A spherical polylactic acid powder with a wide sintering window, its preparation method and application

Abstract

This invention proposes a spherical polylactic acid (PLA) powder with a wide sintering window, its preparation method, and its application. The preparation method includes the following steps: S1, mixing a poor solvent and PLA granules, adding the mixture to a reaction vessel, stirring, first heating, then holding at that temperature; S2, then cooling to 100-118°C, holding at that temperature for 30-60 minutes; S3, removing the product, washing and drying the product to obtain the spherical PLA powder. This invention is based on thermally induced phase separation, adding a spheroidization step to increase the crystallinity and sphericity of the PLA powder. The prepared spherical PLA powder has a particle size distribution of 10-120 μm and a bulk density of 0.3-0.45 g / cm³. 3 The powder possesses high fluidity, a wide sintering window, and excellent sintering performance, while retaining the excellent greenness and biocompatibility of polylactic acid. PLA parts formed by LPBF printing using the spherical PLA powder prepared in this invention exhibit good mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of advanced polymer powder preparation technology, and in particular to a spherical polylactic acid powder with a wide sintering window, its preparation method, and its application. Background Technology

[0002] Polylactic acid (PLA), a biodegradable and bioabsorbable polymer, is considered one of the most promising biopolymers due to its excellent biocompatibility and mechanical properties. PLA's superior characteristics not only avoid potential immune responses triggered by exogenous enzymes but also significantly reduce the risk of peri-implant inflammation, laying a crucial foundation for its advantages in tissue engineering scaffolds. In polymer scaffold manufacturing technology, laser powder bed fusion (LPBF) is renowned for its high degree of freedom in manufacturing due to the self-supporting properties of the powder bed. In recent years, an increasing number of polymers have been used in LPBF to break free from the limitations of device fabrication and target a wider range of complex applications. Combining PLA with LPBF makes it possible to manufacture PLA scaffolds that precisely match the geometry of bone defects in patients and meet specific biomechanical requirements.

[0003] LPBF (Liquid Polymer Packaging) is a complex process, and its forming quality depends not only on the precise control of process parameters but also fundamentally on the properties of the material itself. LPBF places stringent requirements on the properties of polymer powders; sintering behavior and powder layer spreading are the most critical aspects. A suitable sintering window (determined by the crystallization and melting initiation points measured by differential scanning calorimetry) is a prerequisite for the sintering capability of polymer powders, a characteristic unique to semi-crystalline polymers. For PLA, a wider sintering window generally facilitates stable and controllable sintering during LPBF. Good spreading behavior is key to obtaining a high-quality deposited layer; powder flow and spreading behavior are constrained by particle size distribution and shape. Spherical or near-spherical particles ensure low adhesion and friction between particles, enhancing powder flowability. Maintaining excellent flowability and packing density is essential for obtaining high-quality printed parts.

[0004] The properties of PLA powder are mainly affected by the preparation method. PLA powder preparation methods include mechanical grinding, spray drying, evaporation phase separation, and solvent precipitation. Among these, low-temperature mechanical grinding produces PLA powder with irregular morphology and poor flowability, resulting in uneven powder bed spreading and making multi-layer printing impossible. Atomization drying equipment can produce PLA microspheres, but due to rapid solvent evaporation, PLA has a high tendency to amorphize. Clearly, for polymer-based LPBF (Liquid-Based Printing), semi-crystalline polymers must have a sintering window for stable sintering; amorphous powders or powders without a sintering window cannot be used for LPBF printing. Emulsion evaporation is an effective method for preparing PLA submicron or nanocomposite microspheres. However, if used to prepare micron-sized PLA microspheres, the yield would be too low to be acceptable. In high-yield conditions, the microspheres are submicron or nanoscale, but their small size causes premature melting during the high-temperature preheating stage of LPBF processing, making them unusable for LPBF processing. Solvent precipitation is currently a common method for preparing LPBF-specific polyamide (such as PA11 and PA12) powders. PA11 or PA12 raw materials are heated to a high temperature in an autoclave and held for 15 minutes to form a homogeneous solution, followed by cooling (1.5~3℃ / min) to achieve phase separation and particle precipitation. However, PLA's inherent crystallization kinetics are very slow. Using this process to produce PLA powder will result in powder amorphization, the absence of a sintering window, and its unsuitability for LPBF printing. Furthermore, this process itself relies on a high-pressure environment to maintain the ethanol in a liquid state, posing a potential safety hazard during production.

[0005] Therefore, existing powder preparation technologies cannot provide PLA powder specifically for LPBF. Developing a spherical powder that is non-toxic, micron-sized, and has a wide sintering window is a prerequisite for combining PLA with LPBF and targeting bone tissue engineering. Summary of the Invention

[0006] In view of this, the present invention proposes a spherical polylactic acid (PLA) powder with a wide sintering window, its preparation method, and its applications. By using PLA as a non-toxic and undesirable solvent, and based on the thermally induced phase separation (TIPS) method, the solution concentration, composition, and temperature are controlled to allow the PLA-solvent binary system to stably separate into PLA-rich droplets in the metastable region via a nucleation-growth mechanism. The addition of a spheroidizing process increases the crystallinity and sphericity of the PLA powder. The particle size is controlled by the stirring rate. Finally, cooling precipitates a particle size distribution of 10–120 μm and a bulk density of 0.3–0.45 g / cm³. 3Spherical PLA powder with high flowability (angle of repose 27~34°) and a wide sintering window exhibits excellent sintering performance, meeting the stringent requirements of laser powder bed fusion (LPBF) technology for PLA powder. Furthermore, it retains the excellent environmental friendliness and biocompatibility of PLA. PLA parts printed using the spherical PLA powder prepared by this invention via LPBF possess good mechanical properties.

[0007] The technical solution of this invention is implemented as follows: In a first aspect, the present invention provides a method for preparing spherical polylactic acid powder with a wide sintering window, comprising the following steps: S1, mixing a poor solvent and PLA granules, adding the mixture to a reaction vessel, stirring, heating first, and then holding at the temperature; S2. Then cool down to 100~118℃ and keep warm for 30~60 minutes; S3. Take out the product, wash and dry the product to obtain the spherical polylactic acid powder.

[0008] This invention utilizes thermodynamic induction to induce a suitable concentration of polylactic acid (PLA)-solvent system to undergo nucleation-growth in the metastable region, resulting in PLA-rich droplets. After Ostwald ripening and droplet coalescence, PLA microspheres solidify and precipitate when the PLA saturation limit within the droplets is exceeded. By controlling the spheroidization time to regulate the crystallinity and morphology of the powder, and by controlling the stirring rate to control the particle size distribution, particle sizes ranging from 10 to 120 μm and bulk densities from 0.3 to 0.45 g / cm³ can be obtained. 3 High fluidity (angle of repose 27~34) ° This invention prepares spherical polylactic acid powder with excellent sintering properties, while retaining the excellent greenness and biocompatibility of polylactic acid.

[0009] Thermally induced phase separation (TIPS) occurs in the PLA-solvent binary system, where the system spontaneously separates into two coexisting liquid phases with different compositions due to thermodynamic instability. The concentration composition of the PLA-solvent binary system is within a suitable range. Under controlled concentration and temperature, the system is confined to cross the double-nodal line in its binary phase diagram and enter the metastable region. The nucleation-growth mechanism followed by phase separation in the metastable region is a prerequisite for obtaining spherical PLA powder.

[0010] If the system concentration and temperature are not properly controlled, the system will enter the spinoline region in the phase diagram, placing it in a thermodynamically unstable region. Phase separation will primarily follow the spinolysis mechanism. Any slight change in composition will decrease the system's free energy, thus triggering rapid phase separation. Under the combined effects of PLA's slow crystallization kinetics and rapid phase separation, interpenetrating, co-continuous lamellar or network structures will form. As the system temperature decreases, this irregular structure is retained, resulting in irregularly shaped PLA particles. Step S1 ensures that the PLA-solvent binary system enters the metastable region, achieving stable phase separation.

[0011] Under normal cooling, after TIPS (twist-inducing process) occurs, a large number of discrete PLA-rich droplets are formed under stirring and shearing. These droplets are not perfectly spherical. A sphericity process is introduced to improve the droplet sphericity. Holding at 100–118 °C for 30–60 min is selected, as this temperature range represents the fastest PLA crystallization rate, ensuring excellent crystallinity of the PLA powder and thus providing a wide sintering window. Simultaneously, at this temperature, the PLA chain segments have sufficient mobility, and the droplet viscosity is low. During this period, the droplets undergo Ostwald ripening (small droplets diffuse towards larger droplets; to minimize surface energy, the droplets undergo a self-sphericization process under interfacial tension for a certain period) or coalescence (under stirring, some small droplets coalesce to form larger spherical droplets). Under Ostwald ripening or coalescence, the droplets gradually improve their sphericity. When the saturation limit in the PLA-rich droplets is exceeded, nucleation and growth into spherical microparticles occur under supercooling.

[0012] The added spheroidizing process enables PLA powder to have both good sphericity and excellent crystallinity, giving PLA powder both excellent flowability and wide sintering window characteristics.

[0013] At the same time, the selected poor solvent is also a plasticizer. By inserting between polymer chains, it weakens the interaction forces between chains (van der Waals forces), making the chains easier to move, thereby making hard and brittle PLA soft and tough, and increasing the processability of PLA.

[0014] Based on the above technical solution, the stirring rate is further specified as 50~300 r / min.

[0015] This invention controls the particle size distribution of PLA powder by controlling the stirring rate.

[0016] The low stirring rate results in insufficient shear force, and the newly generated PLA-enriched phase droplets cannot be effectively dispersed. Due to Brownian motion, they undergo uncontrollable collisions and mergers, eventually forming powder particles with wide particle size distribution, irregular shape, and severe agglomeration.

[0017] Excessive stirring rate introduces excessive shear energy, causing droplets to be excessively broken into submicron particles and significantly increasing the free energy within the system, inducing secondary agglomeration of the powder.

[0018] Secondly, 300 r / min is the upper limit of rotational speed that can be achieved by reaction vessel equipment with a typical capacity of liters.

[0019] Based on the above technical solutions, the reaction vessel further includes temperature regulation, high-temperature sealing, and stirring functions.

[0020] The average pressure inside the reaction vessel is 0~0.1 MPa, which reduces safety hazards during the preparation process.

[0021] Based on the above technical solution, the heating in step S1 further includes heating to 170-200°C at a rate of 2-5°C / min.

[0022] Based on the above technical solution, the heating in step S1 further includes heating from room temperature to 170-200°C at a rate of 2-5°C / min.

[0023] 170~200℃ is the saturation temperature of PLA in unsuitable solvents, and the saturation temperature does not exceed the boiling point of the unsuitable solvent.

[0024] Based on the above technical solution, the heat preservation time in step S1 is further 30~90 minutes.

[0025] The holding time is 30-90 minutes to ensure that PLA can be fully dissolved, and a homogeneous solution is formed in the reaction vessel under the action of stirring and shearing.

[0026] Based on the above technical solutions, furthermore, the cooling rate in step S2 is 0.3~1℃ / min.

[0027] The liquid-liquid phase separation process in the PLA-solvent binary system entering the metastable region of its binary phase diagram is slow. In order to allow the system to remain in the metastable region for a sufficient time, the cooling process must be carried out slowly.

[0028] If the cooling rate is too low, the preparation efficiency will be low; if the cooling rate is too high, the system will quickly cross the metastable region and enter the thermodynamically unstable region, triggering the spindle decomposition mechanism, which will easily form irregular plate-like particles.

[0029] Based on the above technical solutions, the diameter of the PLA granules is further specified to be 4-5 mm.

[0030] Based on the above technical solutions, the undesirable solvent further includes any one of triethyl citrate, tributyl citrate, methyl oleate, glyceryl triacetate, or cyclohexanedicarboxylate.

[0031] The unsuitable solvent selected in this invention is an ester reagent that is mild on the human body and has good biodegradability. The boiling point of the unsuitable solvent is 200~290℃, and it is analytical grade with a purity of 99.5%. It has a plasticizing effect and can improve the hardness and brittleness of PLA, making it soft and tough.

[0032] Thermally induced phase separation requires the use of a poor solvent to prevent PLA from dissolving at room temperature. This ensures that the poor solvent transforms into a good solvent for PLA after being heated to a high temperature, allowing for complete dissolution. After cooling, the solvent gradually becomes a poor solvent for PLA again, thus achieving phase separation.

[0033] Spheroidization occurs after phase separation and must be carried out at a high temperature; good solvents cannot be used for spheroidization.

[0034] Based on the above technical solution, further, taking the sum of the mass percentages of the defective solvent and PLA granules as 100%, it includes 10~30wt% PLA granules and 70~90wt% defective solvent.

[0035] PLA concentration is a key factor affecting particle morphology and particle size. Concentration directly changes the thermodynamic state and kinetic phase separation pathway of the system, resulting in PLA powders with different properties.

[0036] When the PLA concentration is too low, phase separation mainly follows the spinolysis mechanism, with most particles exhibiting irregular shapes and only a small number of spherical particles being produced.

[0037] Excessive PLA concentration leads to the generation of more droplets during phase separation. With the addition of stirring, the probability of collisions between droplets increases significantly, resulting in droplet aggregation and the formation of larger, irregularly shaped powder particles.

[0038] Based on the above technical solution, step S3 further includes removing the product when the temperature drops to below 65°C.

[0039] After cooling, PLA undergoes a melting-recrystallization process, which eliminates any redundant thermal history that the raw material may have contained, making it more conducive to LPBF printing.

[0040] The product can be removed when the temperature inside the reaction vessel drops to 65°C or below, because PLA is difficult to crystallize or has very low crystallization efficiency at temperatures of 65°C or below.

[0041] Alcohol can be used for cleaning. Ester reagents are soluble in alcohol, while PLA is insoluble in alcohol at room temperature. Alcohol can be used to clean the PLA effectively. Alcohol is highly volatile and will not introduce a second phase, making it an economical and efficient cleaning solution.

[0042] The drying temperature should not exceed 70℃ to prevent the PLA powder from vitrifying during the drying process, and the powder should be completely dried in 48 hours.

[0043] Secondly, the present invention provides spherical polylactic acid powder with a wide sintering window prepared by the preparation method described above.

[0044] Thirdly, the present invention also provides an application of the spherical polylactic acid powder in laser powder bed melting.

[0045] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention uses PLA and poor solvents as raw materials, and based on thermally induced phase separation, through Ostwald ripening and droplet coalescence spheroidization steps, to obtain particles with a size distribution of 10~120 μm and a bulk density of 0.3~0.45 g / cm³. 3 High fluidity (angle of repose 27~34) ° The spherical polylactic acid powder with a wide sintering window is beneficial for uniform powder spreading during the LPBF process.

[0046] (2) The present invention significantly improves the crystallinity of the powder by spheroidizing and holding it at the temperature range (100~118℃) where PLA crystallization is fastest, forming a wide sintering window (110~148℃), thus ensuring stable and controllable sintering during the LPBF process.

[0047] (3) By adjusting the stirring rate, holding time and cooling program, the present invention can accurately control the particle size, crystallinity and morphology of powder, and has wide process adaptability and good repeatability.

[0048] (4) The present invention uses ester solvents with high boiling point (200~290℃) which have plasticizing effect, making hard and brittle PLA soft and tough. The interaction between molecular chains is weakened, which reduces the melt viscosity during sintering and is more conducive to LPBF processing.

[0049] (5) In the preparation process of the present invention, PLA undergoes a melting-recrystallization process, which eliminates the original thermal history of the raw material and is more conducive to LPBF forming. The printed PLA parts have good mechanical properties and have application prospects in bone tissue engineering. Attached Figure Description

[0050] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0051] Figure 1 This is a flowchart illustrating the preparation of spherical polylactic acid powder according to the present invention; Figure 2 SEM image of the spherical polylactic acid powder prepared in Example 1; Figure 3 The sintering window of the spherical polylactic acid powder prepared in Example 3; Figure 4 This is a drawing of a PLA part prepared by LPBF in Example 2. Detailed Implementation

[0052] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0053] This invention provides a spherical polylactic acid powder with a wide sintering window and a method for preparing the same. The preparation method includes the following steps: Weigh the poor solvent and PLA granules separately using a balance, mix them, and then add them to a reaction vessel (equipped with a stirrer); Turn on the stirrer, set the heating program, heat to 170~200℃, and keep warm for 30~90 minutes; After the heat preservation is completed, the cooling rate is adjusted to an average cooling rate of 0.3~1℃ / min. During the cooling process, a droplet spheroidizing step (spheroidizing time 30-60 min) is added at the temperature point where PLA is prone to crystallization (100~118℃) to ensure that the powder has good crystallinity and sphericity. After cooling is complete, remove the product and wash and filter it 3 or more times until the residual solvent is washed away. The washed powder is placed in a vacuum drying oven at 50~70℃ and dried for 48 hours. The sintering window is then tested, and the powder can be used for LPBF printing.

[0054] The flowchart of the method for preparing spherical polylactic acid powder with a wide sintering window provided by the present invention is as follows: Figure 1 As shown.

[0055] In the following specific embodiments, room temperature refers to the temperature inside the reaction vessel, typically between 25°C and 35°C.

[0056] In the following specific implementation, the PLA was purchased from NatureWorks, Inc., USA, and its grade was 4032D.

[0057] Example 1 This embodiment provides a spherical polylactic acid powder with a wide sintering window and its preparation method. The raw materials include 80 wt% methyl oleate and 20 wt% PLA granules, with the sum of the mass percentages being 100%. The mass of the PLA granules is 400 g.

[0058] The methyl oleate was 99.5% analytical grade, and the average diameter of the PLA granules was 4.5 mm.

[0059] The preparation method includes the following steps: S1. Mix methyl oleate and PLA granules and pour them into a 5L reaction vessel. S2. Set the stirring speed of the reaction vessel to 150 r / min, and heat it from room temperature to 180℃ at a rate of 3℃ / min, and then hold it at that temperature for 50 min. S3. After the heat preservation is completed, the cooling rate is adjusted by setting the cooling time through the program. The cooling rate is maintained at 0.6℃ / min throughout the entire cooling process. S4. When the temperature drops to the point where PLA is prone to crystallization (110℃), keep it at that temperature for 40 minutes to carry out the droplet spheroidization process, ensuring that the powder has good crystallinity and sphericity. S5. Wait for the temperature inside the reactor to drop below 65°C, remove the product, wash and filter it three times with alcohol as a cleaning agent, place the washed powder in a vacuum drying oven at 60°C and dry for 48 hours to obtain spherical polylactic acid powder.

[0060] SEM images of the spherical PLA powder prepared in Example 1 were observed, and the results are as follows: Figure 2 As shown, by Figure 2 It is known that the PLA powder prepared by the present invention is mainly composed of spherical or near-spherical particles, and a very small number of particles exhibit adhesion due to unavoidable collisions of droplets.

[0061] Example 2 This embodiment provides a spherical polylactic acid powder with a wide sintering window and its preparation method. The raw materials include 90 wt% triethyl citrate and 10 wt% PLA granules, with the sum of the mass percentages being 100%. The mass of the PLA granules is 300 g.

[0062] The triethyl citrate was 99.5% analytical grade, and the average diameter of the PLA granules was 4 mm.

[0063] The preparation method includes the following steps: S1. Mix triethyl citrate and PLA granules and pour into a 5L reaction vessel. S2. Set the stirring speed of the reaction vessel to 50 r / min, and heat it from room temperature to 200℃ at a rate of 2℃ / min, and then hold it at that temperature for 30 min. S3. After the heat preservation is completed, the cooling rate is adjusted by setting the cooling time through the program. The cooling rate is maintained at 0.3℃ / min throughout the entire cooling process. S4. When the temperature drops to the point where PLA easily crystallizes (100℃), keep it at that temperature for 30 minutes to carry out the droplet spheroidization process to ensure that the powder has good crystallinity and sphericity. S5. Wait for the temperature inside the reactor to drop below 65°C, remove the product, wash and filter it three times with alcohol as a cleaning agent, place the washed powder in a vacuum drying oven at 50°C and dry for 48 hours to obtain spherical polylactic acid powder.

[0064] Based on the sintering window, a printing size of 10×10×10 mm was achieved at a laser powder bed temperature of 115℃. 3 and the dimensions are 20×20×20 mm 3 Triply periodic minimal surfaces (TPMS) porous scaffolds, such as Figure 4 As shown, the compressive strength measured by the electronic universal testing machine can reach 22.14 MPa.

[0065] Example 3 This embodiment provides a spherical polylactic acid powder with a wide sintering window and its preparation method. The raw materials include 70 wt% triacetin and 30 wt% PLA granules, with the sum of the mass percentages being 100%. The mass of the PLA granules is 500 g.

[0066] The triacetin was 99.5% analytical grade, and the average diameter of the PLA granules was 5 mm.

[0067] The preparation method includes the following steps: S1. Mix triacetyl ester and PLA granules and pour into a 5L reaction vessel; S2. Set the stirring speed of the reaction vessel to 300 r / min, and heat it from room temperature to 170℃ at a rate of 5℃ / min, and then hold it at that temperature for 90 min. S3. After the heat preservation is completed, the cooling rate is adjusted by setting the cooling time through the program. The cooling rate is maintained at 1℃ / min throughout the entire cooling process. S4. When the temperature drops to the point where PLA is prone to crystallization (118℃), keep it at that temperature for 60 minutes to carry out the droplet spheroidization process to ensure that the powder has good crystallinity and sphericity. S5. Wait for the temperature inside the reactor to drop below 65°C, remove the product, wash and filter it three times with alcohol as a cleaning agent, place the washed powder in a vacuum drying oven at 70°C and dry for 48 hours to obtain spherical polylactic acid powder.

[0068] Differential scanning calorimetry (DSC) measurements yielded the sintering window as follows: Figure 3 As shown, the temperature ranges from 113.4 to 147.6℃.

[0069] Example 4 The difference between this embodiment and Embodiment 1 is that tributyl citrate is used instead of triethyl citrate.

[0070] Example 5 The difference between this embodiment and Example 1 is that cyclohexanedicarboxylate is used instead of triethyl citrate.

[0071] Example 6 The difference between this embodiment and Embodiment 1 is that the temperature for droplet spheroidization in step S4 is 100°C.

[0072] Example 7 The difference between this embodiment and Embodiment 1 is that the temperature at which the droplets spheroidize in step S4 is 118°C.

[0073] Example 8 The difference between this embodiment and Embodiment 1 is that the heat preservation time in step S4 is 30 minutes.

[0074] Example 9 The difference between this embodiment and Embodiment 1 is that the heat preservation time in step S4 is 60 minutes.

[0075] Comparative Example 1 The difference between this comparative example and Example 1 is that droplet spheroidization is not performed in this comparative example, i.e., step S4 is not included.

[0076] Comparative Example 2 The difference between this comparative example and Example 1 is that the temperature for droplet spheroidization in step S4 is too high, at 130°C.

[0077] Comparative Example 3 The difference between this comparative example and Example 1 is that the droplet spheroidization temperature in step S4 is too low, at 90°C.

[0078] Comparative Example 4 The difference between this comparative example and Example 1 is that the droplet spheroidization time in step S4 is too long, that is, the heat preservation time is 2 hours.

[0079] Comparative Example 5 The difference between this comparative example and Example 1 is that the droplet spheroidization time in step S4 is too short, that is, the heat preservation time is 20 minutes.

[0080] Comparative Example 6 The difference between this comparative example and Example 1 is that in step S3, the cooling rate is too slow, at 0.1℃ / min.

[0081] Comparative Example 7 The difference between this comparative example and Example 1 is that in step S3, the cooling rate is too fast, at 3℃ / min.

[0082] Performance testing 16. The spherical PLA powders prepared in Examples 1-9 and Comparative Examples 1-7 were subjected to performance testing. This included measuring the particle size of the spherical PLA powders using a laser particle size analyzer, measuring the bulk density using a Hall effect flowmeter, measuring the angle of repose using a powder angle of repose tester, and measuring the sintering window using a differential scanning calorimeter. The test results are shown in Table 1.

[0083] Table 1 Performance test results of spherical PLA powders in Examples 1-9 and Comparative Examples 1-7

[0084] 17. Based on the sintering window, the spherical PLA powders prepared in Examples 1-9 and Comparative Examples 1-7 were printed using LPBF to form shapes with dimensions of 20×20×20 mm. 3 A three-period minimal curved porous support, a tensile specimen of model 1BB in GB / T 1040.2-2022, with dimensions of 80×10×4 mm. 3 The bending specimens were tested for compressive strength, tensile strength and bending strength using an electronic universal testing machine. The test results are shown in Table 2.

[0085] Table 2. Mechanical property test results of spherical PLA powders in Examples 1-9 and Comparative Examples 1-7

[0086] As shown in Table 1, this invention utilizes thermodynamic induction to induce a suitable concentration of polylactic acid (PLA)-solvent system to undergo nucleation-growth in the metastable region, resulting in PLA-rich droplets. After Ostwald ripening and droplet coalescence, PLA microspheres solidify and precipitate when the PLA saturation limit within the droplets is exceeded. By controlling the spheroidization time to regulate the crystallinity and morphology of the powder, and by controlling the stirring rate to control the particle size distribution, particle sizes ranging from 10 to 120 μm and bulk densities from 0.3 to 0.45 g / cm³ can be obtained. 3 High fluidity (angle of repose 27~34) ° ) and spherical polylactic acid powder with a wide sintering window.

[0087] As shown in Table 2, the PLA parts formed by LPBF printing using the spherical polylactic acid powder prepared by the present invention have good mechanical properties.

[0088] As can be seen from the comparison between Example 1 and Comparative Example 1, without the Ostwald curing and droplet coalescence spheroidization process, the PLA powder has an irregular morphology, resulting in a large angle of repose, poor flowability, uneven powder spreading, reduced interlayer density and surface quality in LPBF printing, and worsened mechanical properties of the parts.

[0089] A comparison of Example 1 and Comparative Example 2 shows that when the droplet spheroidization temperature is too high and the system viscosity is too low, the surface tension is insufficient to effectively spheroidize the droplets and maintain their shape, leading to easy aggregation and elongation. Cooling and precipitation result in the formation of deformed agglomerates, a wider powder particle size distribution, and poorer flowability, which is detrimental to LPBF processing.

[0090] As can be seen from the comparison between Example 1 and Comparative Example 3, if the spheroidization temperature is too low, the PLA particles will begin to solidify and precipitate. If the viscosity of the system is too high, only a very small portion of the droplets will complete spheroidization, resulting in poor overall sphericity of the powder.

[0091] As can be seen from the comparison between Example 1 and Comparative Example 4, if the droplet spheroidization time is too long, it will lead to excessive Ostwald ripening of small droplets, forming larger droplets. At the same time, due to the stirring effect, the probability of collision between droplets increases, forming a large number of irregularly aggregated particles.

[0092] As can be seen from the comparison between Example 1 and Comparative Example 5, the spheroidization of droplets is not instantaneous; a large number of droplets in the reaction vessel require sufficient time to spheroidize. If the droplet spheroidization time is too short, the spheroidization process of the system is incomplete, resulting in some irregularly shaped particles in the powder product, affecting the powder flowability and hindering LPBF processing.

[0093] A comparison of Example 1 and Comparative Example 6 shows that if the cooling rate is too slow, PLA has ample time for its molecular chains to fold and align. The crystallization process of PLA will preferentially or dominantly separate the phases, and droplets may shrink or coalesce before spheroidization due to internal inhomogeneity, resulting in poor powder sphericity. Secondly, if the high-temperature stage is prolonged, the molecular weight of PLA may decrease due to the extended thermal history.

[0094] As can be seen from the comparison between Example 1 and Comparative Example 7, under rapid cooling, the phase separation process produces violent cyclotron decomposition, resulting in extremely irregular PLA powder morphology and the lowest packing density. The irregular morphology of the particles reduces the powder's flowability and uniformity of powder spreading, and the existing flaky particles are also prone to sintering splashes and pores.

[0095] In summary, this invention proposes a spherical polylactic acid (PLA) powder with a wide sintering window, its preparation method, and its applications. By using PLA as a non-toxic and undesirable solvent, and based on the thermally induced phase separation (TIPS) method, the solution concentration, composition, and temperature are controlled to allow the PLA-solvent binary system to stably separate into PLA-rich droplets in the metastable region via a nucleation-growth mechanism. The addition of a spheroidizing process increases the crystallinity and sphericity of the PLA powder. The particle size is controlled by the stirring rate. Finally, cooling precipitates a particle size distribution of 10–120 μm and a bulk density of 0.3–0.45 g / cm³. 3 Spherical PLA powder with high flowability (angle of repose 27~34°) and a wide sintering window exhibits excellent sintering performance, meeting the stringent requirements of laser powder bed fusion (LPBF) technology for PLA powder. Furthermore, it retains the excellent environmental friendliness and biocompatibility of PLA. PLA parts printed using the spherical PLA powder prepared by this invention via LPBF possess good mechanical properties.

[0096] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for producing a spherically shaped polylactic acid powder having a wide sintering window, characterized by, The method comprises the following steps: S1, mixing a poor solvent and PLA granules, then adding them into a reaction container, keeping stirring, first increasing temperature, and then keeping temperature; S2, then decreasing temperature to 100-118℃, keeping temperature for 30-60min; S3, taking out the product, washing and drying the product, and obtaining the spherical polylactic acid powder.

2. The production method according to claim 1, wherein The temperature increasing in the step S1 comprises increasing temperature to 170-200℃ at a rate of 2-5℃ / min.

3. The production method according to claim 1, wherein The keeping temperature time in the step S1 is 30-90min.

4. The production method according to claim 1, wherein The decreasing temperature rate in the step S2 is 0.3-1℃ / min.

5. The production method according to claim 1, wherein The stirring rate is 50-300r / min.

6. The production method according to claim 1, wherein The poor solvent comprises any one of triethyl citrate, tributyl citrate, methyl oleate, glyceryl triacetate or cyclohexane dimethylate.

7. The production method according to claim 1, wherein The sum of the mass percentages of the poor solvent and the PLA granules is 100%, comprising 10-30wt% of the PLA granules and 70-90wt% of the poor solvent.

8. The production method according to claim 1, wherein The step S3 comprises taking out the product when the temperature is decreased to below 65℃ at a rate of 0.3-1℃ / min.

9. The spherical polylactic acid powder with a wide sintering window prepared by the preparation method in any one of claims 1-8.

10. The application of the spherical polylactic acid powder in claim 9 in laser powder bed melting.

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