Polylactic acid article post-treatment method and polylactic acid component

Abstract

The application discloses a kind of polylactic acid product post-processing method and polylactic acid component, wherein, polylactic acid product post-processing method includes the following steps, step one: the polylactic acid product is placed in the closed cavity with mixed gas, the mixed gas includes the carbon dioxide gas with volume fraction greater than 0 and less than or equal to 30%.Step two: the air pressure in closed cavity is kept in set range, and the set range is 0.08MPa to 0.12MPa.Closed cavity is heated, and polylactic acid product is converted from glass state to high elastic state.Step three: keep the air pressure of closed cavity in set range, and closed cavity is cooled, and polylactic acid product is returned from high elastic state to glass state, and polylactic acid component is formed.In the application, the chain segment movement ability is greatly increased after polylactic acid product is heated into high elastic state, and carbon dioxide molecules can interact with ester group of polylactic acid at this time, and plasticizing effect is realized.

Description

Technical Field

[0001] This invention relates to the field of material processing, and more particularly to a post-processing method for polylactic acid (PLA) products and PLA components. Background Technology

[0002] Polylactic acid (PLA), a biodegradable aliphatic polyester, has been widely used in medical polymer products such as surgical sutures, bone fixation devices, tissue engineering scaffolds, medical catheters, and repair membranes due to its excellent biocompatibility, non-toxicity, non-irritation, and good processing properties.

[0003] Currently, polylactic acid (PLA) granules are typically manufactured into PLA products through injection molding, extrusion, blow molding, spinning, and 3D printing processes.

[0004] However, polylactic acid (PLA) products manufactured through processes such as injection molding generally suffer from problems such as high residual internal stress, low and uneven crystallinity, and disordered molecular chain orientation. These directly result in significant defects in the mechanical properties, deformation resistance, and degradation performance of PLA products, making it difficult to meet the mechanical reliability requirements of medical products such as bone fixation devices. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention discloses a post-processing method for polylactic acid (PLA) products and PLA components, thereby improving the mechanical properties of PLA components.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A post-processing method for polylactic acid (PLA) products includes the following steps: Step 1: placing the PLA product in a closed cavity containing a mixed gas, wherein the mixed gas contains carbon dioxide gas with a volume fraction greater than 0 and less than or equal to 30%; Step 2: maintaining the gas pressure in the closed cavity within a set range, wherein the set range is 0.08 MPa to 0.12 MPa; heating the closed cavity, causing the PLA product to change from a glassy state to a highly elastic state; Step 3: maintaining the gas pressure in the closed cavity within the set range, cooling the closed cavity, causing the PLA product to return from the highly elastic state to the glassy state, forming a PLA component.

[0008] Furthermore, in the mixed gas, the volume fraction of carbon dioxide gas is greater than or equal to 10%.

[0009] Furthermore, in step two, the process of heating the sealed cavity is as follows: first heating, first heating and holding, second heating, and second heating and holding, with the second heating and holding temperature being 95 to 120 degrees Celsius.

[0010] Furthermore, the heating rate of the first heating is greater than the heating rate of the second heating, and the holding time of the first heating is less than the holding time of the second heating.

[0011] Furthermore, in step three, the process of cooling the sealed cavity is as follows: first cooling, first cooling and heat preservation, and second cooling. The first cooling and heat preservation temperature is 60 to 80 degrees Celsius.

[0012] Furthermore, the cooling rate of the first cooling step is 1 to 4 degrees Celsius per minute.

[0013] Furthermore, the mixed gas consists of an inert gas and a carbon dioxide gas.

[0014] Furthermore, before step one, the polylactic acid product is placed into the sealed cavity, then the sealed cavity is evacuated, and then the mixed gas is introduced into the sealed cavity to complete the gas replacement in the sealed cavity.

[0015] Furthermore, before the polylactic acid product is placed into the sealed cavity, the surface of the polylactic acid product is first cleaned.

[0016] A polylactic acid component is prepared by post-processing the polylactic acid product as described in any one of the above-mentioned methods.

[0017] Compared with existing technologies, the advantages of this invention are as follows: By placing polylactic acid (PLA) products in a closed cavity containing a mixed gas with a carbon dioxide volume fraction of less than or equal to 30%, and simultaneously controlling the gas pressure within the closed cavity within a set range, the cavity is first heated and then cooled. After the PLA product enters a highly elastic state upon heating, the chain segment mobility increases significantly, and the free volume expands substantially. At this point, carbon dioxide molecules can rapidly diffuse into the inter-chain gaps, undergoing sufficient physical interaction with the ester groups of PLA, thus achieving a plasticizing effect. PLA components formed after this process exhibit better mechanical properties. Attached Figure Description

[0018] Figure 1 This is a schematic flowchart of the post-processing method for polylactic acid products of the present invention; Figure 2 This is a photograph of a medical bone nail sample made of polylactic acid injection molding according to the present invention after being processed by the relevant steps in Example 1; Figure 3 This is a physical image of the medical bone nail sample made of polylactic acid injection molding according to the present invention after processing in the relevant steps of Comparative Example 1. Detailed Implementation

[0019] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] In the description of this invention, it should be noted that the terms "inner" and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0021] Currently, medical polylactic acid (PLA) products are typically manufactured using conventional processes such as injection molding, extrusion, blow molding, spinning, and 3D printing. However, the resulting PLA products often suffer from technical problems such as high residual internal stress, insufficient mechanical properties, and uncontrollable degradation rates, making it difficult to meet the requirements for PLA products as medical implants.

[0022] Therefore, this invention discloses a post-processing method for polylactic acid (PLA) products, used to prepare PLA components that meet the mechanical performance requirements for medical implants. The PLA components may specifically include intramedullary nails, suture anchors, and bone plates.

[0023] Before performing the post-processing method for polylactic acid (PLA) products according to the present invention, the following operations are performed: Using polylactic acid (PLA) granules, several PLA products are prepared through injection molding, extrusion, blow molding, spinning, or 3D printing. The PLA products can be shaped like intramedullary nails, wire anchors, bone plates, or any other medical implant. Simultaneously, a heating device with a closed cavity is prepared. This heating device includes a heating system, a ventilation system, and a pressure-maintaining system. The heating system is used to increase the temperature within the closed cavity, thereby heating the items within the closed cavity. The ventilation system is used to replace the gas within the closed cavity. The pressure-maintaining system is used to maintain the gas pressure within the closed cavity within a set range. The heating device also includes a hatch and a valve, with the opening area of ​​the valve being smaller than the opening area of ​​the hatch. Opening the hatch and / or the valve allows the closed cavity to communicate with the outside air. Similarly, closing the hatch and the valve simultaneously isolates the closed cavity from the outside air. When a pressure difference exists between the air pressure inside the sealed cavity and the outside air pressure, the valve opens, allowing the air pressure inside the sealed cavity to gradually approach the outside air pressure. The hatch opens primarily to place polylactic acid (PLA) products into the sealed cavity. The valves, hatches, heating systems, ventilation systems, and pressure-holding systems mentioned in this paragraph all utilize existing products; therefore, their detailed structures will not be elaborated upon here.

[0024] After the operator has prepared several polylactic acid (PLA) products, they open the hatch and place the PLA products into a closed cavity, ensuring that the PLA products within the closed cavity are not stacked or compressed. The hatch is then closed. Next, the PLA product post-processing method of this invention is executed.

[0025] like Figure 1 As shown, the post-processing method for polylactic acid products of the present invention includes the following steps in detail: Step 1: Place the polylactic acid product in a closed cavity containing a mixed gas containing carbon dioxide gas with a volume fraction greater than 0 and less than or equal to 30%. Specifically, a ventilation system is used to replace the air in the closed cavity with the mixed gas.

[0026] Step Two: The pressure within the sealed cavity is maintained within a set range using a pressure-holding system, specifically 0.10 ± 0.02 MPa. Next, the sealed cavity is heated using a heating system until the polylactic acid product transitions from a glassy state to a highly elastic state. During the heating process, the mixed gas expands due to heat, but the pressure within the sealed cavity remains stable within the set range thanks to the pressure-holding system.

[0027] Step 3: Continue using the pressure-holding system to maintain the air pressure in the sealed cavity within the set range. Simultaneously, cool the sealed cavity until the polylactic acid product returns from its elastic state to a glassy state, forming the polylactic acid component. During the cooling process of the sealed cavity, the mixed gas will contract upon cooling. Again, due to the presence of the pressure-holding system, the air pressure within the sealed cavity remains stable within the set range.

[0028] Step 4: After the polylactic acid component has cooled to room temperature, open the valve to balance the pressure inside and outside the sealed cavity with the outside air. Then, open the hatch and take out the polylactic acid component.

[0029] This invention primarily utilizes the interaction between carbon dioxide (CO2) in a mixed gas and the ester groups (-COO-) within polylactic acid (PLA) products to improve the mechanical properties of the PLA products. Details: Carbon dioxide and the ester groups of polylactic acid (PLA) do not react chemically at room temperature or under heating conditions. The interaction between them is physical, not chemical bonding or bond breaking. The carbon dioxide molecule has a significant quadrupole moment, while the carbonyl group in the PLA ester group carries a partially negative charge, and the carbon atom carries a partially positive charge. This results in dipole-quadrupole interactions and van der Waals forces between them.

[0030] At room temperature: Physical interactions already exist between carbon dioxide and the ester groups of polylactic acid. Carbon dioxide molecules can reversibly adsorb near the ester groups, but due to the glassy state of the polylactic acid molecular chains at room temperature (Tg approximately 60°C), where Tg refers to the glass transition temperature, the ester group segments are frozen, and the diffusion rate of carbon dioxide into the bulk is extremely slow, resulting in a macroscopic perception of "no reaction".

[0031] When the sealed cavity is heated, polylactic acid enters a highly elastic state, greatly increasing the mobility of the ester chain segments and significantly expanding the free volume. At this time, carbon dioxide molecules can rapidly diffuse into the gaps between the molecular chains, engaging in sufficient physical interaction with the ester groups to achieve a plasticizing effect.

[0032] Carbon dioxide exhibits unique properties in response to polylactic acid (PLA): While CO2 also plasticizes various polymers such as polystyrene and polycarbonate, a phenomenon widely studied in the foaming industry, PLA's response to CO2 is significantly different. First, the extremely high density of ester groups in PLA molecular chains (one ester group for every two carbon atoms) provides abundant specific interaction sites for CO2. Second, the large free volume of PLA's amorphous regions allows CO2 molecules to diffuse more easily into the PLA chains. CO2 acts like countless tiny "molecular-level lubricants," penetrating the gaps between PLA molecular chains, expanding space, shielding attraction, and reducing friction, making the originally "clumsy" molecular chains more "flexible," thus allowing for smoother movement under external forces or heat and eliminating internal stress. Third, when PLA is in a highly elastic state, the solubility of CO2 is approximately 1 to 3 wt% (mass percentage concentration), placing PLA and CO2 in a thermodynamically stable region between "forming a continuous lubricating layer" and "supersaturated precipitation."

[0033] Meanwhile, not all ester-containing polymers can achieve good mechanical properties using the polylactic acid post-processing method of this invention. This is because other ester-based polymers, such as polyethylene terephthalate (PET) with rigid chains and small free volume, and polycaprolactone (PCL) with overly flexible chains and high solubility, cannot simultaneously meet the requirements of sufficient plasticization and defect-free foaming. Polylactic acid (PLA), on the other hand, exhibits the best performance and has an irreplaceable structural compatibility.

[0034] The reason why the air pressure in the sealed cavity needs to be maintained within a set range of 0.10±0.02MPa during the post-processing of polylactic acid (PLA) products is that 0.08MPa is the lower threshold for forming an effective molecular lubrication layer; below 0.08MPa, insufficient lubrication by carbon dioxide molecules will occur. Meanwhile, 0.12MPa is the safe upper limit to avoid oversaturation precipitation during the cooling process; above 0.12MPa, the PLA components may "foam," equivalent to a foaming process. Details: The interaction between carbon dioxide and the ester groups of polylactic acid (PLA) is a dipole-quadrupole physical interaction, and the dissolution process follows Henry's law. At very low pressures (less than 0.08 MPa), the number of carbon dioxide molecules dissolving into the amorphous region per unit time is insufficient to form a continuous "lubricating layer" between the molecular chains. The free volume increase is minimal, and the dipole attraction between the PLA ester chains is not effectively shielded. Therefore, the activation energy of PLA ester chain segment motion remains higher than the thermal motion energy. Even in the elastic state, PLA molecular chains cannot achieve sufficient conformational relaxation and internal stress relief, resulting in insufficient carbon dioxide dissolution within PLA.

[0035] Excessive pressure (greater than 0.12 MPa) causes the amount of carbon dioxide dissolved in polylactic acid at high temperatures to far exceed the equilibrium solubility at that temperature. During subsequent cooling, the solubility of carbon dioxide actually increases (an exothermic dissolution process). However, if the material was already supersaturated before cooling, the decrease in solubility (or temperature fluctuations) will drive rapid homogeneous nucleation and phase separation of carbon dioxide, forming a large number of submicron-sized pores—i.e., "microfoaming." These micropores disrupt the continuous phase structure of the material, becoming stress concentration points, and simultaneously disturb the regular arrangement of near-surface molecular chains, macroscopically manifesting as decreased mechanical properties and surface ripples.

[0036] In the post-processing of polylactic acid (PLA) products, the volume fraction of carbon dioxide in the mixed gas must be less than or equal to 30% because the dissolution of carbon dioxide in PLA is an exothermic process, and its solubility decreases with increasing temperature (van ter Hoff equation). When PLA is in a highly elastic state, when the volume fraction of carbon dioxide exceeds 30%, the amount of carbon dioxide dissolved approaches the saturation limit at that temperature. During subsequent cooling, the solubility of carbon dioxide increases with decreasing temperature, but because the amount of carbon dioxide dissolved is already high, PLA will enter a supersaturated state. Classical nucleation theory shows that after the supersaturation exceeds the critical value, the bubble nucleation barrier decreases sharply, and the nucleation rate increases exponentially. A large amount of carbon dioxide rapidly and homogeneously nucleates and undergoes phase separation, forming submicron-sized voids—i.e., microfoaming defects. These micropores disrupt the ordered arrangement of molecular chains, becoming stress concentration points, leading to decreased mechanical properties and surface ripples. Similarly, when the volume fraction of carbon dioxide in the mixed gas is greater than 30%, foaming is more likely to occur.

[0037] In step four, since the pressure setting range (0.08 to 0.12 MPa) within the sealed cavity is almost equal to atmospheric pressure (approximately 0.1 MPa), the pressure difference between the inside and outside of the sealed cavity is minimal. Under these conditions, pressure equilibrium can be achieved simply by opening the valve to allow the sealed cavity to communicate with the atmosphere. During the pressure equilibrium process, carbon dioxide molecules dissolved inside the polylactic acid slowly diffuse out of the material surface along the concentration gradient, rather than undergoing violent precipitation driven by a sudden pressure change. This gentle diffusion process effectively prevents rapid carbon dioxide vaporization from forming microcracks or deformations inside the product, ensuring the structural integrity and dimensional accuracy of the final product.

[0038] This invention involves placing polylactic acid (PLA) products in a closed cavity containing a mixed gas, wherein the volume fraction of carbon dioxide gas in the mixed gas is less than or equal to 30%. Simultaneously, the gas pressure within the closed cavity is controlled within a set range, and the cavity is first heated and then cooled. After the PLA product enters a highly elastic state upon heating, the chain segment mobility increases significantly, and the free volume expands substantially. At this point, carbon dioxide molecules can rapidly diffuse into the inter-chain gaps, undergoing sufficient physical interaction with the ester groups of the PLA, thus achieving a plasticizing effect.

[0039] In the post-processing method for polylactic acid products of the present invention, many technical features, such as the volume fraction of carbon dioxide gas and the method of heating in a closed cavity, have multiple implementations. Below, for each of these technical features, including the volume fraction of carbon dioxide gas, one implementation method is mainly selected for detailed description. This embodiment is referred to as "this embodiment." Other implementations of the features, including the volume fraction of carbon dioxide gas, are referred to as "other embodiments," which are briefly described below.

[0040] In this embodiment, the volume fraction of carbon dioxide gas in the gas mixture is greater than or equal to 10%. That is, the volume fraction of carbon dioxide gas in the gas mixture is greater than or equal to 10% and less than or equal to 30%.

[0041] When the volume fraction of carbon dioxide gas is below 10%, the amount of carbon dioxide dissolved in polylactic acid (PLA) products is insufficient to reach the critical concentration required for effective swelling and plasticization. The increase in free volume between PLA molecular chains is limited, and the reduction in activation energy for chain segment movement is not significant. Even in the highly elastic state, the resistance to chain segment movement in PLA remains relatively high, making it difficult to achieve sufficient chain segment relaxation and deorientation, resulting in incomplete elimination of internal stress.

[0042] When the volume fraction of carbon dioxide gas exceeds 30%, polylactic acid (PLA) absorbs far more carbon dioxide than usual under high-temperature, high-elasticity conditions, forming a high-concentration homogeneous system. However, the solubility of carbon dioxide in the polymer decreases with increasing temperature. This thermodynamic instability drives the rapid aggregation and nucleation of carbon dioxide molecules, leading to phase separation and vaporization from the polymer matrix, resulting in numerous irregular voids—microfoaming defects—at the microscale.

[0043] In this invention, carbon dioxide plays a dual role in the post-processing method of polylactic acid (PLA) products. On the one hand, carbon dioxide molecules can diffuse into the PLA chains, interacting specifically with the PLA ester groups through the quadrupole moment of the carbon dioxide molecules, increasing free volume, reducing chain segment movement resistance, and promoting internal stress release and crystallization regulation. On the other hand, carbon dioxide is also a foaming agent at high temperatures. When the volume fraction of carbon dioxide in the mixed gas is too high, micropores or even macropores are easily formed inside the PLA product, compromising its density and mechanical properties. This invention limits the volume fraction of carbon dioxide in the mixed gas to 10% to 30% because below 10% the swelling and plasticizing effect is insufficient, and above 30% the risk of microfoaming increases significantly. When the mixed gas is entirely carbon dioxide, under high-temperature conditions of 0.08 to 0.12 MPa, the PLA product post-processing process will evolve into a foaming process, which cannot meet the medical load-bearing requirements of PLA components.

[0044] When the volume fraction of carbon dioxide gas is between 0 and 10%, the number of carbon dioxide molecules dissolved into the polylactic acid (PLA) chains is sufficient, significantly increasing the free volume, shielding inter-chain interactions, and greatly reducing the activation energy of chain segment movement. Simultaneously, the amount of carbon dioxide dissolved within this concentration range does not exceed the thermodynamic stability limit of PLA at that temperature, maintaining a homogeneous state—that is, carbon dioxide is completely dissolved in the PLA matrix with no tendency for phase separation. This invention, by limiting the volume fraction of carbon dioxide gas in the mixed gas to between 0 and 10%, prevents insufficient plasticizing effect of swelling carbon dioxide while preventing foaming. In other embodiments, the volume fraction of carbon dioxide gas in the mixed gas may be greater than 0 and less than 10, but the mechanical properties of the resulting PLA components are not as good as in this embodiment.

[0045] In this embodiment, the heating process of the sealed cavity in step two is as follows: first heating, first heating and holding, second heating, and second heating and holding, with the second heating and holding temperature being 95 to 120 degrees Celsius. The purpose of the first heating is to bring the polylactic acid (PLA) product into a state of thermal motion. The purpose of the first heating and holding is to homogenize the internal temperature of the PLA product, ensuring that the molecular chains in all regions of the PLA product simultaneously enter a state of thermal motion, preventing non-uniform expansion and secondary internal stress caused by temperature differences. The second heating is required to bring the PLA into a stable, highly elastic state. In the highly elastic state, the PLA molecular chain segments gain sufficient energy (higher than the activation energy of chain segment motion) to overcome intermolecular forces, resulting in large-scale chain segment relaxation motion; the oriented molecular chains gradually deorient, and the internal stress frozen during the molding process is uniformly released. During the second heating and holding process, polylactic acid (PLA) remains in a highly elastic state. The temperature of 95 to 120 degrees Celsius falls precisely within the subcritical / supercritical state of carbon dioxide, where its diffusion and swelling capabilities are at their peak. This facilitates the entry of carbon dioxide molecules into the PLA product. Driven by thermal motion, the carbon dioxide molecules and PLA molecular chains continuously adjust their conformation, arranging themselves in an orderly manner to form crystal nuclei and grow into uniform and dense crystalline regions. This invention, through a combination of gradient heating and holding, achieves complete elimination of internal stress and uniform crystallization in the treated PLA component. In other embodiments, a process of rapid initial heating followed by slow heating can be used for the PLA product, maintaining the PLA in a highly elastic state during the slow heating process. However, without the initial heating and holding process, the treated PLA component may experience non-uniform expansion and secondary internal stress due to temperature differences.

[0046] In this embodiment, the heating rate of the first heating is greater than that of the second heating, and the holding time of the first heating is less than that of the second heating. Specifically, the parameters for the first heating are: a heating rate of 2 to 8 degrees Celsius per minute, raising the temperature from room temperature (which can be 25 degrees Celsius) to 60 to 90 degrees Celsius; the heating rate is controlled at 2 to 8 degrees Celsius per minute to avoid localized overheating that could lead to thermal degradation of the polylactic acid (PLA) molecular chains. The heating rate of the second heating is 1 to 3 degrees Celsius per minute, raising the temperature to 95 to 120 degrees Celsius, at which point the heating rate decreases to ensure the PLA product remains in a stable, highly elastic state. When the temperature exceeds 120 degrees Celsius, the PLA molecular chains move too violently, increasing the risk of ester bond breakage and decreasing the molecular weight; while when the temperature is below 95 degrees Celsius, the PLA molecular chain segments lack sufficient mobility, internal stress is not completely eliminated, and crystallization is incomplete. It is worth noting that although there is some overlap between 2 to 8 degrees Celsius per minute and 1 to 3 degrees Celsius per minute, in actual use, it is necessary to strictly maintain that the heating rate of the first heating is greater than that of the second heating. For example, if the heating rate of the first heating is 2 degrees Celsius per minute, then the heating rate of the second heating should be 1 degree Celsius per minute.

[0047] Regarding the holding times for the first and second heating cycles, the first heating cycle should last 10 to 30 minutes, and the second heating cycle should last 30 to 120 minutes. Similarly, in actual use, although the holding times for the first and second heating cycles overlap, the first heating cycle must be strictly shorter than the second heating cycle. For example, if the first heating cycle is 30 minutes, the second heating cycle should be 60 minutes. The second holding time is adjusted according to the thickness of the polylactic acid (PLA) product. For relatively thin PLA products, the second heating cycle should last 30 to 60 minutes; while for relatively thick PLA products, it should last 60 to 120 minutes. A longer second heating cycle allows sufficient time for carbon dioxide molecules to penetrate the PLA product. The initial heating and holding time is set within 10 to 30 minutes, which allows the molecular chains in all areas of the polylactic acid product to enter a state of thermal motion simultaneously, preventing uneven expansion and secondary internal stress caused by temperature differences.

[0048] The first heating and subsequent holding phase primarily preheat the polylactic acid (PLA) product and do not involve crystallization; therefore, rapid heating and short-term holding are permissible. However, when carbon dioxide molecules need to enter the PLA product, slow heating and prolonged holding are required. This is mainly because at this temperature, the PLA product is in its critical crystallization temperature range, requiring slow and thorough stress release, molecular relaxation, and uniform crystallization to ensure dimensional stability and mechanical properties. In other embodiments, the first heating rate can be set to be equal to the second heating rate.

[0049] In this embodiment, the cooling process of the sealed cavity in step three consists of a first cooling, a first cooling and holding period, and a second cooling. The first cooling and holding temperature is 60 to 80 degrees Celsius. After the second cooling, the polylactic acid (PLA) product changes from a highly elastic state to a glassy state, forming a PLA component. The purpose of setting a holding period between the two cooling cycles, and setting the first cooling and holding temperature at 60 to 80 degrees Celsius, is to further mitigate thermal stress in the PLA product's molecular chains during the cooling process by setting a holding period near the glass transition temperature of PLA, thus completing a smooth transition from the highly elastic state to the glassy state, ensuring a uniform internal temperature field for the PLA product, and preventing warping deformation caused by temperature differences. In other embodiments, the first cooling and holding process may be omitted, and the sealed cavity may be naturally cooled to room temperature in step three.

[0050] In this embodiment, the cooling rate for the first cooling step is set to 1 to 4 degrees Celsius per minute. Slow cooling allows sufficient time for the polylactic acid (PLA) molecular chains to transition from the elastic state to the glassy state, ensuring the stability of the ordered crystalline regions and preventing the PLA molecular chains from "freezing" in a twisted or stretched state due to rapid cooling, which would generate secondary internal stress. For the second cooling step, natural cooling to room temperature (which could be 25 degrees Celsius) is used. In other embodiments, the cooling rate for the first cooling step can be greater than 4 degrees Celsius per minute, but this carries the risk of generating secondary internal stress.

[0051] In this embodiment, the mixed gas consists of an inert gas and carbon dioxide. The inert gas is preferably nitrogen or argon. The introduction of the inert gas eliminates oxygen within the sealed cavity, effectively inhibiting the thermo-oxidative degradation of the ester bonds in polylactic acid (PLA) at high temperatures and protecting the integrity of the PLA molecular chain. This invention effectively inhibits the thermo-oxidative degradation of PLA ester bonds at high temperatures by introducing an inert gas. In other embodiments, the mixed gas may also be a combination of carbon dioxide and oxygen, but this will result in thermal degradation of PLA at high temperatures.

[0052] In this embodiment, before step one, the polylactic acid product is placed into the sealed cavity. Then, the sealed cavity is evacuated to remove the air, resulting in a pressure within the sealed cavity of less than or equal to -0.09 MPa. Subsequently, a mixed gas is introduced into the sealed cavity, bringing the pressure within the cavity down to a predetermined range, thus completing the gas replacement within the sealed cavity. This invention uses a method of evacuating the cavity before introducing the mixed gas, which facilitates gas replacement within the sealed cavity. In other embodiments, gas replacement can also be achieved by simultaneously purging the air from the original sealed cavity and injecting the mixed gas into the sealed cavity.

[0053] In this embodiment, the polylactic acid (PLA) product is surface-cleaned before being placed into the sealed cavity to remove surface impurities and dust. This cleaning process prevents impurities and dust from causing stress concentration or degradation during the post-processing of the PLA product. In other embodiments, cleaning of impurities and dust may be omitted.

[0054] The post-processing methods for polylactic acid products in this embodiment are summarized in detail as follows: S1: Clean the molded polylactic acid product; S2: Place the cleaned polylactic acid product into the closed cavity; S3: Evacuate the sealed cavity, and then introduce a mixed gas consisting of inert gas and carbon dioxide, so that only the mixed gas remains in the sealed cavity. The volume fraction of carbon dioxide in the mixed gas is 10% to 30%, and the gas pressure in the sealed cavity is maintained within the set range of 0.10±0.02MPa. S4: Maintain the gas pressure within the set range, and in a mixed gas environment, raise the temperature inside the sealed cavity from room temperature (which can be 25 degrees Celsius) to 60 to 90 degrees Celsius at a heating rate of 2 to 8 degrees Celsius per minute, and then hold the temperature for 10 to 30 minutes. S5: Maintain the air pressure within the set range, and in a mixed gas environment, raise the temperature inside the sealed cavity to 95 to 120°C at a heating rate of 1 to 3 degrees Celsius per minute, and then hold the temperature for 30 to 120 minutes. S6: Keep the air pressure within the set range, and in a mixed gas environment, reduce the temperature to 60 to 80 degrees Celsius at a cooling rate of 1 to 4 degrees Celsius per minute, then keep it at that temperature for 20 to 40 minutes, and then let it cool naturally to room temperature (which can be 25 degrees Celsius). S7: After the temperature inside the sealed cavity drops to room temperature, the polylactic acid product inside the sealed cavity is finished and polylactic acid component is formed. At this time, the polylactic acid component can be taken out from the sealed cavity.

[0055] The polylactic acid (PLA) components produced by the post-processing method described in the previous paragraph, when used in medical implants, offer the following beneficial effects: I. Significantly Improved Mechanical Properties: The mixed gas environment avoids thermo-oxidative degradation, the swelling and plasticizing effect of carbon dioxide allows for more complete movement of molecular chains, and the gradient heating achieves complete elimination of internal stress and uniform crystallization. The bending strength of the product is increased by 20% to 40%, the impact toughness by 30% to 50%, the tensile strength by 15% to 30%, and the brittleness is significantly reduced, meeting the load-bearing requirements of medical implantable devices.

[0056] II. Significantly improved resistance to deformation and dimensional stability: The heat shrinkage rate of the product is reduced to below 1%, the creep is reduced by 60% to 80%, there is no warping or rebound, and the dimensional accuracy retention rate is ≥99% under body temperature (37℃) and body fluid environment, making it suitable for precision medical devices.

[0057] III. Controllable degradation performance and clinical suitability: Crystallinity is uniformly controlled to 30% to 50%, the degradation rate of the product in vivo is slow, the mechanical retention rate is ≥80% in the early stage (1 to 3 months) to avoid premature failure; in the later stage (6 to 12 months) degradation is complete, acidic products are slowly released, there is no local accumulation, and the biocompatibility is excellent.

[0058] IV. Clean process and suitable for medical requirements: No additives, no chemical modification, no surface damage throughout the process. The mixed gas environment avoids oxidation and pollution, complies with the GMP production standards for medical products, and can be directly used for the finishing of sterile medical polylactic acid products.

[0059] V. Strong process versatility: It is suitable for medical polylactic acid products prepared by various molding processes such as injection molding, extrusion, 3D printing, and spinning, including thin-walled parts, thick-walled parts, and complex structural parts (such as bone nails, scaffolds, and repair membranes). The process is simple to operate and easy to promote industrialization.

[0060] VI. No foaming phenomenon: Polylactic acid components do not produce foam structure, which will not damage the density and mechanical properties of the product; the processing temperature is in the high elastic state (95 to 120℃) rather than below the glassy state, which can achieve full rearrangement of molecular chains and elimination of internal stress; the gradient heating and cooling process is adopted, resulting in more uniform crystallization and better dimensional stability.

[0061] The beneficial effects of the polylactic acid (PLA) product post-processing method and PLA components involved in this invention were experimentally verified. The detailed experimental process is as follows: Experimental materials: Medical-grade polylactic acid composite material (melting temperature 170-190℃), which was injection molded into standard mechanical specimens and medical bone nail specimens (φ4.5×14.0mm).

[0062] Test equipment: drying oven, BABYPLAST injection molding machine, electronic universal testing machine, simply supported beam impact testing machine, heat distortion Vicat tester, programmed temperature controlled heat setting chamber, gel permeation chromatography (GPC), and precision micrometer.

[0063] The standard mechanical spline and medical bone nail sample of polylactic acid injection molding were processed according to the following steps, with a total of five examples and eight comparative examples. The processing methods for each example and comparative example are as follows: Example 1 (includes the following steps in sequence): Pretreatment: The standard mechanical specimens and medical bone nail specimens, which are injection molded from polylactic acid, are wiped with sterile anhydrous ethanol to remove impurities and dust. They are then placed in a polytetrafluoroethylene heat-resistant fixture, ensuring that the specimens and bone nails are not stacked, squeezed, or in contact. Finally, the polylactic acid standard mechanical specimens and medical bone nail specimens are placed in a closed cavity.

[0064] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (20% carbon dioxide and 80% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.10 MPa.

[0065] Gradient heating heat setting: Maintaining the gas pressure stable at 0.10 MPa, in a mixed gas environment, the internal temperature is raised from room temperature (24°C) to 70°C at a heating rate of 5°C / min, and held for 20 minutes to ensure uniform internal temperature between the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen; then, the temperature is raised to 100°C at a heating rate of 2°C / min and held for 60 minutes to complete the heat setting.

[0066] Gradient cooling: After the shaping is completed, the gas pressure is kept stable at 0.10 MPa. In a mixed gas environment, the temperature is reduced to 70 degrees Celsius at a cooling rate of 2 degrees Celsius / minute and held for 30 minutes. Then, heating is stopped and the temperature is allowed to drop naturally to 25 degrees Celsius.

[0067] Pressure balancing and discharge: After the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen have cooled to room temperature, open the valve to balance the internal and external pressure of the sealed cavity with the outside air, and then take out the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen.

[0068] Example 2 (includes the following steps in sequence): Pretreatment: Same as in Example 1.

[0069] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (10% carbon dioxide and 90% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.12 MPa.

[0070] Gradient heating heat setting: Maintaining the gas pressure stable at 0.12 MPa, in a mixed gas environment, the internal temperature is raised from room temperature (26°C) to 60°C at a heating rate of 3°C / min, and held for 30 minutes to ensure uniform internal temperature between the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen; then, the temperature is raised to 95°C at a heating rate of 1°C / min and held for 90 minutes to complete the heat setting.

[0071] Gradient cooling: After the shaping is completed, the gas pressure is kept stable at 0.12 MPa. In a mixed gas environment, the temperature is reduced to 60 degrees Celsius at a cooling rate of 1 degree Celsius / minute and held for 40 minutes. Then, heating is stopped and the temperature is allowed to drop naturally to 26 degrees Celsius.

[0072] Pressure balancing and discharge: Same as in Example 1.

[0073] Example 3 (includes the following steps in sequence): Pretreatment: Same as in Example 1.

[0074] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (30% carbon dioxide and 70% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.08 MPa.

[0075] Gradient heating heat setting: Maintaining the gas pressure stable at 0.08 MPa, in a mixed gas environment, the internal temperature is raised from room temperature (25°C) to 80°C at a heating rate of 8°C / min, and held for 10 minutes to ensure uniform internal temperature between the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen; then, the temperature is raised to 120°C at a heating rate of 3°C / min and held for 120 minutes to complete the heat setting.

[0076] Gradient cooling: After the shaping is completed, the gas pressure is kept stable at 0.08 MPa. In a mixed gas environment, the temperature is reduced to 80 degrees Celsius at a cooling rate of 4 degrees Celsius / minute and held for 20 minutes. Then, heating is stopped and the temperature is allowed to cool naturally to 26 degrees Celsius.

[0077] Pressure balancing and discharge: Same as in Example 1.

[0078] Example 4 (includes the following steps in sequence): Pretreatment: Same as in Example 1.

[0079] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (15% carbon dioxide and 85% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.09 MPa.

[0080] Gradient heating heat setting: Maintaining the gas pressure stable at 0.09 MPa, in a mixed gas environment, the internal temperature is raised from room temperature (27°C) to 75°C at a heating rate of 4°C / min, and held for 25 minutes to ensure uniform internal temperature between the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen; then, the temperature is raised to 105°C at a heating rate of 2°C / min and held for 80 minutes to complete the heat setting.

[0081] Gradient cooling: After the shaping is completed, the gas pressure is kept stable at 0.09 MPa. In a mixed gas environment, the temperature is reduced to 75 degrees Celsius at a cooling rate of 3 degrees Celsius / minute and held for 25 minutes. Then, heating is stopped and the temperature is allowed to drop naturally to 24 degrees Celsius.

[0082] Pressure balancing and discharge: Same as in Example 1.

[0083] Example 5 (includes the following steps in sequence): Pretreatment: Same as in Example 1.

[0084] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (25% carbon dioxide and 75% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.11 MPa.

[0085] Gradient heating heat setting: Maintaining the gas pressure stable at 0.11 MPa, in a mixed gas environment, the internal temperature is raised from room temperature (24°C) to 65°C at a heating rate of 6°C / min, and held for 15 minutes to ensure uniform internal temperature between the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen; then, the temperature is raised to 110°C at a heating rate of 2°C / min and held for 50 minutes to complete the heat setting.

[0086] Gradient cooling: After the shaping is completed, the gas pressure is kept stable at 0.11 MPa. In a mixed gas environment, the temperature is reduced to 65 degrees Celsius at a cooling rate of 2 degrees Celsius / minute and held for 35 minutes. Then, heating is stopped and the temperature is allowed to drop naturally to 25 degrees Celsius.

[0087] Pressure balancing and discharge: Same as in Example 1.

[0088] Comparative Example 1 (without post-treatment of polylactic acid products, including the following steps): Take standard mechanical specimens of polylactic acid and medical polylactic acid bone nail specimens from the same batch, and only perform the steps in Example 1 of wiping the surface with sterile anhydrous ethanol to remove impurities and dust, without performing vacuum and gas replacement, gradient heating heat setting and gradient cooling.

[0089] Comparative Example 2 (no mixed gas, no pressure control in the closed cavity, including the following steps in sequence): Pretreatment: Same as in Example 1.

[0090] Gradient heating heat setting: In natural air environment, the temperature inside the cavity is raised from room temperature (25 degrees Celsius) to 70 degrees Celsius at a heating rate of 5 degrees Celsius / minute and held for 20 minutes; then the temperature is raised to 100 degrees Celsius at a heating rate of 2 degrees Celsius / minute and held for 60 minutes.

[0091] Gradient cooling: The temperature is reduced to 70 degrees Celsius at a rate of 2 degrees Celsius per minute and held for 30 minutes; finally, it is allowed to cool naturally to 25 degrees Celsius.

[0092] Pressure balancing and discharge: Same as in Example 1.

[0093] Comparative Example 3 (using pure nitrogen instead of the mixed gas, and including the following steps in sequence): Pretreatment: Same as in Example 1.

[0094] Vacuum and gas replacement: Pure nitrogen gas is introduced to replace the air in the sealed cavity, so that the pressure in the sealed cavity is stabilized to 0.10 MPa.

[0095] Gradient heating heat setting: Same as in Example 1.

[0096] Gradient cooling: Same as in Example 1.

[0097] Pressure balancing and discharge: Same as in Example 1.

[0098] Comparative Example 4 (foaming process, including the following steps in sequence): Pretreatment: Same as in Example 1.

[0099] Carbon dioxide impregnation: The gas in the sealed cavity is replaced with pure carbon dioxide gas, and the gas pressure in the sealed cavity is maintained at 2.0 MPa, and the temperature in the sealed cavity is maintained at 40 degrees Celsius for 4 hours.

[0100] Heating and foaming: The temperature inside the sealed cavity is raised to 120 degrees Celsius, and then kept at that temperature for 10 minutes, so that the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen are vaporized, expanded and foamed.

[0101] Cooling and unloading: Allow the sealed cavity to cool naturally to room temperature, and release the pressure to remove the polylactic acid standard mechanical specimen and the medical polylactic acid bone nail specimen.

[0102] Comparative Example 5 (The air pressure inside the closed cavity is lower than the lower limit of the set range, including the following steps in sequence): Pretreatment: Same as in Example 1.

[0103] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (20% carbon dioxide and 80% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.04 MPa.

[0104] Gradient heating heat setting: Same as in Example 1.

[0105] Gradient cooling: Same as in Example 1.

[0106] Pressure balancing and discharge: Same as in Example 1.

[0107] Comparative Example 6 (The air pressure inside the closed cavity is higher than the upper limit of the set range, including the following steps in sequence): Pretreatment: Same as in Example 1.

[0108] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (20% carbon dioxide and 80% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.15 MPa.

[0109] Gradient heating heat setting: Same as in Example 1.

[0110] Gradient cooling: Same as in Example 1.

[0111] Pressure balancing and discharge: Same as in Example 1.

[0112] Comparative Example 7 (the volume fraction of carbon dioxide is less than 10%, and includes the following steps in sequence): Pretreatment: Same as in Example 1.

[0113] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (5% carbon dioxide and 95% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.10 MPa.

[0114] Gradient heating heat setting: Same as in Example 1.

[0115] Gradient cooling: Same as in Example 1.

[0116] Pressure balancing and discharge: Same as in Example 1.

[0117] Comparative Example 8 (the volume fraction of carbon dioxide is higher than 30%, and includes the following steps in sequence): Pretreatment: Same as in Example 1.

[0118] Vacuum and gas replacement: First, evacuate the sealed cavity to ≤-0.09 MPa, then introduce a mixed gas (40% carbon dioxide and 60% nitrogen) into the sealed cavity to replace the air in the sealed cavity and stabilize the gas pressure in the sealed cavity to 0.10 MPa.

[0119] Gradient heating heat setting: Same as in Example 1.

[0120] Gradient cooling: Same as in Example 1.

[0121] Pressure balancing and discharge: Same as in Example 1.

[0122] Tensile strength, flexural strength, impact strength and elongation at break tests were performed on the polylactic acid standard mechanical specimens formed after the five examples and eight comparative examples mentioned above. The detailed test methods for each test are as follows.

[0123] Tensile strength test: The test shall be conducted in accordance with the standard GB / T 1040.1-2018, "Plastics - Determination of tensile properties - Part 1: General".

[0124] Bending strength test: The test shall be conducted in accordance with the "Determination of Bending Properties of Plastics" (GB / T 9341-2008).

[0125] Impact strength test: The test was conducted according to the standard "Determination of impact strength of plastic cantilever beams" (GB / T 1843-2008).

[0126] Elongation at break test: The test shall be conducted in accordance with the standard GB / T 1040.1-2018, "Plastics - Determination of tensile properties - Part 1: General".

[0127] The test results of the polylactic acid standard mechanical specimens for Examples 1 to 5 and Comparative Examples 1 to 8 are as follows:

[0128] For the medical polylactic acid bone nail samples after the five examples and eight comparative examples mentioned above, the following tests were conducted: heat shrinkage rate test, 37°C creep test, initial fixation strength test, three-month fixation strength test, three-month mechanical retention rate test, initial screwing torque test, initial maximum torque test, six-month mass loss rate test, and product appearance inspection. The detailed test methods for each test are as follows.

[0129] Heat shrinkage rate test: The temperature was maintained at 47℃±2℃ for 60 minutes, then cooled to room temperature. The heat shrinkage rate was calculated using the formula: Heat shrinkage rate = (L0...) / L0... The calculation is L1) / L0×100%, where L0 is the initial length and L1 is the length after heating and cooling.

[0130] Creep test at 37℃: The medical polylactic acid (PLA) bone screw sample was first completely immersed in a 37℃ constant-temperature simulated body fluid bath filled with phosphate buffered saline (PBS) solution. The axis of the PLA bone screw sample was positioned along the direction of gravity. A constant pressure of 50N was applied to the PLA bone screw sample (using weights placed above the sample; the weights' gravity created the 50N pressure). The strain of the material was precisely measured over 72 hours. The final creep length was calculated as (L0-L1) / L0×100%, where L0 is the initial length and L1 is the final length after 72 hours.

[0131] Initial fixation strength test: The test shall be conducted in accordance with Section 5.5.4 of "Sports Medicine Implantable Devices with Wire Anchors" (YY / T 1867-2023).

[0132] Three-month fixation strength test: After the medical polylactic acid bone nail sample was completely immersed in phosphate buffer solution (PBS solution) at a constant temperature of 37°C for three months, the test was carried out in accordance with Section 5.5.4 of "Sports Medicine Implantable Devices with Wire Anchors" (YY / T 1867-2023).

[0133] Three-month mechanical retention rate test: calculated by dividing the fixation strength after three months by the initial fixation strength.

[0134] Initial screw-in torque test: The test shall be conducted in accordance with Section 5.5.2 of "Sports Medicine Implantable Devices with Wire Anchors" (YY / T 1867-2023).

[0135] Initial maximum torque test: Tested according to Section 5.5.1 of "Sports Medicine Implantable Devices with Wire Anchors" (YY / T 1867-2023).

[0136] 6-month mass loss rate test: The test was conducted according to the "Evaluation Methods for In Vitro Degradation Performance of Biomedical Materials Part 1: Degradable Polyesters" (YY / T 1806.1-2021).

[0137] Product appearance inspection: visual inspection.

[0138] The test results for the medical polylactic acid bone nail samples in Examples 1 to 5 and Comparative Examples 1 to 8 are as follows:

[0139] like Figure 2 and Figure 3 As shown, Figure 2This is a physical image of a polylactic acid injection-molded medical bone nail sample after processing in the relevant steps of Example 1. It is an appearance image of a polylactic acid component formed after the polylactic acid product post-processing method of the present invention. The surface of the medical bone nail sample appears smooth. Figure 3 The image shown is of a medical bone nail sample made of polylactic acid injection molding according to the present invention after processing in the relevant steps of Comparative Example 1. That is, it is an appearance image of a polylactic acid product that has not been processed by the post-processing method of the present invention. The surface of the medical bone nail sample is not as smooth as the image of the sample after processing in the relevant steps of Example 1.

[0140] The reason why polylactic acid (PLA) components formed by the PLA post-processing method of this invention have a smoother surface compared to PLA products not treated by this method is that the molecular chain orientation and internal stress "frozen" on the surface during the injection molding, extrusion, blow molding, spinning, and 3D printing processes of the PLA product are completely released under high elasticity through molecular chain rearrangement and molecular lubrication by carbon dioxide. This repairs microscopic defects on the surface, naturally improving the macroscopic smoothness. Therefore, a smooth surface can reduce the screwing friction resistance when the anchor is implanted into the bone tunnel, making the implantation operation smoother and the torque more controllable. At the same time, a smooth surface can reduce mechanical stimulation and inflammatory response to surrounding soft tissues, reduce the risk of postoperative complications, and weaken the adhesion of bacterial biofilms, thereby improving the biocompatibility and long-term safety of the implant.

[0141] In summary, the post-processing method and polylactic acid (PLA) component of the present invention, by placing the PLA product in a closed cavity containing a mixed gas, limiting the volume fraction of carbon dioxide gas and the pressure range within the closed cavity, and then heating the PLA product to a high-elasticity state, allows carbon dioxide molecules to rapidly diffuse into the gaps between molecular chains, achieving a plasticizing effect. By limiting the volume fraction of carbon dioxide gas in the mixed gas to between 10% and 30%, insufficient plasticizing effect of swelling carbon dioxide is prevented, while also preventing foaming. Furthermore, the combined gradient heating and heat preservation process ensures that the treated PLA component achieves complete elimination of internal stress and uniform crystallization. The rapid heating and short heat preservation during the first heating and heat preservation phases allow for rapid preheating of the PLA product; while the slow heating and long heat preservation during the second heating and heat preservation phases allow for sufficient release of internal stress during the entry of carbon dioxide molecules into the PLA product. Finally, by setting a heat preservation stage near the glass transition temperature of PLA, the molecular chains of the PLA product further mitigate thermal stress during the cooling process. By setting the initial cooling rate to 1 to 4 degrees Celsius per minute, rapid cooling is prevented from causing the polylactic acid (PLA) molecular chains to "freeze" in a twisted or stretched state, thus avoiding secondary internal stress. Introducing an inert gas effectively inhibits the thermo-oxidative degradation of PLA ester bonds at high temperatures. The method of first evacuating the vacuum chamber and then introducing the mixed gas facilitates gas replacement within the sealed cavity. Furthermore, removing surface impurities and dust before placing PLA products into the sealed cavity reduces the probability of stress concentration or degradation during post-processing.

[0142] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A post-processing method for polylactic acid products, characterized in that, Includes the following steps, Step 1: Place the polylactic acid product in a closed cavity containing a mixed gas, wherein the mixed gas contains carbon dioxide gas with a volume fraction greater than or equal to 10% and less than or equal to 30%. Step 2: Maintain the air pressure inside the sealed cavity within a set range, which is 0.08 MPa to 0.12 MPa; heat the sealed cavity, causing the polylactic acid product to transform from a glassy state to a highly elastic state; Step 3: Maintain the air pressure of the sealed cavity within the set range, cool the sealed cavity, and the polylactic acid product returns from the high elastic state to the glassy state, forming a polylactic acid component; In step two, the process of heating the sealed cavity is as follows: first heating, first heating and holding, second heating, and second heating and holding, with the second heating and holding temperature being 95 to 120 degrees Celsius. The mixed gas consists of an inert gas and carbon dioxide gas; Before the polylactic acid product is placed into the closed cavity, the surface of the polylactic acid product is first cleaned.

2. The post-processing method for polylactic acid products according to claim 1, characterized in that, The heating rate of the first heating is greater than the heating rate of the second heating, and the holding time of the first heating is less than the holding time of the second heating.

3. The post-processing method for polylactic acid products according to claim 1, characterized in that, In step three, the process of cooling the sealed cavity is as follows: first cooling, first cooling and heat preservation, and second cooling. The first cooling and heat preservation temperature is 60 to 80 degrees Celsius.

4. The post-processing method for polylactic acid products according to claim 3, characterized in that, The cooling rate for the first cooling step is 1 to 4 degrees Celsius per minute.

5. The post-processing method for polylactic acid products according to claim 1, characterized in that, Before step one, the polylactic acid product is placed into the sealed cavity, then the sealed cavity is evacuated, and then the mixed gas is introduced into the sealed cavity to complete the gas replacement in the sealed cavity.

6. A polylactic acid component, characterized in that, It is prepared by using the post-processing method for polylactic acid products according to any one of claims 1 to 5.

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