixture was stirred continuously with a magnetic stirrer at 25 °C. After the system stabilized, the chloroform PLA solution was added dropwise to the methanol at a rate of 4 mL / min. After 2 min of addition, ultrasonic crystallization was induced at a frequency of 30 kHz for 30 min. The remaining chloroform solution was then added dropwise at a rate of 1 mL / min. After the addition was complete, the crystals were kept at a constant temperature for 40 min. The crystals were filtered and dried under vacuum at 40 °C. X-ray powder diffraction confirmed that the obtained PLA spherulites were consistent with the crystal form of Example 1, with a yield of 93.5% and a purity of 99.96% as determined by gas chromatography.

[0060] Example 6

[0061] In this embodiment, another method for preparing the polylactic acid spherulites described in Example 1 is provided, comprising the following steps:

[0062] 2 g of polylactic acid (PLA) was dissolved in 25 mL of chloroform. 45 mL of ethanol was added to a crystallizer, and the mixture was stirred continuously with a magnetic stirrer at 10°C. After the system stabilized, the chloroform PLA solution was added dropwise to the ethanol at a rate of 5 mL / min. After 3 min of addition, ultrasonic induction crystallization was initiated at a frequency of 25 kHz for 20 min. The remaining chloroform solution was then added dropwise at a rate of 1 mL / min. After the addition was complete, the crystals were kept at a constant temperature for 30 min. The crystals were filtered and dried under vacuum at 40°C. X-ray powder diffraction confirmed that the obtained PLA spherulites were consistent with the crystal form of Example 1, with a yield of 97.8% and a purity of 99.94% as determined by gas chromatography.

[0063] Example 7

[0064] In this embodiment, another method for preparing the polylactic acid spherulites described in Example 1 is provided, comprising the following steps:

[0065] 2 g of polylactic acid (PLA) was dissolved in 20 mL of chloroform. 32 mL of methanol was added to a crystallizer, and the mixture was stirred continuously with a magnetic stirrer at 20 °C. After the system stabilized, the chloroform PLA solution was added dropwise to the methanol at a rate of 6 mL / min. After 2 min of addition, ultrasonic crystallization was induced at a frequency of 40 kHz for 25 min. The remaining chloroform solution was then added dropwise at a rate of 3 mL / min. After the addition was complete, the crystals were kept at a constant temperature for 50 min. The crystals were filtered and dried under vacuum at 40 °C. X-ray powder diffraction confirmed that the obtained PLA spherulites were consistent with the crystal form of Example 1, with a yield of 98.1% and a purity of 99.96% as determined by gas chromatography.

[0066] Liquidity test

[0067] According to the static angle of repose determination method in the "Guidelines for Determination of Powder Flowability" issued by the National Pharmacopoeia Commission, the polylactic acid sample obtained in the embodiments of this invention was compared with the raw material, and the results are as follows:

[0068] Table 1. Angle of repose of polylactic acid spherulites obtained in Examples 1-7 and raw polylactic acid.

[0069]

[0070] According to the results in Table 1, the polylactic acid spherulites described in Examples 1-7 have a smaller angle of repose. This means that the spherulites have better flowability, lower interparticle frictional resistance, and are easier to uniformly fill the mold during processing. They are suitable for injection molding, extrusion, and other molding processes, effectively reducing energy consumption and improving production efficiency, especially for processing complex structures. Furthermore, improved flowability also means that the spherulites are less prone to clumping during transportation and mixing, allowing for uniform blending with other materials.

[0071] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A type of polylactic acid spherulite, characterized in that, The X-ray powder diffraction pattern of the polylactic acid spherulites under Cu-Kα radiation shows a diffraction angle of 2... θ Characteristic peaks are observed at 17.9±0.2°, 20.2±0.2°, 23.5±0.2°, and 32.7±0.2°. The preparation method of polylactic acid spherulites includes the following steps: S1: Prepare a polylactic acid (PLA) chloromethane solution, wherein the dosage ratio of PLA to chloromethane is 2g:15~50mL; keep the crystallizer at a low constant temperature, add an alcohol reagent and continuously stir during the subsequent reaction, add 30%~60% of the total volume of the PLA chloromethane solution to the alcohol solvent dropwise, and induce crystallization by ultrasonic treatment, wherein the volume of the alcohol solvent is 1.4~1.8 times the volume of the PLA chloromethane solution prepared in S1; the PLA chloromethane solution is prepared as follows: add PLA to the chloromethane solvent and stir continuously until the solid is completely dissolved to obtain a clear solution, which is the final product; The frequency of the ultrasound is 20~40 KHz, and the ultrasound treatment lasts for 15~30 min. The chloromethane is selected from one of dichloromethane and trichloromethane, or a combination of both in any proportion; The alcohol solvent is a low-carbon alcohol solvent, selected from one or a combination of methanol, ethanol, propanol or isopropanol; S2: Continue to add the remaining polylactic acid chloromethane solution dropwise. After the addition is complete, allow crystals to grow. Filter, retain the solid portion, and dry to obtain the final product.

2. The polylactic acid spherulites as described in claim 1, characterized in that, The chloromethane is dichloromethane, and the dosage ratio of polylactic acid to dichloromethane is 2g:30~50mL; Alternatively, the chloromethane may be trichloromethane, and the dosage ratio of polylactic acid to dichloromethane may be 2g:15~25mL.

3. The polylactic acid spherulites as described in claim 1, characterized in that, In the preparation method, the reactions from adding alcohol reagent in S1 to crystal growth in S2 all occur in the crystallizer, and the reaction process is maintained at a low temperature, which is 5~25℃.

4. The polylactic acid spherulites as described in claim 1, characterized in that, In step S1, the dropping rate of the polylactic acid chloromethane solution is 4~6 mL / min.

5. The polylactic acid spherulites as described in claim 1, characterized in that, In step S2, the dripping rate of the remaining polylactic acid in chloromethane solution is 1~3 mL / min.

6. The polylactic acid spherulites as described in claim 1, characterized in that, In step S2, the crystal growth time is 30~60 minutes.

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