A polypropylene porous membrane and a method for preparing the same

Abstract

The application discloses a kind of polypropylene porous membrane and its preparation method, preparation method includes: taking long chain branched polypropylene resin or long chain branched polypropylene resin and linear polypropylene melt extrusion forms film base material, film base material is stretched, and is shaped to obtain polypropylene porous membrane;Wherein, long chain branched polypropylene resin long chain branched point content is 0.05~0.50 / 1000C, melting point is greater than 160 DEG C.The long chain branched polypropylene of the application has higher melt strength, in melt extrusion process, it can effectively avoid the problem of uneven film thickness caused by melt flowability too strong;In the stretching stage, long chain branched polypropylene makes film base material under the action of tension can be uniformly deformed, so as to overcome the defect that local fracture or non-uniform pore formation appears in the stretching process of traditional linear polypropylene film;In the shaping stage, the risk of pore size shrinkage and deformation can be significantly reduced.

Description

Technical Field

[0001] This invention belongs to the field of polymer material processing technology, specifically, it relates to a polypropylene porous membrane and its preparation method. Background Technology

[0002] Polypropylene porous membranes are widely used in filtration, separation, and lithium-ion battery separators due to their excellent chemical resistance, high temperature resistance, and high mechanical strength. However, the current manufacturing processes for polypropylene porous membranes still have many shortcomings. These problems not only affect the overall performance of the product but also limit its promotion in high-end applications. Common problems include uneven pore size distribution, complex processing techniques, and high environmental pollution risks, making it difficult for existing technologies to meet the growing demand for high-performance, multifunctional polypropylene porous membranes.

[0003] Currently, some technical solutions attempt to optimize the pore structure of membranes by improving processing techniques. For example, Chinese patent CN1525907A discloses a method for preparing porous membranes by adding a β-crystal nucleating agent to promote the formation of polypropylene β crystals and using a continuous biaxial stretching process. Although this method performs well in terms of tear resistance and porosity, it still has several prominent problems. First, the high equipment cost and complex processing technology increase the difficulty of production. Second, the pore size distribution is relatively wide, making it difficult to meet the application scenarios with high requirements for pore size uniformity. In addition, this method has strict requirements for processing parameters, and slight carelessness can affect the quality and performance of the final membrane material.

[0004] For example, the supercritical fluid foaming technology used in Chinese patent CN104629180A achieves the preparation of high-flux polypropylene porous membranes through the swelling effect of supercritical carbon dioxide and the induced rupture effect of siloxane-containing porogens. Although this method has significant advantages in membrane permeability and is more environmentally friendly than traditional solvent methods, it has several technical problems. First, the equipment cost is high, and the operation and maintenance of the supercritical fluid system require additional investment. Second, the process has extremely high requirements for parameter control, and the rupture and connectivity of the pore walls during foaming is difficult to stabilize, resulting in large fluctuations in pore size distribution, which affects the consistency of the product and limits its application in fields such as high consistency and precision filtration.

[0005] Furthermore, existing technologies utilize pore-forming agents or compatibilizers added to polypropylene resin to regulate pore structure, such as small organic pore-forming agents, inorganic particles, or polymeric compatibilizers. However, this method often increases formulation complexity and is highly dependent on process conditions. Different batches of production can lead to significant changes in the membrane's microstructure, affecting key indicators such as the final product's mechanical properties, pore size distribution, and permeability. Therefore, the stability and controllability of such methods in industrial production remain challenging, particularly in demanding applications requiring precision filtration or high consistency.

[0006] In view of this, the present invention is hereby proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a polypropylene porous membrane and its preparation method, so as to achieve a simple preparation process, environmental friendliness and precise control of the pore structure of the polypropylene porous membrane, and overcome the problems of uneven pore size distribution, complex processing and poor product consistency in the prior art.

[0008] To solve the above-mentioned technical problems, the basic concept of the technical solution adopted by the present invention is as follows:

[0009] A method for preparing a polypropylene porous membrane involves taking long-chain branched polypropylene resin or long-chain branched polypropylene resin and linear polypropylene and melt-extruding to form a film substrate, and then stretching and shaping the film substrate to obtain a polypropylene porous membrane.

[0010] Among them, the long-chain branching point content of the long-chain branched polypropylene resin is 0.05~0.50 / 1000C, and the melting point is greater than 160℃.

[0011] The long-chain branched polypropylene used in this invention is obtained by hydrolysis after polymerization using a Ziegler-Natta catalyst, and has the following characteristics:

[0012] i) Melting point: The melting point of this long-chain branched polypropylene is above 160°C, which helps to improve the thermal stability and processing performance of the material.

[0013] ii) Long-chain branching content: The long-chain branching point content of long-chain branched polypropylene can be determined by hydrogen nuclear magnetic resonance spectroscopy (NMR spectroscopy). 1 The results are measured by ¹H NMR and expressed as the number of long-chain branching points per 1000 carbon atoms. In this invention, the content of long-chain branching points ranges from 0.05 to 0.50 / 1000C, that is, there are approximately 0.05 to 0.50 long-chain branching points per 1000 carbon atoms.

[0014] This invention controls the content of long-chain branching points between 0.05 and 0.50 / 1000C, so that the material has both good melt strength and suitable fluidity. The pore formation process is optimized in the stretching and shaping steps, thereby preparing a high-performance polypropylene porous membrane with uniform pore structure and excellent mechanical properties.

[0015] The long-chain branched polypropylene of this invention has a melting point above 160°C, which makes the porous membrane of this application less prone to softening or deformation in high-temperature processing or application environments (such as filtering high-temperature fluids, lithium-ion battery separators, etc.); during high-temperature extrusion and stretching, the film substrate can maintain good structural integrity and avoid deformation or tensile breakage due to excessively low melting point.

[0016] Furthermore, the long-chain branching point content of the long-chain branched polypropylene resin is 0.15 to 0.20 / 1000°C.

[0017] Further, by mass parts, the long-chain branched polypropylene resin is 50-100 parts by mass and the linear polypropylene is 0-50 parts by mass, wherein the melting point of the linear polypropylene is 155-170°C.

[0018] The addition of linear polypropylene can increase the flexibility of porous membranes and improve their elongation at break, making them less prone to breakage under external forces, thereby enhancing their durability.

[0019] Further, by weight parts, the long-chain branched polypropylene resin is 70-100 parts by weight, the linear polypropylene is 0-30 parts by weight, the compatibilizer is 1-10 parts by weight, and the pore-forming agent is 5-20 parts by weight.

[0020] Compatibilizers can improve the interfacial compatibility between long-chain branched polypropylene resin and linear polypropylene, ensuring uniform mixing and improving the processing stability of the melt.

[0021] By adding an appropriate amount of pore-forming agent, uniformly distributed pores can be formed during the stretching process of the film substrate, thereby improving air permeability and filtration performance.

[0022] Furthermore, the compatibilizer is selected from one or more of maleic anhydride-grafted polypropylene, acrylic acid-grafted polypropylene, glycidyl methacrylate-grafted polypropylene, styrene-maleic anhydride copolymer, and ethylene-vinyl acetate copolymer.

[0023] Furthermore, the pore-forming agent is selected from one or more of polyethylene glycol, polypropylene oxide, polysiloxane, calcium carbonate, titanium dioxide, and silicon dioxide.

[0024] Furthermore, long-chain branched polypropylene resin or a mixture of long-chain branched polypropylene resin and linear polypropylene is extruded using a twin-screw extruder to form a film substrate, with an extrusion temperature of 170–230°C and a screw speed of 30–80 rpm.

[0025] Furthermore, the extruded film substrate is stretched at a temperature of 25–50°C, a stretching rate of 5–100 mm / min, and a stretching ratio of 1–6 times.

[0026] Furthermore, the stretched film substrate is subjected to heat treatment at a temperature of 100–150°C.

[0027] The present invention also provides a polypropylene porous membrane, which is prepared by any of the preparation methods described in the above technical solutions. The pore size distribution range of the polypropylene porous membrane is 0.05 to 4.0 μm, and the Young's modulus is ≥460 MPa.

[0028] By adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art.

[0029] The long-chain branched polypropylene of the present invention has high melt strength, which can effectively avoid the problem of uneven film thickness caused by excessive melt flow during melt extrusion. In addition, the long-chain branched polypropylene has a moderate crystallization rate, which can form a more uniform spherulite structure during cooling, providing a good microstructure basis for the subsequent stretching stage and helping to improve the pore size uniformity of the film.

[0030] During the stretching stage, long-chain branched polypropylene enables the film substrate to deform uniformly under tensile force, effectively preventing the non-uniform porosity problem found in traditional linear polypropylene films. Because long-chain branched polypropylene forms a stable fibrous structure during stretching, the pores expand along a predetermined direction under stress, avoiding uneven pore size distribution caused by stress concentration. Furthermore, the presence of the long-chain branched structure can increase the film's stretch ratio range, making the film's microstructure more easily controllable and thus adapting to different porosity requirements.

[0031] During the shaping stage, long-chain branched polypropylene ensures that the pore structure formed during stretching remains stable during subsequent heat treatment, reducing the possibility of pore shrinkage or deformation. Compared with traditional linear polypropylene membranes, porous membranes prepared using long-chain branched polypropylene can better maintain the uniformity of pore size during the shaping process, and further improve the overall mechanical properties of the membrane, such as Young's modulus and tear strength, after heat stabilization treatment. This not only improves the durability of the membrane, but also enhances its performance in demanding applications such as precision filtration and lithium battery separators.

[0032] The specific embodiments of the present invention will be described in further detail below.

[0033] It should be noted that the textual descriptions in these embodiments are not intended to limit the scope of the invention in any way, but rather to illustrate the concepts of the invention to those skilled in the art. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0035] The present invention tests the performance of the prepared porous membrane, and the test method is as follows:

[0036] Young's modulus test: The prepared polypropylene porous membrane was subjected to tensile testing using a universal testing machine. The test conditions were room temperature (25℃), tensile rate of 5 mm / min, and test direction parallel to the surface of the porous membrane. The Young's modulus of the material was calculated using the stress-strain curve.

[0037] Pore ​​structure size test: The surface morphology of the prepared polypropylene porous membrane was observed using a scanning electron microscope (SEM) to obtain high-resolution images of the pore structure. The pore size and distribution were quantitatively analyzed using image analysis software (ImageJ). The size distribution range of the pores was obtained by statistical analysis of multiple regions.

[0038] Evaluation of the spatial distribution of void structures:

[0039] Surface morphology images of multiple regions of the sample were acquired using SEM, and the spatial distribution characteristics of the voids were evaluated using image analysis software (ImageJ). The evaluation criterion was Nearest Neighbor Distance (NND). NND was used to calculate the average distance between adjacent voids. and standard deviation (σ) d ), and further calculate its coefficient of variation (CV) NND ):

[0040]

[0041] According to CV NND The value of categorizes the spatial distribution of voids into the following four types:

[0042] Uniform porosity distribution (CV<0.3): Porosity is uniformly distributed across the entire membrane surface, with no obvious aggregated or dispersed areas;

[0043] The voids are relatively uniformly distributed (0.3≤CV<0.5): The voids are generally uniformly distributed, but there may be slight aggregation or dispersion in local areas;

[0044] The porosity distribution is relatively dispersed (0.5≤CV<0.7): the porosity is unevenly distributed on the membrane surface, with large areas of porosity aggregation or absence in some local areas;

[0045] The voids are distributed in a dispersed manner (CV≥0.7): the voids are distributed in a distinct regional pattern, with large areas of void aggregation or sparse areas.

[0046] The long-chain branched polypropylene used in this invention is obtained by hydrolysis after polymerization using a Ziegler-Natta catalyst, and has the following characteristics:

[0047] i) Melting point: The melting point of this long-chain branched polypropylene is above 160°C. The excellent melting point characteristics help to improve the thermal stability and processing performance of the material.

[0048] ii) Long-chain branching content: The long-chain branching point content of long-chain branched polypropylene can be determined by hydrogen nuclear magnetic resonance spectroscopy (NMR spectroscopy). 1 The results are measured by ¹H NMR and expressed as the number of long-chain branching points per 1000 carbon atoms. In this invention, the content of long-chain branching points ranges from 0.05 to 0.50 / 1000C, that is, there are approximately 0.05 to 0.50 long-chain branching points per 1000 carbon atoms.

[0049] Example 1

[0050] Raw material: Long-chain branched polypropylene resin with a branching point content of 0.30 / 1000°C and a melting point of 162°C;

[0051] Preparation steps:

[0052] (1) Extrusion: Long-chain branched polypropylene resin is fed into a twin-screw extruder at an extrusion temperature of 200°C and a screw speed of 50 rpm to form a film substrate.

[0053] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 30 mm / min, a stretching ratio of 2 times, and a stretching temperature of room temperature (25℃). An initial pore structure is formed on the film substrate.

[0054] (3) Heat setting treatment: The stretched film is heat-treated at 130°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0055] Example 2

[0056] Raw material: Long-chain branched polypropylene resin with a branching point content of 0.25 / 1000°C and a melting point of 161°C;

[0057] Preparation steps:

[0058] (1) Extrusion: Long-chain branched polypropylene resin is fed into a twin-screw extruder at an extrusion temperature of 200℃ and a screw speed of 40rpm to form a film substrate.

[0059] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 25 mm / min, a stretching ratio of 3 times, and a stretching temperature of 40℃, forming an initial pore structure on the film.

[0060] (3) Heat setting treatment: The stretched film is heat-treated at 135°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0061] Example 3

[0062] Raw material: Long-chain branched polypropylene resin with a branching point content of 0.2 / 1000°C and a melting point of 160°C;

[0063] Preparation steps:

[0064] (1) Extrusion: Long-chain branched polypropylene resin is fed into a twin-screw extruder at an extrusion temperature of 190°C and a screw speed of 50 rpm to form a film substrate;

[0065] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 35 mm / min, a stretching ratio of 2.5 times, and a stretching temperature of 35°C, forming an initial pore structure on the film.

[0066] (3) Heat setting treatment: The stretched film is heat-treated at 130°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0067] Example 4

[0068] Raw materials: 70 parts by weight of long-chain branched polypropylene resin with a long-chain branching point content of 0.35 / 1000C, and 30 parts by weight of linear polypropylene, wherein the melting point of long-chain branched polypropylene is 163℃ and the melting point of linear polypropylene is 160℃.

[0069] Preparation steps:

[0070] (1) Extrusion: Long-chain branched polypropylene resin and linear polypropylene are mixed and fed into a twin-screw extruder after being mixed evenly. The extrusion temperature is 200℃ and the screw speed is 55rpm to form a film substrate.

[0071] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 30 mm / min, a stretching ratio of 2.5 times, and a stretching temperature of 40℃, forming an initial pore structure on the film.

[0072] (3) Heat setting treatment: The stretched film is heat-treated at 130°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0073] Example 5

[0074] Raw materials: 60 parts by weight of long-chain branched polypropylene resin with a long-chain branching point content of 0.4 / 1000C, and 40 parts by weight of linear polypropylene, wherein the melting point of long-chain branched polypropylene is 164℃ and the melting point of linear polypropylene is 168℃.

[0075] Preparation steps:

[0076] (1) Extrusion: Long-chain branched polypropylene resin and linear polypropylene are mixed and fed into a twin-screw extruder after being mixed evenly. The extrusion temperature is 200℃ and the screw speed is 60rpm to form a film substrate.

[0077] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 25 mm / min, a stretching ratio of 3 times, and a stretching temperature of 50℃, forming an initial pore structure on the film.

[0078] (3) Heat setting treatment: The stretched film is heat-treated at 135°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0079] Example 6

[0080] Raw materials: 50 parts by weight of long-chain branched polypropylene resin with a long-chain branching point content of 0.45 / 1000C, and 50 parts by weight of linear polypropylene, wherein the melting point of long-chain branched polypropylene is 165℃ and the melting point of linear polypropylene is 158℃.

[0081] Preparation steps:

[0082] (1) Extrusion: Long-chain branched polypropylene resin and linear polypropylene are mixed and fed into a twin-screw extruder after being mixed evenly. The extrusion temperature is 190℃ and the screw speed is 45rpm to form a film substrate.

[0083] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 30 mm / min, a stretching ratio of 2 times, and a stretching temperature of 40℃, forming an initial pore structure on the film.

[0084] (3) Heat setting treatment: The stretched film is heat-treated at 130°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0085] Example 7

[0086] Raw materials: 87 parts by weight of long-chain branched polypropylene resin with a long-chain branching point content of 0.2 / 1000C, 3 parts by weight of compatibilizer ethylene-vinyl acetate copolymer, and 10 parts by weight of pore-forming agent calcium carbonate; wherein, the melting point of long-chain branched polypropylene is 160℃.

[0087] Preparation steps:

[0088] (1) Extrusion: The long-chain branched polypropylene resin, compatibilizer and pore-forming agent are mixed evenly and then fed into a twin-screw extruder. The extrusion temperature is 210℃ and the screw speed is 55rpm to form a film substrate. During the extrusion process, the shear rate is controlled to ensure the uniform dispersion of the raw materials, especially the distribution of the pore-forming agent.

[0089] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 20 mm / min, a stretching ratio of 2.5 times, and a stretching temperature of 50°C, forming an initial pore structure on the film.

[0090] (3) Heat setting treatment: The stretched film is heat-treated at 140°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0091] Example 8

[0092] Raw materials: 79 parts by weight of long-chain branched polypropylene resin with a long-chain branching point content of 0.15 / 1000C, 6 parts by weight of compatibilizer glycidyl methacrylate grafted polypropylene, and 15 parts by weight of pore-forming agent silica; wherein, the melting point of long-chain branched polypropylene is 160℃.

[0093] Preparation steps:

[0094] (1) Extrusion: The long-chain branched polypropylene resin, compatibilizer and pore-forming agent are mixed evenly and then fed into a twin-screw extruder. The extrusion temperature is 205℃ and the screw speed is 60rpm to form a film substrate. During the extrusion process, the shear rate is controlled to ensure the uniform dispersion of the raw materials, especially the distribution of the pore-forming agent.

[0095] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 40 mm / min, a stretching ratio of 2 times, and a stretching temperature of room temperature (around 25°C), forming an initial pore structure on the film.

[0096] (3) Heat setting treatment: The stretched film is heat-treated at 125°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0097] Example 9

[0098] Raw materials: 74 parts by weight of long-chain branched polypropylene resin with a long-chain branching point content of 0.2 / 1000C, 10 parts by weight of linear polypropylene, 8 parts by weight of maleic anhydride grafted polypropylene compatibilizer, and 8 parts by weight of polyethylene glycol porogen; wherein, the melting point of long-chain branched polypropylene is 160℃ and the melting point of linear polypropylene is 165℃.

[0099] Preparation steps:

[0100] (1) Extrusion: The long-chain branched polypropylene resin, linear polypropylene, compatibilizer and pore-forming agent are mixed evenly and then fed into a twin-screw extruder. The extrusion temperature is 210℃ and the screw speed is 55rpm to form a film substrate. During the extrusion process, the shear rate is controlled to ensure the uniform dispersion of the raw materials, especially the distribution of the pore-forming agent.

[0101] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 30 mm / min, a stretching ratio of 3 times, and a stretching temperature of room temperature (around 25°C), forming an initial pore structure on the film.

[0102] (3) Heat setting treatment: The stretched film is heat-treated at 130°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0103] Example 10

[0104] Raw materials: 74 parts by weight of long-chain branched polypropylene resin with a long-chain branching point content of 0.05 / 1000C, 10 parts by weight of linear polypropylene, 1 part by weight of maleic anhydride grafted polypropylene compatibilizer, and 20 parts by weight of polyethylene glycol porogen; wherein, the melting point of long-chain branched polypropylene is 160℃ and the melting point of linear polypropylene is 155℃.

[0105] Preparation steps:

[0106] (1) Extrusion: The long-chain branched polypropylene resin, linear polypropylene, compatibilizer and pore-forming agent are mixed evenly and then fed into a twin-screw extruder. The extrusion temperature is 210℃ and the screw speed is 55rpm to form a film substrate. During the extrusion process, the shear rate is controlled to ensure the uniform dispersion of the raw materials, especially the distribution of the pore-forming agent.

[0107] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 30 mm / min, a stretching ratio of 3 times, and a stretching temperature of room temperature (around 25°C), forming an initial pore structure on the film.

[0108] (3) Heat setting treatment: The stretched film is heat-treated at 130°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0109] Example 11

[0110] Raw materials: 74 parts by weight of long-chain branched polypropylene resin with a long-chain branching point content of 0.5 / 1000C, 10 parts by weight of linear polypropylene, 10 parts by weight of maleic anhydride grafted polypropylene compatibilizer, and 5 parts by weight of polyethylene glycol porogen; wherein, the melting point of long-chain branched polypropylene is 160℃ and the melting point of linear polypropylene is 170℃.

[0111] Preparation steps:

[0112] (1) Extrusion: The long-chain branched polypropylene resin, linear polypropylene, compatibilizer and pore-forming agent are mixed evenly and then fed into a twin-screw extruder. The extrusion temperature is 210℃ and the screw speed is 55rpm to form a film substrate. During the extrusion process, the shear rate is controlled to ensure the uniform dispersion of the raw materials, especially the distribution of the pore-forming agent.

[0113] (2) Stretching and preliminary shaping: The film substrate is stretched at a stretching rate of 30 mm / min, a stretching ratio of 3 times, and a stretching temperature of room temperature (around 25°C), forming an initial pore structure on the film.

[0114] (3) Heat setting treatment: The stretched film is heat-treated at 130°C to further stabilize the pore size and pore structure, enhance the overall performance of the porous membrane, and finally obtain a polypropylene porous membrane.

[0115] Comparative Example 1

[0116] The only difference between this comparative example and Example 1 is the raw material. The raw material for this comparative example is linear polypropylene with a melting point of 160°C.

[0117] Comparative Example 2

[0118] The only difference between this comparative example and Example 1 is that the branching point content of the long-chain branched polypropylene resin is different. The branching point content of the long-chain branched polypropylene resin in this comparative example is 0.03 / 1000C, which is lower than that of the long-chain branched polypropylene resin in Example 1.

[0119] Comparative Example 3

[0120] The only difference between this comparative example and Example 1 is that the branching point content of the long-chain branched polypropylene resin is different. The branching point content of the long-chain branched polypropylene resin in this comparative example is 0.6 / 1000C, which is higher than that of the long-chain branched polypropylene resin in Example 1.

[0121] Comparative Example 4

[0122] The only difference between this comparative example and Example 7 is that "long-chain branched polypropylene resin" is replaced with an equal number of parts by mass of "linear polypropylene".

[0123] Comparative Example 5

[0124] The only difference between this comparative example and Example 7 is that "long-chain branched polypropylene resin with a long-chain branching point content of 0.2 / 1000C" is replaced with an equal mass fraction of "long-chain branched polypropylene resin with a long-chain branching point content of 0.03 / 1000C".

[0125] Comparative Example 6

[0126] The only difference between this comparative example and Example 7 is that "long-chain branched polypropylene resin with a long-chain branching point content of 0.2 / 1000C" is replaced with an equal number of parts by mass of "long-chain branched polypropylene resin with a long-chain branching point content of 0.6 / 1000C".

[0127] The performance of the porous membranes obtained in Examples 1 to 9 and Comparative Examples 1 to 6 was tested, and the results are shown in Table 1 below:

[0128] Table 1

[0129]

[0130]

[0131] Example 1 uses long-chain branched polypropylene, which exhibits significant advantages over the linear polypropylene used in Comparative Example 1 in terms of melt processing, pore structure control, and mechanical properties. Due to its long-chain branched structure, long-chain branched polypropylene has higher melt strength and maintains a more stable morphology during melt processing. During twin-screw extrusion, the melt of long-chain branched polypropylene can be more uniformly dispersed, forming fine and uniform spherulitic structures, providing stable microstructural support for the subsequent stretching process. In contrast, linear polypropylene lacks long-chain branching points, resulting in higher melt fluidity and a slower crystallization rate, leading to the formation of larger spherulitic structures during crystallization. This makes the porous membrane prone to uneven pore size and excessively wide pore size distribution during the stretching stage. The membrane prepared in Example 1 has a concentrated pore size distribution (0.05–2.0 μm) and good uniformity, while the membrane prepared from linear polypropylene has a larger pore range (1–15 μm) and a more dispersed distribution, making it difficult to meet the requirements of high-precision filtration or lithium-ion battery separators.

[0132] The main difference between Example 1 and Comparative Example 2 lies in the branching point content of the long-chain branched polypropylene. The branching point content of the long-chain branched polypropylene in Example 1 was 0.30 / 1000C, while that in Comparative Example 2 was only 0.03 / 1000C, significantly lower than that in Example 1. Although the long-chain branched polypropylene in Comparative Example 2 still possessed a certain degree of branching structure, its melt strength decreased significantly due to the excessively low branching point content. Furthermore, the lower branching content led to a slower crystallization rate and larger spherulites, resulting in a more uneven microstructure of the membrane. Particularly during the stretching process, this resulted in larger pore sizes, wider pore size distribution, and poorer pore structure consistency, affecting the membrane's filtration accuracy and permeability.

[0133] In terms of mechanical properties, the Young's modulus of Example 1 was 550 MPa, while that of Comparative Example 2, due to its lower branching point content and insufficient melt strength, decreased to 390 MPa, resulting in a decline in mechanical properties. Long-chain branched polypropylene with a higher branching point content can provide better molecular chain orientation and stress dispersion during stretching, thereby improving the overall rigidity and tear resistance of the membrane. However, due to insufficient branching structure, the molecular chains in Comparative Example 2 struggled to form a stable orientation structure during stretching, leading to weaker mechanical properties and a greater susceptibility to localized deformation or breakage under stress. Furthermore, the larger spherulitic structure also affected the overall uniformity of the membrane, making it prone to dimensional shrinkage or deformation under heat or stress, thus affecting its service life. In summary, an excessively low branching point content leads to uneven pore structure, decreased mechanical properties, and poor dimensional stability in the membrane. Therefore, the performance of Comparative Example 2 is significantly inferior to that of Example 1, failing to meet the application requirements of a high-performance porous membrane.

[0134] The main difference between Example 1 and Comparative Example 3 lies in the branching point content of the long-chain branched polypropylene (LCB-PP). Example 1 has a branching point content of 0.30 / 1000C, while Comparative Example 3 has a higher branching point content of 0.60 / 1000C. Although an appropriate amount of long-chain branching can improve melt strength and processing stability, excessively high branching point content leads to increased gel content and reduced melt flowability, negatively impacting processing and stretching.

[0135] Due to the excessively high content of long-chain branching points in Comparative Example 3, its melt exhibits low fluidity, limiting the material's deformation capacity during extrusion and stretching. Furthermore, materials with high branching content tend to form gel structures, which are fragile and prone to breakage during stretching, resulting in a structure with large and widely distributed pore sizes. This affects the membrane's pore size uniformity and stability. This situation leads to an unstable membrane pore structure, making the pore morphology easily disrupted during stretching, thus making pore size control of the final membrane difficult and affecting its consistency and reliability in applications such as high-precision filtration.

[0136] In terms of mechanical properties, the Young's modulus of Example 1 was 550 MPa, while that of Comparative Example 3, due to its excessively high branching point content, resulted in a brittle gel structure in the membrane, making it difficult to form a stable stress distribution during stretching. This ultimately led to a decrease in the Young's modulus of the membrane to 420 MPa, indicating lower mechanical properties than Example 1. The high branching point content makes it difficult for the material to achieve uniform orientation during stretching, resulting in reduced tensile and tear resistance, affecting its durability and structural stability. Furthermore, the excessively high degree of branching may cause the membrane's micropore structure to become uneven due to gelation, reducing its filtration performance and making it unsuitable for high-end applications. Therefore, the excessively high branching point content not only failed to improve performance but also led to unstable pore structure and decreased mechanical properties, making the membrane of Comparative Example 3 significantly limited in practical applications and inferior to the superior overall performance of Example 1.

[0137] The main difference between Example 7 and Comparative Example 4 lies in the matrix resin. Example 7 used long-chain branched polypropylene (LCB-PP), while Comparative Example 4 used an equal part by mass of linear polypropylene (PP) instead of LCB-PP. Although both formulations contain a compatibilizer (ethylene-vinyl acetate copolymer) and a pore-forming agent (calcium carbonate), the difference in the matrix resin has a significant impact on the final performance of the membrane.

[0138] In Example 7, the long-chain branched polypropylene (LCB-PP) has a suitable long-chain branching point (0.2 / 1000°C), high melt strength, and can form a stable melt network structure during extrusion and stretching, resulting in a more uniform pore distribution and good stability after heat setting. In contrast, in Comparative Example 4, linear polypropylene, lacking a long-chain branched structure, is more prone to forming uneven pores during stretching, leading to a wider pore size distribution and poorer pore structure stability in the membrane. Although the addition of compatibilizers and pore-forming agents improved the membrane's pore structure to some extent, making it slightly more uniform than pure linear polypropylene (PP), the formation and stability of pores during stretching were still inferior to those of long-chain branched polypropylene (LCB-PP), ultimately reducing the membrane's filtration accuracy and permeability.

[0139] The Young's modulus of Example 7 is higher than that of Comparative Example 4, indicating that long-chain branched polypropylene (LCB-PP) provides better tensile deformation capability, making the membrane less prone to breakage under stress and exhibiting stronger overall rigidity. In contrast, Comparative Example 4, due to the lack of sufficient stress support during the stretching process of pure linear polypropylene (PP), resulted in a lower Young's modulus and decreased mechanical properties in its porous membrane. Furthermore, the high melt strength of long-chain branched polypropylene (LCB-PP) makes the membrane more dimensionally stable, maintaining a good pore structure even during heat setting. Pure linear polypropylene (PP), on the other hand, is prone to significant pore size changes during heat treatment due to shrinkage or deformation, affecting the final membrane's stability. Therefore, although Comparative Example 4 improved the pore distribution of the membrane to some extent by adding compatibilizers and porogens, the weaker melt properties of the matrix resin resulted in lower pore uniformity, mechanical properties, and stability compared to Example 7, failing to achieve its superior overall performance.

[0140] The main difference between Example 7 and Comparative Example 5 lies in the branching point content of the long-chain branched polypropylene (LCB-PP). Example 7 used long-chain branched polypropylene (LCB-PP) with a branching point content of 0.2 / 1000C, while the long-chain branched polypropylene (LCB-PP) in Comparative Example 5 had a branching point content of only 0.03 / 1000C, which is much lower than that in Example 7. Although Comparative Example 5 still used long-chain branched polypropylene (LCB-PP), its lower branching point content affected the melt properties, crystallization behavior, and the pore structure and mechanical properties of the final film.

[0141] Regarding the pore structure, the long-chain branched polypropylene (LCB-PP) in Example 7 had a moderate branching point content, resulting in high melt strength and enabling the formation of a stable pore structure during stretching. The pores formed after stretching were small and uniformly distributed, and the pore structure maintained good stability after heat setting. In contrast, in Comparative Example 5, the branching point content of the long-chain branched polypropylene (LCB-PP) was too low, and the crystallization rate was slow, resulting in larger spherulites. During stretching, pore formation was less uniform, leading to a larger pore size and wider size distribution in the final membrane, and poorer pore stability. Although the addition of compatibilizers and porogens improved the pore structure to some extent, making it slightly more uniform than pure linear polypropylene (PP), it still could not compensate for the instability caused by the low branching point content of the long-chain branched polypropylene (LCB-PP), ultimately affecting the membrane's filtration performance and consistency.

[0142] In terms of mechanical properties, Example 7 exhibits a higher Young's modulus, indicating stronger rigidity and tensile strength. In contrast, the membrane in Comparative Example 5, due to insufficient branching point content in long-chain branched polypropylene (LCB-PP), struggles to form a stable molecular orientation structure during stretching, leading to decreased mechanical strength. Furthermore, the long-chain branched polypropylene (LCB-PP) in Example 7 maintains better dimensional stability during heat setting, while the membrane in Comparative Example 5 is prone to shrinkage or pore deformation during heat treatment, further affecting its overall performance. In summary, although Comparative Example 5 still uses long-chain branched polypropylene (LCB-PP), its low branching point content results in lower pore uniformity, mechanical properties, and dimensional stability compared to Example 7, failing to meet ideal performance standards.

[0143] The main difference between Example 7 and Comparative Example 6 lies in the branching point content of the long-chain branched polypropylene (LCB-PP). The branching point content of the LCB-PP in Example 7 was 0.2 / 1000C, while that in Comparative Example 6 was higher, at 0.6 / 1000C. Although an appropriate amount of long-chain branching can improve melt strength and pore stability, excessively high branching point content reduces material fluidity and introduces a gel structure, negatively impacting the processing stability and final performance of the film.

[0144] Regarding the pore structure, the long-chain branched polypropylene (LCB-PP) in Example 7 has a moderate branching point content, resulting in high melt strength while maintaining appropriate fluidity. The pore structure formed during extrusion and stretching is stable and uniformly distributed, ensuring the membrane's filtration accuracy and permeability. In contrast, in Comparative Example 6, the excessively high branching point content leads to a gel structure, resulting in poor melt fluidity and difficulty in forming stable pore expansion during stretching. Especially at high stretching ratios, the gel structure is brittle and prone to breakage, resulting in an uneven pore structure with large and widely distributed pores. Even with the addition of compatibilizers and pore-forming agents, it is difficult to homogenize the pore structure, leading to decreased membrane filtration accuracy and limited permeability.

[0145] In terms of mechanical properties, the membrane of Example 7 exhibits a higher Young's modulus, providing better tensile strength and tear resistance, ensuring good mechanical stability during use. In contrast, the membrane of Comparative Example 6, due to its fragile gel structure, suffers from uneven stress distribution during stretching, leading to a decrease in Young's modulus and deterioration in mechanical properties. Furthermore, the high branching content of long-chain branched polypropylene (LCB-PP) is easily affected by the gel structure during heat setting, resulting in poor dimensional stability and difficulty in maintaining stable pore morphology, thus affecting the consistency of the final product. Therefore, although Comparative Example 6 optimized the pore structure to some extent through compatibilizers and pore-forming agents, the excessively high branching point content of the long-chain branched polypropylene (LCB-PP) results in membranes with inferior pore uniformity, mechanical properties, and dimensional stability compared to Example 7, failing to meet the requirements of high-performance filtration and diaphragm applications.

[0146] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a porous polypropylene membrane, characterized in that: Long-chain branched polypropylene resin or long-chain branched polypropylene resin and linear polypropylene are melt-extruded to form a film substrate. The film substrate is stretched and shaped to obtain a polypropylene porous membrane. Among them, the long-chain branching point content of the long-chain branched polypropylene resin is 0.05~0.50 / 1000C, and the melting point is greater than 160℃.

2. The method for preparing a polypropylene porous membrane according to claim 1, characterized in that: The long-chain branching point content of the long-chain branched polypropylene resin is 0.15 to 0.20 per 1000°C.

3. A method for preparing a polypropylene porous membrane according to claim 1 or 2, characterized in that: The long-chain branched polypropylene resin comprises 50-100 parts by weight and the linear polypropylene comprises 0-50 parts by weight, wherein the melting point of the linear polypropylene is 155-170°C.

4. A method for preparing a polypropylene porous membrane according to claim 3, characterized in that: By weight, the long-chain branched polypropylene resin is 70-100 parts by weight, the linear polypropylene is 0-30 parts by weight, the compatibilizer is 1-10 parts by weight, and the pore-forming agent is 5-20 parts by weight.

5. A method for preparing a polypropylene porous membrane according to claim 4, characterized in that: The compatibilizer is selected from one or more of maleic anhydride-grafted polypropylene, acrylic acid-grafted polypropylene, glycidyl methacrylate-grafted polypropylene, styrene-maleic anhydride copolymer, and ethylene-vinyl acetate copolymer.

6. A method for preparing a polypropylene porous membrane according to claim 4 or 5, characterized in that: The pore-forming agent is selected from one or more of polyethylene glycol, polypropylene oxide, polysiloxane, calcium carbonate, titanium dioxide, and silicon dioxide.

7. A method for preparing a polypropylene porous membrane according to any one of claims 1 to 6, characterized in that: Long-chain branched polypropylene resin or a mixture of long-chain branched polypropylene resin and linear polypropylene is extruded into a film substrate using a twin-screw extruder. The extrusion temperature is 170–230°C and the screw speed is 30–80 rpm.

8. A method for preparing a polypropylene porous membrane according to claims 1-7, characterized in that: The extruded film substrate is stretched at a temperature of 25–50°C, a stretching rate of 5–100 mm / min, and a stretching ratio of 1–6 times.

9. A method for preparing a polypropylene porous membrane according to claims 1 to 8, characterized in that: The stretched film substrate is heat-treated at a temperature of 100–150°C.

10. A porous polypropylene membrane, characterized in that, The polypropylene porous membrane prepared by any one of the preparation methods described in claims 1 to 9 has a pore size distribution range of 0.05 to 4.0 μm and a Young's modulus ≥ 460 MPa.

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