Heat resistant porous meltblown material and method of making

Abstract

The application provides a heat-resistant porous melt-blown material and a preparation method thereof, and comprises the following steps: S1: dissolving PLLA and PDLA in a DMC solvent, sufficiently dissolving, and then evaporating the DMC solvent to obtain a filter cake, and drying the filter cake to obtain SC crystal micro-powder; S2: mixing the SC crystal micro-powder, PLLA and a composite foaming agent, and then performing melt blending and granulation to obtain melt-blown master batches; and S3: performing melt-blown treatment on the melt-blown master batches to obtain the heat-resistant porous melt-blown material, high-efficiency preparation and uniform dispersion of SC crystals are realized under a low PDLA addition amount, meanwhile, the processability of the melt-blown process is considered, and the heat resistance of the material matrix and the structural thermal stability are synergistically improved.

Description

Technical Field

[0001] This invention relates to the field of meltblown materials, and in particular to a heat-resistant porous meltblown material and its preparation method, which can be widely used in high-temperature filtration, medical protection, heat insulation, industrial oil absorption, food contact and other fields. Background Technology

[0002] Bio-based polylactic acid (PLA), with its complete biodegradability, excellent biocompatibility, good processability, and mechanical properties, has become a core preferred material to replace traditional polypropylene (PP) meltblown materials. PLA meltblown nonwoven materials combine the advantages of high porosity, high specific surface area, and ultrafine fiber structure unique to meltblown materials, showing great potential for industrial application in fields such as medical dressings, mask filters, oil-absorbing materials, and sound and heat insulation.

[0003] However, since PLA is a semi-crystalline aliphatic polyester with a glass transition temperature of only about 60°C, the molecular chains in the amorphous region are prone to relaxation at high temperatures, leading to significant thermal shrinkage, structural collapse, and degradation of mechanical properties, resulting in extremely poor heat resistance. At the same time, traditional PLA meltblown materials are mostly solid fiber tightly packed structures, which concentrate internal thermal stress when heated, making them prone to fiber adhesion and pore closure, further losing the material's filtering, air permeability, and heat insulation functions, severely limiting its large-scale application in high-temperature scenarios.

[0004] To overcome the heat resistance defects of PLA meltblown materials, the mainstream modification route in existing technologies is to blend L-polylactic acid (PLLA) with D-polylactic acid (PDLA) to form high-melting-point stereocomposite crystals through hydrogen bonding between molecular chains, thereby utilizing the high-temperature resistance of SC crystals to improve the thermal stability of the material. However, to ensure the full formation of SC, current direct melt blending methods usually require the addition of relatively high or even equal amounts of PDLA, leading to a significant increase in raw material costs. In addition, because the molecular chain movement of PLLA and PDLA is restricted during direct melt blending, the crystallization efficiency of SC is low, resulting in limited and uneven distribution of SC crystals. This leads to a less-than-expected improvement in heat resistance, waste of PDLA, and may even cause severe molecular chain entanglement, a sharp increase in the complex viscosity of the melt, and deterioration of rheological properties, failing to meet the rheological requirements of meltblown processes. Traditional meltblown materials are mostly solid fiber stacks with high packing density. Due to the concentration of internal thermal stress when heated, fiber adhesion or pore closure easily occurs at high temperatures, limiting their application in high-temperature fields.

[0005] Given the inherent defects of PLA material in the above meltblown scheme, such as insufficient SC formation efficiency, high meltblown fiber density, and the tendency for direct melt blending to result in excessively high melt viscosity, it is of great application value to achieve efficient formation and uniform dispersion of SC crystals while maintaining the processability of the meltblown process, and to prepare PLA meltblown nonwoven materials with high heat resistance and stable fiber structure. Summary of the Invention

[0006] The purpose of this invention is to provide a heat-resistant porous meltblown material and its preparation method, breaking through the technical inertia of existing direct melt blending of PLLA / PDLA, achieving efficient preparation and uniform dispersion of SC crystals with low PDLA addition, while taking into account the processability of the meltblown process, and achieving a synergistic improvement in the heat resistance of the material matrix and the thermal stability of the structure, ultimately obtaining a lightweight, highly heat-resistant, highly breathable, and fully biodegradable meltblown nonwoven material.

[0007] To achieve the above objectives, this technical solution provides a method for preparing a heat-resistant porous meltblown material, comprising the following steps: S1: Dissolve PLLA and PDLA in DMC solvent until fully dissolved, then evaporate the DMC solvent to obtain filter cake, and dry the filter cake to obtain SC crystalline powder; S2: After fully mixing SC crystalline micro powder, PLLA and composite foaming agent, melt blending and granulation are carried out to obtain meltblown masterbatch; S3: Meltblown masterbatch is meltblown to obtain heat-resistant porous meltblown material.

[0008] This scheme employs a process of pre-forming SC crystalline micropowder, melt blending and foaming, and secondary pore formation via meltblowing to create ultrafine porous fibers with interconnected micropores and SC microspheres anchoring the pore walls. Through the high-temperature resistance of SC crystals and the thermal stress buffering effect of the porous structure, the material's heat resistance and structural stability are synergistically improved. It should be noted that during the melt blending and granulation process after thoroughly mixing SC crystalline micropowder, PLLA, and the composite foaming agent, the SC microspheres act as nucleating agents, inducing the crystallization of the PLLA matrix while guiding bubbles to preferentially nucleate and grow on its surface. After cooling, the SC microspheres naturally anchor to the pore walls. Finally, during the meltblowing stage of the masterbatch, under the action of a high-temperature, high-speed airflow field, the remaining foaming agent completely decomposes, and the initially formed closed micropores in the masterbatch are fully stretched, ultimately forming a heat-resistant porous meltblown material with high porosity.

[0009] This solution method uses the green solvent DMC to dissolve PLLA and PDLA, achieving uniform mixing of PLLA and PDLA at the molecular level, thus significantly improving the crystallization efficiency of SC. In the traditional molten state, the molecular chains of PLLA and PDLA are highly entangled, severely restricting chain segment movement. Therefore, only a small amount of pairing can occur at the interface, failing to achieve uniform mixing at the molecular level. This results in SC crystals forming only at the interface with extremely low crystallinity (typically only 30%~50%). Only by adding a high proportion of PDLA and prolonged high-temperature heat treatment can the SC crystal content be barely increased, accompanied by severe molecular chain entanglement and a surge in melt viscosity. This solution, using DMC as a solvent, completely dissolves PLLA and PDLA, completely de-entangleing the two polymers and uniformly dispersing them as single molecules in the solvent system, truly achieving uniform mixing at the molecular level.

[0010] In some embodiments, the mass ratio of PLLA to PDLA is 7:3 to 1:1.

[0011] Preferably, the mass ratio of PLLA to PDLA is 1:1. It should be noted that the essence of SC crystalline micropowder, which forms a high-melting-point stereocomposite crystal with PLLA and PDLA, is that the left-handed and right-handed polylactic acid molecular chains form an ordered stereocomposite structure with a 1:1 pairing through hydrogen bonding. Since this method uses a solution-based processing, the mass ratio of PLLA to PDLA can be controlled at 1:1, enabling efficient formation of SC with a relatively low PDLA content, thus overcoming the technological inertia of high PDLA addition.

[0012] In some embodiments, PLLA and PDLA are dissolved in DMC solvent and the solution is stirred in a magnetic stirrer until fully dissolved. The stirring temperature is between room temperature and 50°C, the stirring rate is 200 to 600 rpm, and the stirring time is generally 4 to 6 hours until the solutes are fully dissolved.

[0013] In some embodiments, the fully dissolved solution is placed in a fume hood and allowed to ventilate naturally until the solvent has completely evaporated to obtain a filter cake.

[0014] Preferably, natural ventilation is carried out at room temperature (20-25°C) and horizontal ventilation velocity (0.2-0.5 m / s) until the solvent is completely evaporated and the material reaches constant weight, resulting in a loose and uniform solid filter cake. This process provides sufficient kinetic conditions for the stereocomplex pairing of PLLA and PDLA molecular chains through a mild and controllable solvent evaporation rate, ensuring maximum conversion to high-purity stereocomplex crystals and avoiding phase separation and homopolymer formation caused by rapid solvent removal.

[0015] In some embodiments, the filter cake is placed in a vacuum drying oven and dried for 12 to 24 hours at 60 to 80°C and a vacuum of -0.08 MPa to -0.1 MPa to obtain SC crystalline micro powder.

[0016] Preferably, the filter cake is placed in a vacuum drying oven and dried at 80°C and a vacuum of -0.09 MPa for 24 hours to obtain SC crystalline micro powder.

[0017] This method involves preparing SC crystalline micropowder using a solvent method, followed by melt blending of the SC crystalline micropowder, PLLA matrix, and composite foaming agent. It should be noted that because this method involves adding the pre-prepared SC crystalline micropowder to PLLA for melt blending, it avoids the problem of excessive SC formation leading to a sudden increase in melt viscosity and reduced spinnability, thus ensuring melt flowability. Furthermore, in this structure, the high-melting-point SC microspheres pin the pore walls, forming a high-temperature resistant second phase, effectively restricting the thermal motion of molecular chains in the amorphous region and improving the material's heat resistance.

[0018] In some embodiments, the mass ratio of SC crystalline micro powder, PLLA and composite foaming agent is (20-30):(68-78):(2-3), and the total mass of the three is 100%.

[0019] Preferably, the mass ratio of SC crystalline micro powder, PLLA and composite foaming agent is 20 wt%, 77 wt% and 3 wt%.

[0020] In some embodiments, the composite foaming agent is a mixture of citric acid and sodium bicarbonate. This composite foaming agent foams onto the PLLA matrix to form micropores. Compared to traditional solid fibers, the porous structure reduces the fiber packing density, providing a buffer for volume expansion at high temperatures, reducing warping or cracking caused by thermal expansion and contraction, and improving performance in high-temperature applications. Furthermore, the porous structure itself also provides nonwoven materials with lightweight, high air permeability, and high thermal insulation properties.

[0021] In some embodiments, the mass ratio of citric acid to sodium bicarbonate is 6:4 to 1:1.

[0022] Preferably, the mass ratio of citric acid to sodium bicarbonate is 1:1.

[0023] In some embodiments, SC crystalline micropowder, PLLA and composite foaming agent are thoroughly mixed and added to a twin-screw extruder for melt blending and granulation to obtain meltblown masterbatch. The temperature of the twin-screw extruder needs to be lower than the melting point of SC and higher than the melting point of PLLA, and the composite foaming agent needs to be fully foamed. The advantage of this is that it ensures that the SC microcrystals are not completely melted and are uniformly dispersed in the PLLA melt.

[0024] In some embodiments, the five zone temperatures of the twin-screw extruder are set to 180°C, 190°C, 200°C, 200°C, and 190°C, and the die temperature is 220°C (die temperature is a set value).

[0025] .like Figure 1 As shown, this method first mixes PLLA and PDLA to obtain SC crystalline micropowder. Then, the SC crystalline micropowder, PLLA, and composite foaming agent are added to a twin-screw extruder for melt extrusion. The extruded fibers are then granulated after passing through a cooling and conveying device to obtain meltblown masterbatch. As mentioned earlier, because the SC crystalline micropowder is uniformly distributed within the PLLA matrix during melt blending, and because the composite foaming agent forms micropores within the PLLA fibers, the SC crystalline micropowder can naturally anchor itself to the pore walls of the PLLA matrix. Therefore, due to the synergistic effect of the SC crystalline micropowder and the micropores, the heat resistance and thermal insulation performance of the meltblown material produced from the subsequent meltblown masterbatch are improved.

[0026] This method involves secondary granulation of the molten masterbatch after its preparation. The advantage of this is that it allows for complete deagglomeration of SC crystal micropowder agglomerates through secondary controllable melt shearing, achieving nanoscale uniform dispersion of SC crystal micropowder within the PLLA matrix and ensuring the continuity of the high-temperature resistant support network throughout. Simultaneously, it precisely controls the pre-reaction process of the composite foaming agent, achieving uniform bubble nucleation and in-situ pinning of SC microspheres to the pore walls. Furthermore, secondary granulation enables complete and uniform plasticization of the PLLA matrix, stabilizing melt rheological properties, deeply removing volatile impurities from the system, and eliminating the risks of internal stress and phase separation. Ultimately, this yields a specialized masterbatch with uniform composition, stable performance, and perfect compatibility with meltblown processes, solving the core pain points of existing SC crystal-modified PLA systems, such as uneven dispersion, poor melt flowability, poor spinnability, and poor batch stability.

[0027] In some embodiments, the prepared meltblown masterbatch is poured into the hopper of the masterbatch meltblown machine. The meltblown masterbatch is heated by a twin-screw extruder to become a melt. The melt is extruded from the spinning hole in the meltblown die and stretched into ultrafine fibers by hot air. The ultrafine fibers accumulate on the receiving screen and self-bond by the residual heat of the fibers to form a heat-resistant porous meltblown material.

[0028] In some embodiments, the meltblown treatment conditions are as follows: heating zone one and zone two temperatures are 200 ℃ and 210 ℃ respectively, mold heating temperature is 220 ℃, air heating zone temperature is 80 ℃, and main machine speed is 500 rpm.

[0029] Secondly, this solution provides a heat-resistant porous meltblown material prepared according to the preparation method of the heat-resistant porous meltblown material mentioned in the first aspect, wherein the heat resistance is increased by 20 °C compared with the pure PLLA meltblown material, and the air permeability reaches 800 mm / s.

[0030] Compared with existing technologies, this technical solution has the following characteristics and beneficial effects: 1. The solution method is used to pre-prepare SC crystallized micro powder, which significantly improves the crystallization efficiency of SC, reduces the amount of PDLA used, and solves the problems of low efficiency and high cost of traditional melt blending of SC.

[0031] 2. The route of first preparing SC crystalline micro powder and then melt blending is adopted to avoid excessive SC generation leading to a sudden increase in melt viscosity, thus ensuring melt flowability and meltblown spinnability.

[0032] 3. Constructing a porous fiber structure with pinned pore walls of SC microspheres achieves a dual improvement in both matrix heat resistance and structural heat resistance, with high-temperature dimensional stability significantly superior to traditional PLA meltblown materials.

[0033] 4. The porous structure endows the material with lightweight, high air permeability, high heat insulation and high specific surface area characteristics, making it superior to solid fiber meltblown materials in all aspects.

[0034] 5. The entire system uses green solvents and food-grade foaming agents, and all components are biodegradable PLA, making it environmentally friendly and safe, suitable for high-end fields such as medical, food, and high-temperature filtration. Attached Figure Description

[0035] Figure 1 This is a micrograph of the meltblown material prepared in Example 1.

[0036] Figure 2 This is a micrograph of the meltblown material prepared in Comparative Example 1.

[0037] Figure 3 This is a graph showing the results of a heat resistance test. Detailed Implementation

[0038] This section provides a more detailed description of the technical solution of the present invention through specific embodiments, but does not limit the invention to the following embodiments. Any obvious substitutions without departing from the concept of the invention fall within the protection scope of the invention. In the embodiments of the present invention, unless otherwise specified, the raw materials and reagents used are all commercially available conventional products, and the methods used are all conventional experimental methods in the art.

[0039] Example 1 Step 1: Dissolve PLLA:PDLA in DMC solvent at a mass ratio of 1:1. Stir the solution in a magnetic stirrer until fully dissolved. Place the solution in a fume hood and allow it to air dry naturally until the solvent evaporates completely. Place the resulting filter cake in a vacuum drying oven and dry it at 80°C and a vacuum of -0.09 MPa for 24 hours to obtain SC crystalline micro powder. Step 2: Mix SC crystalline micro powder, PLLA, and citric acid / sodium bicarbonate composite foaming agent thoroughly at 20 wt%, 77 wt%, and 1.5 wt% / 1.5 wt%, and then add the mixture to a twin-screw extruder for secondary granulation. With other parameters remaining unchanged, set the temperatures of the five zones to 180℃, 190℃, 200℃, 200℃, and 190℃, and the die temperature to 220℃ to prepare meltblown masterbatch. Step 3: The prepared meltblown masterbatch is placed in the meltblown machine hopper. The masterbatch is heated by a twin-screw extruder to become a melt, which is then extruded through the spinning holes in the meltblown die and drawn into ultrafine fibers by hot air. The ultrafine fibers accumulate on the receiving screen and self-bond using residual heat to form the meltblown material. The meltblown process parameters are: heating zones 1 and 2 temperatures are 200 ℃ and 210 ℃ respectively; die heating temperature is 220 ℃; air heating zone temperature is 80 ℃; and the main machine speed is 500 rpm.

[0040] Comparative Example 1 Step 1: Dissolve PLLA:PDLA in DMC solvent at a mass ratio of 1:1. Stir the solution in a magnetic stirrer until fully dissolved. Place the solution in a fume hood and allow it to air dry naturally until the solvent evaporates completely. Place the resulting filter cake in a vacuum drying oven and dry it at 80°C and a vacuum of -0.09 MPa for 24 hours to obtain SC crystalline micro powder. Step 2: Mix SC crystalline micro powder and PLLA thoroughly at 20 wt% and 80 wt%, and then add them to a twin-screw extruder for secondary granulation. With other parameters remaining unchanged, set the temperatures of the five zones to 180℃, 190℃, 200℃, 200℃, and 190℃, and the die temperature to 220℃ to prepare meltblown masterbatch. Step 3: The prepared meltblown masterbatch is placed in the meltblown machine hopper. The masterbatch is heated by a twin-screw extruder to become a melt, which is then extruded through the spinning holes in the meltblown die and drawn into ultrafine fibers by hot air. The ultrafine fibers accumulate on the receiving screen and self-bond using residual heat to form the meltblown material. The meltblown process parameters are: heating zones 1 and 2 temperatures are 200 ℃ and 210 ℃ respectively; die heating temperature is 220 ℃; air heating zone temperature is 80 ℃; and the main machine speed is 500 rpm.

[0041] Comparative Example 2: Step 1: Add PLLA:PDLA at a mass ratio of 1:1 to a twin-screw extruder for secondary granulation. With other parameters remaining unchanged, set the temperatures of the five zones to 190℃, 200℃, 210℃, 210℃, and 200℃, and the die temperature to 220℃ to prepare meltblown masterbatch. Step 2: Thoroughly mix SC melt blend masterbatch, PLLA, and citric acid / sodium bicarbonate composite foaming agent at 20 wt%, 77 wt%, and 1.5 wt% / 1.5 wt%, and then add the mixture to a twin-screw extruder for secondary granulation. With other parameters remaining unchanged, set the temperatures of the five zones to 180℃, 190℃, 200℃, 200℃, and 190℃, and the die temperature to 220℃ to prepare meltblown masterbatch. Step 3: The prepared meltblown masterbatch is placed in the meltblown machine hopper. The masterbatch is heated by a twin-screw extruder to become a melt, which is then extruded through the spinning holes in the meltblown die and drawn into ultrafine fibers by hot air. The ultrafine fibers accumulate on the receiving screen and self-bond using residual heat to form the meltblown material. The meltblown process parameters are: heating zones 1 and 2 temperatures are 200 ℃ and 210 ℃ respectively; die heating temperature is 220 ℃; air heating zone temperature is 80 ℃; and the main machine speed is 500 rpm.

[0042] test: (1) Microscopic morphology characterization: The surface morphology, diameter and distribution of fibers in Example 1 and Comparative Example 1 were observed by scanning electron microscopy (SEM).

[0043] SEM images of the fibers prepared in Example 1 are shown below. Figure 1 As shown, the fibers exhibit a distinct porous structure; the SEM image of the fibers prepared in Comparative Example 1 is shown below. Figure 2 As shown, Comparative Example 1 is a traditional solid fiber.

[0044] (2) Heat resistance test: The heat resistance of Example 1, Comparative Example 1 and Comparative Example 2 at different temperatures was tested using a forced-air drying oven. The sample size was 5 cm × 5 cm. The test temperatures were 60 ℃, 70 ℃, 80 ℃, 90 ℃, 100 ℃, 110 ℃ and 120 ℃ for 20 minutes. Each sample was tested 3 times to reduce experimental error. The test results are shown in the figure below. Figure 3 As shown in Table 1 below, the heat resistance test results of the examples and comparative examples are as follows: Table 1. Results of heat resistance tests for the examples and comparative examples. .

[0046] Table 1 shows that the meltblown nonwoven material in Comparative Example 2 had shrunk to 6.2% of its original volume, and the overall remaining volume fraction was significantly lower than that of Example 1 and Comparative Example 1. This indicates that the PLLA / PDLA solution blending method has a higher efficiency in forming SC and better heat resistance. The heat resistance of Example 1 and Comparative Example 1 is very similar, but after 80 °C, Example 1 has better overall heat resistance, and the remaining volume fraction after heating is higher than that of Comparative Example 1, indicating that the addition of porous structure improves heat resistance.

[0047] Those skilled in the art should understand that the technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments have been described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0048] The above embodiments are merely illustrative of several implementation methods of this application, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for preparing a heat-resistant porous meltblown material, characterized in that, Includes the following steps: S1: Dissolve PLLA and PDLA in DMC solvent until fully dissolved, then evaporate the DMC solvent to obtain a filter cake. Dry the filter cake to obtain SC crystalline powder. S2: After fully mixing SC crystalline micro powder, PLLA and composite foaming agent, melt blending and granulation are carried out to obtain meltblown masterbatch; S3: Meltblown masterbatch is meltblown to obtain heat-resistant porous meltblown material.

2. The method for preparing the heat-resistant porous meltblown material according to claim 1, characterized in that, The mass ratio of PLLA to PDLA is 7:3 to 1:

1.

3. The method for preparing the heat-resistant porous meltblown material according to claim 2, characterized in that, The mass ratio of PLLA to PDLA is 1:

1.

4. The method for preparing the heat-resistant porous meltblown material according to claim 1, characterized in that, The mass ratio of SC crystalline micro powder, PLLA and composite foaming agent is (20-30):(68-78):(2-3), and the total mass of the three is 100%.

5. The method for preparing the heat-resistant porous meltblown material according to claim 1, characterized in that, The composite foaming agent is a mixture of citric acid and sodium bicarbonate.

6. The method for preparing the heat-resistant porous meltblown material according to claim 1, characterized in that, The mass ratio of citric acid to sodium bicarbonate is 6:4 to 1:

1.

7. The method for preparing the heat-resistant porous meltblown material according to claim 1, characterized in that, SC crystalline micro powder, PLLA and composite foaming agent are thoroughly mixed and added to a twin-screw extruder for melt blending and granulation to obtain meltblown masterbatch. The temperature of the twin-screw extruder must be lower than the melting point of SC and higher than the melting point of PLLA.

8. The method for preparing the heat-resistant porous meltblown material according to claim 7, characterized in that, The five zone temperatures of the twin-screw extruder are set to 180℃, 190℃, 200℃, 200℃, and 190℃, and the die temperature is 220℃.

9. The method for preparing the heat-resistant porous meltblown material according to claim 1, characterized in that, The prepared meltblown masterbatch is poured into the hopper of the masterbatch meltblown machine. The meltblown masterbatch is heated by a twin-screw extruder to become a melt. The melt is extruded from the spinning hole in the meltblown die and stretched into ultrafine fibers by hot air. The ultrafine fibers accumulate on the receiving screen and self-bond by the residual heat of the fibers to form a heat-resistant porous meltblown material.

10. The method for preparing the heat-resistant porous meltblown material according to claim 9, characterized in that, The conditions for meltblown treatment are: heating zone 1 and zone 2 temperatures of 200 ℃ and 210 ℃ respectively, mold heating temperature of 220 ℃, air heating zone temperature of 80 ℃, and main machine speed of 500 rpm.

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