A polyolefin porous membrane and a method for producing the same

Abstract

The application discloses a polyolefin porous membrane, wherein the Gurley value of the polyolefin porous membrane is less than or equal to 25 s / 100 mL, and the tortuosity is less than 1.5; the thickness of the polyolefin porous membrane is 5-100 microns, the porosity is 65-80%, and the pore size is 50-200 nm. The polyolefin porous membrane has excellent permeability, and the preparation process has continuity, so that the permeability use requirement of the polyolefin porous membrane in the fields of separation and filtration is ensured.

Description

Technical Field

[0001] This invention belongs to the field of polyolefin porous membrane manufacturing technology, specifically relating to a polyolefin porous membrane and its preparation method. Background Technology

[0002] Porous membranes, especially polyolefin microporous membranes, are widely used in various technical fields such as battery separators, capacitor separators, fuel cell materials, and various separation membranes. Thermally induced phase separation (TIPS) is the most commonly used method for preparing polyolefin microporous membranes. In recent years, with the development of the petrochemical industry, the production of polyolefins has increased dramatically, with polyethylene (PE) accounting for 25%.

[0003] To broaden the application fields of polyethylene, existing technologies typically employ stretching equipment to re-stretch the finished raw material membrane, or use polyethylene of different molecular weights to prepare a three-layer laminated polyethylene microporous membrane with different orientation structures via the TIPS process. However, existing technologies involve complex preparation processes with no continuity, resulting in polyethylene porous membranes with high Gurley values ​​and high tortuosity, which cannot meet the requirements of porous membranes in certain specific fields. Therefore, it is urgent to solve the problems of high Gurley values ​​and high tortuosity in polyolefin porous membranes. Summary of the Invention

[0004] The present invention aims to at least partially solve at least one of the technical problems existing in the prior art. To this end, the present invention provides a polyolefin porous membrane with low Gurley value and tortuosity value, exhibiting excellent permeability performance, and meeting the permeability requirements for use in separation filtration and other fields.

[0005] In a first aspect, the present invention provides a polyolefin porous membrane having a Gurley value ≤ 25s / 100mL and a tortuosity <1.5.

[0006] The Gurley value is an important parameter for measuring the permeability of porous membranes; a lower Gurley value indicates better permeability. A Gurley value ≤ 25 s / 100mL indicates good permeability, allowing gas or liquid to pass smoothly through the membrane pores. This enables rapid separation and screening of substances during filtration. For example, in water treatment, water molecules can quickly pass through the membrane pores, while impurities are retained, thus purifying the water. A tortuosity < 1.5 indicates a relatively straight pore path and low flow resistance within the membrane, further improving filtration efficiency. Due to its good permeability and low flow resistance, lower energy consumption is required during filtration and separation processes. For example, in gas separation, lower pressure is not required for gas to pass through the membrane, reducing energy consumption and equipment operating costs. Therefore, the polyolefin porous membrane of this invention exhibits excellent permeability performance, meeting the permeability requirements for applications in separation and filtration.

[0007] In some implementations, the polyolefin porous membrane has a Gurley value of 10–25 s / 100 mL and a tortuosity of 1–1.5.

[0008] In some implementations, the polyolefin porous membrane satisfies at least one of the following: (1) The thickness of the polyolefin porous membrane is 5 ~ 100 μm; (2) The porosity of the polyolefin porous membrane is 65-80%; (3) The pore size of the polyolefin porous membrane is 50~200 nm.

[0009] In a second aspect, the present invention provides a method for preparing a polyolefin porous membrane, comprising the following steps: Polyethylene resin and a pore-forming agent are heat-treated to form a melt; The melt solidifies to form an intermediate; The intermediate is subjected to a first longitudinal stretch and a first transverse stretch in sequence to form a microporous oil film; Remove the pore-forming agent from the microporous oil film to form a thin film; The film is then stretched laterally a second time to form a microporous membrane. Microporous membranes are heat-set to form polyethylene porous membranes.

[0010] Polyethylene resin and a porogen are thoroughly mixed and melted to obtain a melt, completing the mixing of raw materials. The melt is extruded through a die and falls onto a cooling roller to rapidly reduce its temperature and solidify, yielding an intermediate. The intermediate undergoes a first longitudinal stretch and a first transverse stretch to form a microporous oil film. Stretching orients the molecular chains in the microporous oil film in a specific direction, resulting in a film material with a specific pore structure and thickness. During stretching, the pore size is controlled by adjusting the stretching rate and stretching ratio. For example, at a higher stretching ratio, due to the presence of a large amount of porogen within the microporous oil film, the small porogen droplets aggregate to form larger droplets to reduce intermolecular free energy during stretching, resulting in larger micropores after the first stretch to remove the porogen. Conversely, controlling the stretching rate allows for a longer period of molecular chain movement, causing the small porogen droplets between spherulites to aggregate into larger droplets, resulting in a film with a larger pore size after the porogen is removed. After the first stretch, removing the porogen from the microporous oil film results in a larger microporous structure, forming a thin film. The film is then subjected to a second transverse stretching to form a microporous membrane. This not only increases the overall stretching ratio of the film, causing the pores between the spherulites to be elongated laterally, but also further enlarges the coarse fiber structure of the film, reducing the Gurley value and further improving the membrane's permeability. During the heat setting process of the microporous membrane, reducing the shrinkage ratio minimizes changes in the pore structure during heat setting, ensuring that the resulting polyethylene porous membrane has excellent porosity and pore size while also possessing a low Gurley value and tortuosity. Therefore, the pore size, thickness, and Gurley value of the polyethylene porous membrane are determined by multiple process parameters, including the stretching rate, stretching ratio, and shrinkage ratio. Consequently, the polyethylene porous membrane exhibits a low Gurley value and tortuosity, resulting in improved permeability.

[0011] In some implementations, during the formation of a microporous oil film by sequentially subjecting the intermediate to a first longitudinal stretching and a first transverse stretching, the control parameters satisfy at least one of the following: (a) The stretching rate of the first longitudinal stretching is 70~100 mm / min, preferably 80~90 mm / min; (b) The stretching ratio of the first longitudinal stretch is 4 to 9 times, preferably 5 to 7 times; (c) The stretching rate of the first transverse stretching is 120~160 mm / min, preferably 130~150 mm / min; (d) The stretching ratio of the first transverse stretch is 4 to 9 times, preferably 5 to 7 times.

[0012] In some embodiments, in the process of forming a microporous membrane by a second transverse stretching of the film, the stretching ratio of the second transverse stretching is 1 to 2 times, preferably 1.6 to 2 times.

[0013] In some implementations, during the heat setting of the microporous membrane to form a porous membrane, the control parameters satisfy at least one of the following: (A) Control the retraction ratio to be 0-10%; (B) The heat setting temperature is 120~135℃; (C) The heat setting time is 1~2 min.

[0014] In some implementations, during the formation of a microporous oil film by sequentially subjecting the intermediate to a first longitudinal stretching and a first transverse stretching, the control parameters satisfy at least one of the following: (i) The total stretching ratio is 20 to 80 times; (ii) The stretching temperature is 85~150℃.

[0015] In some embodiments, during the second lateral stretching of the film to form a microporous membrane, the control parameters satisfy at least one of the following: (1) The stretching rate is 90~130 mm / min; (2) The stretching temperature is 110~145℃.

[0016] In some embodiments, polyethylene resin and a pore-forming agent are heat-treated to form a melt, satisfying at least one of the following: (I) The weight-average molecular weight of polyethylene resin is 400,000 to 600,000; (II) The mass percentage of polyethylene resin to the pore-forming agent is (20~35):(65~80); (III) The pore-forming agent includes at least one of hydrocarbons, alcohols and esters; (IV) The heat treatment temperature is 200 ~ 230℃.

[0017] Compared with the prior art, the beneficial technical effects achieved by the present invention are as follows: (1) The polyethylene porous membrane of the present invention has a Gurley value ≤ 25s / 100mL and a tortuosity <1.5. Therefore, the polyolefin porous membrane of the present invention has low permeability and tortuosity, which can meet the permeability requirements for its use in separation filtration and other fields.

[0018] (2) The porous membrane of the present invention is prepared by thermally induced phase separation (TIPS). After one longitudinal and transverse stretching and extraction, a second transverse stretching is performed, which can further improve the porosity of the porous membrane and reduce the permeability of the porous membrane. Moreover, the preparation process is continuous and improves production efficiency.

[0019] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Detailed Implementation

[0020] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0021] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0022] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0023] In this document, the terms “comprising” or “including” are open-ended expressions, meaning they include the contents specified in this invention but do not exclude other aspects.

[0024] In a first aspect of the present invention, the present invention provides a polyolefin porous membrane having a Gurley value ≤ 25s / 100mL and a tortuosity <1.5.

[0025] The Gurley value is an important parameter for measuring the permeability of porous membranes; a lower Gurley value indicates better permeability. A Gurley value ≤ 25 s / 100mL indicates good permeability, allowing gas or liquid to pass smoothly through the membrane pores. This enables rapid separation and screening of substances during filtration. For example, in water treatment, water molecules can quickly pass through the membrane pores, while impurities are retained, thus purifying the water. A tortuosity < 1.5 indicates a relatively straight pore path and low flow resistance within the membrane, further improving filtration efficiency. Due to its good permeability and low flow resistance, lower energy consumption is required during filtration and separation processes. For example, in gas separation, lower pressure is not required for gas to pass through the membrane, reducing energy consumption and equipment operating costs. Therefore, the polyolefin porous membrane of this invention exhibits excellent permeability performance, meeting the permeability requirements in separation and filtration applications.

[0026] In some embodiments of the present invention, the polyolefin porous membrane has a Gurley value of 10-25 s / 100 mL and a tortuosity of 1-1.5. Therefore, the polyolefin porous membrane exhibits excellent permeability and tortuosity, further meeting its permeability requirements in applications such as separation and filtration.

[0027] For example, Gurley values ​​are 10 s / 100mL, 12 s / 100mL, 14 s / 100mL, 16 s / 100mL, 18 s / 100mL, 20 s / 100mL, 22 s / 100mL, 24 s / 100mL, 25 s / 100mL, etc.

[0028] For example, the tortuosity is 1, 1.2, 1.3, 1.4, 1.5, etc.

[0029] In some embodiments of the present invention, the polyolefin porous membrane satisfies at least one of the following: (1) The thickness of the polyolefin porous membrane is 5 ~ 100 μm; (2) The porosity of the polyolefin porous membrane is 65-80%; (3) The pore size of the polyolefin porous membrane is 50~200 nm.

[0030] Therefore, the polyolefin porous membrane of the present invention has excellent performance and can meet the permeability requirements for use in separation filtration and other fields.

[0031] For example, the thickness of the porous membrane is 5μm, 15μm, 25μm, 35μm, 45μm, 55μm, 65μm, 75μm, 85μm, 95μm, 100μm, etc.

[0032] For example, the porosity of the porous membrane is 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, etc.

[0033] For example, the pore size of the porous membrane is 50 nm, 70 nm, 90 nm, 110 nm, 130 nm, 150 nm, 170 nm, 190 nm, 200 nm, etc.

[0034] In a second aspect of the present invention, a method for preparing a polyolefin porous membrane is provided, comprising the following steps: Polyethylene resin and a pore-forming agent are heat-treated to form a melt; The melt solidifies to form an intermediate; The intermediate is subjected to a first longitudinal stretch and a first transverse stretch in sequence to form a microporous oil film; Remove the pore-forming agent from the microporous oil film to form a thin film; The film is then stretched laterally a second time to form a microporous membrane. Microporous membranes are heat-set to form porous membranes.

[0035] Polyethylene resin and a porogen are thoroughly mixed and melted to obtain a melt, completing the mixing of raw materials. The melt is extruded through a die and falls onto a cooling roller to rapidly reduce its temperature and solidify, yielding an intermediate. The intermediate undergoes a first longitudinal stretch and a first transverse stretch to form a microporous oil film. Stretching orients the molecular chains in the microporous oil film in a specific direction, resulting in a film material with a specific pore structure and thickness. During stretching, the pore size is controlled by adjusting the stretching rate and stretching ratio. For example, at a higher stretching ratio, due to the presence of a large amount of porogen within the microporous oil film, the small porogen droplets aggregate to form larger droplets to reduce intermolecular free energy during stretching, resulting in larger micropores after the first stretch to remove the porogen. Conversely, controlling the stretching rate allows for a longer period of molecular chain movement, causing the small porogen droplets between spherulites to aggregate into larger droplets, resulting in a film with a larger pore size after the porogen is removed. After the first stretch, removing the porogen from the microporous oil film results in a larger microporous structure, forming a thin film. The film is then subjected to a second transverse stretching to form a microporous membrane. This not only increases the overall stretching ratio of the film, causing the pores between the spherulites to be elongated laterally, but also further enlarges the coarse fiber structure of the film, reducing the Gurley value and further improving the membrane's permeability. During the heat setting process of the microporous membrane, reducing the shrinkage ratio minimizes changes in the pore structure during heat setting, ensuring that the resulting polyethylene porous membrane has excellent porosity and pore size while also possessing a low Gurley value and tortuosity. Therefore, the pore size, thickness, and Gurley value of the polyethylene porous membrane are determined by multiple process parameters, including the stretching rate, stretching ratio, and shrinkage ratio. Consequently, the polyethylene porous membrane exhibits a low Gurley value and tortuosity, resulting in improved permeability.

[0036] In some embodiments of the present invention, in the process of forming a microporous oil film by sequentially performing a first longitudinal stretching and a first transverse stretching of the intermediate, the control parameters satisfy at least one of the following: (a) The stretching rate of the first longitudinal stretching is 70~100 mm / min, preferably 80~90 mm / min; (b) The stretching ratio of the first longitudinal stretch is 4 to 9 times, preferably 5 to 7 times; (c) The stretching rate of the first transverse stretching is 120~160 mm / min, preferably 130~150 mm / min; (d) The stretching ratio of the first transverse stretch is 4 to 9 times, preferably 5 to 7 times.

[0037] The stretching rate directly affects the pore structure, crystallization behavior, thermal stability, and morphology of porous membranes, thus determining their final application performance. During stretching, micropores between molecular chains and crystal nuclei do not form rapidly but gradually as the stretching process continues. Therefore, the micropores formed initially are stretched into macropores. When the stretching rate for the first longitudinal stretch is 70~100 mm / min, preferably 80~90 mm / min, it avoids both slowing down the regularity of the molecular chain arrangement, causing the molecular chains to loosen, and preventing the molecular chains from failing to keep up with the stretching, which could lead to sample breakage.

[0038] For example, the stretching rate of the first longitudinal stretch is 70 mm / min, 75 mm / min, 80 mm / min, 85 mm / min, 90 mm / min, 95 mm / min, 100 mm / min, etc.

[0039] The initial longitudinal stretching ratio is 4 to 9 times, preferably 5 to 7 times. This avoids the increased crystallinity caused by the increased number of molecular chain segments displaced into the crystal lattice under stress, which would adversely affect the transverse stretching process, thus resulting in a more uniform membrane thickness. Furthermore, the longitudinal stretching produces a coarse fiber bundle structure. The high-ratio transverse stretching can orient this coarse fiber bundle structure along the direction of the external force, improving the permeability of the porous membrane and reducing its tortuosity.

[0040] For example, the stretching ratio of the first longitudinal stretch is 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, etc.

[0041] Since longitudinal stretching first can cause some small solvent molecules and pore-forming agents in the membrane to be squeezed out, and can also increase the orientation and crystallinity of the membrane, the stretching rate of the first transverse stretching should be controlled at 120~160 mm / min, preferably 130~150 mm / min. The mobility of molecular chains is weakened, so the stretching rate of the first transverse stretching needs to be greater than that of the first longitudinal stretching.

[0042] For example, the stretching rate of the first transverse stretch is 120 mm / min, 125 mm / min, 130 mm / min, 135 mm / min, 140 mm / min, 145 mm / min, 150 mm / min, 155 mm / min, 160 mm / min, etc.

[0043] When the stretching ratio of the first transverse stretch is 4 to 9 times, preferably 5 to 7 times, the film thickness is uniform and the film will not break. Therefore, the excellent performance of the polyethylene porous membrane is guaranteed.

[0044] For example, the stretching ratio of the first lateral stretch is 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, etc.

[0045] In some embodiments of the present invention, in the process of forming a microporous membrane by a second transverse stretching of the film, the stretching ratio of the second transverse stretching is 1 to 2 times, preferably 1.6 to 2 times.

[0046] A stretch ratio of 1 to 2 times, preferably 1.6 to 2 times, ensures good tensile properties of the film while achieving suitable pore size for the micropores. This reduces the Gurley value, further improving the air permeability of the porous membrane and reducing its tortuosity.

[0047] For example, the stretching ratio of the second transverse stretch is 1, 1.1, 1.2, 1.3, 1.4, 1.5, 16, 1.7, 1.8, 1.9, 2, etc.

[0048] In some embodiments of the present invention, during the heat setting of the microporous membrane to form a porous membrane, the control parameters satisfy at least one of the following: (A) Control the retraction ratio to be 0-10%; (B) The heat setting temperature is 120~135℃; (C) The heat setting time is 1~2 min.

[0049] Controlling the shrinkage ratio to 0-10% can reduce the displacement of fibers in the microporous membrane during the shrinkage process, thereby reducing changes in the pore structure during heat setting, ensuring that the microporous membrane has a low Gurley value and tortuosity, and improving permeability.

[0050] Heat setting the microporous membrane at 120~135℃ for 1~2 minutes can relax the molecular chains after stretching and orientation, further improve the crystal structure, release internal stress, and enhance the thermal stability of the microporous membrane.

[0051] For example, the shrinkage ratios are 0%, 2%, 4%, 6%, 8%, 10%, etc.

[0052] For example, the heat setting temperature is 120°C, 125°C, 130°C, 135°C, etc.

[0053] For example, the heat setting time is 1 min, 1.2 min, 1.4 min, 1.6 min, 1.8 min, 2 min, etc.

[0054] In some embodiments of the present invention, in the process of forming a microporous oil film by sequentially performing a first longitudinal stretching and a first transverse stretching of the intermediate, the control parameters satisfy at least one of the following: (i) The total stretching ratio is 20 to 80 times, preferably 25 to 48 times; (ii) The stretching temperature is 85~150℃.

[0055] When the total stretching ratio is 20 to 80 times, preferably 25 to 48 times, the resulting porous membrane has sufficient strength, while preventing the membrane from breaking during the stretching process and achieving high productivity.

[0056] For example, the total stretching ratio is 20 times, 25 times, 30 times, 35 times, 40 times, 45 times, 50 times, 55 times, 60 times, 65 times, 70 times, 75 times, 80 times, etc.

[0057] Stretching in a temperature range of 85~150℃ can promote the softening of polyethylene, making it easier to form a microporous structure, which helps to increase the porosity of the porous membrane and thus enhance its air permeability.

[0058] For example, the stretching temperature is 85℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃, 140℃, 145℃, 150℃, etc.

[0059] In some embodiments of the present invention, during the second transverse stretching of the thin film to form a microporous membrane, the control parameters satisfy at least one of the following: (i) The stretching rate of the second transverse stretching is 90~130 mm / min, preferably 100~120 mm / min; (iii) The stretching temperature for the second transverse stretching is 110~145℃.

[0060] The second transverse stretching is less intense than the first, but it increases the overall stretching ratio, laterally elongating the pores between the spherulites, which is beneficial for further improving the permeability of the microporous membrane. The stretching rate of the second transverse stretching is 90–130 mm / min, preferably 100–120 mm / min, which allows for further control of the membrane's pore size and permeability. This further improves the air permeability of the porous membrane and reduces its tortuosity.

[0061] For example, the stretching rate of the second transverse stretching is 90 mm / min, 95 mm / min, 100 mm / min, 105 mm / min, 110 mm / min, 115 mm / min, 120 mm / min, 125 mm / min, 130 mm / min, etc.

[0062] The second transverse stretching temperature is 110~145℃. This temperature range ensures good tensile properties of the film while allowing the micropores to achieve a suitable pore size. This reduces the Gurley value and further improves the film's air permeability.

[0063] For example, the stretching temperature is 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, etc.

[0064] In some embodiments of the present invention, in the process of heat-treating polyethylene resin and a pore-forming agent to form a melt, at least one of the following is satisfied: (I) The weight-average molecular weight of polyethylene resin is 400,000 to 600,000; (II) The mass percentage of polyethylene resin to pore-forming agent is (20~35):(65~80); (III) The pore-forming agent includes at least one of hydrocarbons, alcohols and esters; (IV) The heat treatment temperature is 200~230℃.

[0065] The total mass percentage of polyethylene resin and pore-forming agent is 100%, with a polymer concentration of 20-35% which helps the mixture maintain a suitable viscosity, allowing the droplet phase to coarsen normally and ensuring the normal size of the pores and porosity on the porous membrane surface.

[0066] Pore-forming agents include at least one of hydrocarbons, alcohols, and esters. These substances are non-volatile solvents with good compatibility with polyethylene. When used as pore-forming agents, their melting points are above those of polyethylene, which facilitates the separation of the two after cooling.

[0067] For example, hydrocarbons include, but are not limited to, at least one of paraffin oil, white oil, and mineral oil.

[0068] For example, alcohols include, but are not limited to, at least one of oleyl alcohol and stearyl alcohol.

[0069] For example, esters include, but are not limited to, at least one of dioctyl phthalate and dibutyl phthalate.

[0070] The heat treatment temperature is 200~230℃, which is conducive to the uniform melting of polyethylene into the pore-forming agent.

[0071] For example, the heat treatment temperature is 200℃, 205℃, 210℃, 220℃, 225℃, 230℃, etc.

[0072] The present invention will be explained below with reference to embodiments. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0073] Example 1: (1) Pour 92 parts of polyethylene (weight average molecular weight of 400,000 to 600,000) into the extruder hopper, and add 308 parts of paraffin oil at the same time, i.e., the solid content is 23%. Mix thoroughly to form a melt, and the hot melt temperature is 205℃.

[0074] (2) The melt is pumped to the extrusion die at a rate of 214 kg / h by a metering pump and then extruded onto the cooling roller for cooling and solidification to form an intermediate.

[0075] (3) The intermediate is stretched longitudinally for the first time by multi-point stretching at a stretching rate of 80 mm / min, a stretching ratio of 5.12 times, and a stretching temperature of 102℃. The stretched film is fixed on both sides with clamps and stretched transversely for the first time at a stretching rate of 130 mm / min, a stretching ratio of 5.12 times, and a stretching temperature of 126.8℃ to obtain a microporous oil film.

[0076] (4) The microporous oil film was extracted with dichloromethane and dried to obtain the extracted film.

[0077] (5) The extracted film is subjected to a second transverse stretching at a stretching rate of 120 mm / min, a stretching ratio of 1.95 times, a stretching temperature of 125℃, and a shrinkage ratio of 10% to obtain a microporous membrane.

[0078] (6) Finally, the microporous membrane is heat-set at 125.3℃ for 2 minutes to obtain a polyethylene porous membrane.

[0079] Example 2: The difference between Example 2 and Example 1 is as follows: The solid content of Example 2 is 25%, the extrusion rate is 200 kg / h; the stretching rate of the first longitudinal stretching is 90 mm / min, and the stretching ratio is 6.85 times; the stretching rate of the first transverse stretching is 150 mm / min, and the stretching ratio is 6.73 times; the stretching ratio of the second transverse stretching is 1.7 times, and the shrinkage ratio is 0.

[0080] Example 3: The difference between Example 3 and Example 1 is as follows: The solid content of Example 3 is 28%, the extrusion rate is 70 kg / h; the stretching rate of the first longitudinal stretching is 85 mm / min, and the stretching ratio is 6.63 times; the stretching rate of the first transverse stretching is 145 mm / min, and the stretching ratio is 6.73 times; the stretching ratio of the second transverse stretching is 1.7 times, and the shrinkage ratio is 0.

[0081] Comparative Example 1: Differences between Comparative Example 1 and Example 1: The solid content of Comparative Example 1 is 28%, the extrusion rate is 160 kg / h; the stretching rate of the first longitudinal stretching is 130 mm / min, and the stretching ratio is 6.85 times; the stretching rate of the first transverse stretching is 200 mm / min, and the stretching ratio is 6.85 times; the stretching ratio of the second transverse stretching is 1.2 times, and the shrinkage ratio is 20%.

[0082] Performance testing I. Testing Methods 1. Thickness test: A thickness tester with a resolution of not less than 1 μm was used, and the procedure was performed in accordance with GB / T 6672-2001.

[0083] 2. Average pore size test: A 3H-2000PB bubble pressure method filter membrane pore size distribution meter and BSD16 wetting solution were used. Testing was conducted according to the national standard GB / T 32361-2005, "Test Method for Separation Membrane Pore Size".

[0084] 3. Air permeability test: Use a manual or automatic air permeability meter to perform the test and record the results according to GB / T 458-2008.

[0085] 4. Porosity test: The test was conducted using a weighing method, and the result was calculated using the formula: p = [1 - m / (s × d × ρ0)] × 100%, where: p is the porosity of the porous membrane, expressed as a percentage; m is the mass of the porous membrane, in grams; and s is the area of ​​the porous membrane, in cm². 2 ; d is the thickness of the porous membrane, in cm; ρ0 is the density of the porous membrane, in g / cm³. 3 ; 5. Tortuousness test: The formula yields: t = 5.18 × 10 3 (τ 2 •L) / (d • ε), where: t is the air permeability of the porous membrane, in s / 100mL; L is the thickness of the porous membrane, in μm; d is the pore size of the porous membrane, in nm; ε is the porosity of the porous membrane, expressed as %; and τ is the tortuosity of the porous membrane.

[0086] 6. Water flux test: Because the polyethylene porous membrane is hydrophobic, after immersing the membrane in IPA for 5 minutes, it was tested according to GB / T 32360-2015, and the result was calculated using the following formula: P = V / St, where P is the pure water permeability (L / m). 2 ·h), V is the pure water permeate flow rate (L), and S is the effective membrane filtration area (m²). 2 ), where t is time (h).

[0087] II. Test Results The preparation parameters and performance test results of the above embodiments and comparative examples are shown in Table 1.

[0088] Table 1. Preparation parameters and performance test results of the examples and comparative examples.

[0089] As shown in Table 1, the Gurley value and tortuosity value of the polyethylene porous membranes in Examples 1 to 3 are significantly lower than those in the comparative examples, indicating that the polyethylene porous membranes of the present invention have excellent permeability.

[0090] The polyethylene porous membranes in Examples 1 to 3 have a wide range of thickness and pore size, indicating that by adjusting process parameters such as stretching ratio, stretching rate, and shrinkage ratio during the preparation process, the performance parameters such as pore size and thickness of the polyethylene porous membrane can be flexibly controlled to meet the needs of polyethylene porous membranes in different application scenarios such as separation and filtration.

[0091] The polyethylene porous membranes of Examples 1 to 3 have high water flux values, indicating that they have excellent filtration efficiency, which is crucial for precise separation in water treatment, pharmaceuticals, food processing and other fields.

[0092] This demonstrates that the polyolefin porous membrane of the present invention has excellent permeability, which meets its application requirements in fields such as separation and filtration.

[0093] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0094] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a polyolefin porous membrane, characterized in that, Includes the following steps: Polyethylene resin and a pore-forming agent are heat-treated to form a melt; The melt solidifies to form an intermediate; The intermediate is subjected to a first longitudinal stretch and a first transverse stretch in sequence to form a microporous oil film; The pore-forming agent in the microporous oil film is removed to form a thin film; The film is then subjected to a second lateral stretching to form a microporous membrane; The microporous membrane is heat-set to form a polyethylene porous membrane; In the process of heat-treating polyethylene resin and porogen to form a melt, the following conditions must be met: the weight-average molecular weight of the polyethylene resin is 400,000 to 600,000, and the mass percentage of the polyethylene resin to the porogen is (20-35):(65-80). In the process of sequentially performing a first longitudinal stretching and a first transverse stretching on the intermediate to form a microporous oil film, the control parameters satisfy: The stretching temperature is 85~150℃; The stretching rate of the first longitudinal stretching is 70~100 mm / min; The stretching ratio of the first longitudinal stretching is 4 to 9 times; The stretching rate of the first transverse stretching is 120~160 mm / min; The stretching ratio of the first transverse stretching is 4 to 9 times; In the process of heat-setting the microporous membrane to form a porous membrane, the control parameters satisfy the following: the shrinkage ratio is controlled to be 0-10%; In the process of performing a second transverse stretching of the film to form a microporous film, the control parameters are: stretching rate of 90~130 mm / min, stretching temperature of 110~145℃, and stretching ratio of the second transverse stretching of 1~2 times. The polyolefin porous membrane has a Gurley value ≤ 25s / 100mL and a tortuosity <1.5; The degree of tortuosity is calculated using the formula t = 5.18 × 10⁻⁶. 3 (τ 2 The formula is calculated as follows: t is the air permeability of the porous membrane, in s / 100mL; L is the thickness of the porous membrane, in μm; d is the pore size of the porous membrane, in nm; ε is the porosity of the porous membrane, expressed as %; and τ is the tortuosity of the porous membrane.

2. The preparation method according to claim 1, characterized in that, The polyolefin porous membrane has a Gurley value of 10~25s / 100mL and a tortuosity of 1~1.

25.

3. The preparation method according to claim 1, characterized in that, The polyolefin porous membrane satisfies at least one of the following: (1) The thickness of the polyolefin porous membrane is 5 ~ 100 μm; (2) The porosity of the polyolefin porous membrane is 65-80%; (3) The pore size of the polyolefin porous membrane is 50~200 nm.

4. The preparation method according to claim 1, characterized in that, In the process of sequentially performing a first longitudinal stretching and a first transverse stretching on the intermediate to form a microporous oil film, the control parameters satisfy at least one of the following: (a) The stretching rate of the first longitudinal stretching is 80~90 mm / min; (b) The stretching ratio of the first longitudinal stretch is 5 to 7 times; (c) The stretching rate of the first transverse stretching is 130~150 mm / min; (d) The stretching ratio of the first transverse stretch is 5 to 7 times.

5. The preparation method according to claim 1, characterized in that, In the process of performing a second transverse stretching of the film to form a microporous film, the stretching ratio of the second transverse stretching is 1.6 to 2 times.

6. The preparation method according to claim 1, characterized in that, In the process of heat-setting the microporous membrane to form a porous membrane, the control parameter satisfies at least one of the following: (A) The heat setting temperature is 120~135℃; (B) The heat setting time is 1~2 min.

7. The preparation method according to claim 1, characterized in that, In the process of sequentially performing a first longitudinal stretching and a first transverse stretching on the intermediate to form a microporous oil film, the control parameters satisfy: (i) The total stretching ratio is 20 to 80 times.

8. The preparation method according to claim 1, characterized in that, In the process of heat-treating polyethylene resin and a pore-forming agent to form a melt, at least one of the following conditions must be met: (I) The pore-forming agent includes at least one of hydrocarbons, alcohols, and esters; (II) The temperature of the heat treatment is 200~230℃.

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