Polyolefin porous membrane and preparation method thereof

Abstract

The invention discloses a polyolefin porous membrane. The Gurley value of the polyolefin porous membrane is less than or equal to 25s / 100mL, and the tortuosity is 1t; 1.5 parts; the thickness of the polyolefin porous membrane is 5-100 [mu] m, the porosity is 65-80%, and the aperture is 50-200 nm. The polyolefin porous membrane provided by the invention has excellent permeability, the preparation process has continuity, and the permeability use requirement of the polyolefin porous membrane in the fields of separation, filtration and the like is ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of polyolefin porous film manufacturing, and specifically relates to a polyolefin porous film and a preparation method thereof. Background Art

[0002] Porous membranes, especially polyolefin microporous membranes, are widely used in many 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 petrochemical industry, the output of polyolefins has increased sharply, of which 25% is polyethylene (PE).

[0003] In order to expand the application field of polyethylene in the prior art, a stretching device is usually used to re-stretch the finished raw material film, or polyethylene with different molecular weights is used to prepare a three-layer laminated structure polyethylene microporous membrane with different orientation structures through the TIPS process. However, the preparation process of the prior art is complicated and the preparation process is not continuous. The obtained polyethylene porous membrane has a high Gurley value and high tortuosity, which cannot meet the application of the porous membrane in certain specific fields. Therefore, it is urgent to solve the problem of high Gurley value and high tortuosity of polyolefin porous membranes. Summary of the invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art to a certain extent. To this end, the present invention provides a polyolefin porous membrane, which has a low Gurley value and a tortuosity value, has excellent permeability, and can meet the permeability requirements of separation and filtration and other fields.

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

[0006] Gurley value is an important parameter for measuring the permeability of porous membranes. The lower the Gurley value, the better the permeability of the porous membrane. When the Gurley value is ≤25s / 100mL, the permeability of the porous membrane is good, and gas or liquid can pass through the membrane pores relatively smoothly, which enables it to quickly separate and screen substances during the filtration process. For example, in water treatment, water molecules can quickly pass through the membrane pores, while impurities in the water are trapped, thereby achieving the purpose of purifying water. Tortuousness <1.5 indicates that the path of the membrane pores is relatively straight, and the flow resistance of the fluid in the membrane is small, further improving the filtration efficiency. Due to good permeability and low flow resistance, the energy consumption required in the process of filtration, separation, etc. is low. For example, in the process of gas separation, no excessive pressure is required to separate the gas through the membrane, reducing energy consumption and equipment operating costs. Therefore, the polyolefin porous membrane of the present invention has excellent permeability and can meet its permeability requirements in the fields of separation and filtration.

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

[0008] In some embodiments, the polyolefin porous membrane satisfies at least one of the following:

[0009] (1) The thickness of the polyolefin porous membrane is 5 to 100 μm;

[0010] (2) The porosity of the polyolefin porous membrane is 65 to 80%;

[0011] (3) The pore size of the polyolefin porous membrane is 50 to 200 nm.

[0012] In a second aspect of the present invention, the present invention provides a method for preparing a polyolefin porous membrane, comprising the following steps:

[0013] The polyethylene resin and the porogen are subjected to heat treatment to form a melt;

[0014] The melt solidifies to form an intermediate;

[0015] The intermediate is sequentially subjected to a first longitudinal stretching and a first transverse stretching to form a microporous oil film;

[0016] Remove the porogen in the microporous oil film to form a thin film;

[0017] The film is stretched transversely for a second time to form a microporous membrane;

[0018] The microporous membrane is heat-set to form a polyethylene porous membrane.

[0019] The polyethylene resin and the porogen are fully mixed and melted to obtain a melt. This step completes the mixing of the raw materials. The melt is extruded through the die head of the equipment, falls on the cooling roller, and the temperature is quickly reduced. It is cooled and solidified to obtain an intermediate. The intermediate is sequentially subjected to the first longitudinal stretching and the first transverse stretching to form a microporous oil film. Stretching can orient the molecular chains in the microporous oil film in a specific direction, thereby forming a membrane material with a specific pore structure and thickness. During the stretching process, the pore size is regulated by adjusting the stretching rate and the stretching ratio. For example, at a larger stretching ratio, because there are a large number of porogens in the microporous oil film, during the stretching process, the small droplets of the porogen gather to form large droplets in order to reduce the free energy between molecules, so that larger micropores are formed after the first stretching is completed and the porogen is removed; at the same time, a lower stretching rate is controlled, the molecular chain segments can move for a longer time, and the small droplets of the porogen between the spherulites gather to form large droplets. After removing the porogen, the pore size of the film is also larger. After the first stretching is completed, the porogen in the microporous oil film is removed to produce a larger microporous structure in the microporous oil film to form a film. The film is then stretched transversely for a second time to form a microporous membrane, which not only expands the overall stretching ratio of the film, so that the pores between the internal spherulites are stretched transversely, but also further enlarges the coarse fiber structure of the film, reduces the Gurley value, and further improves the permeability of the membrane. During the heat setting process of the microporous membrane, by reducing the shrinkage ratio, the change of the pore structure during the heat setting process can be reduced, ensuring that the obtained polyethylene porous membrane has excellent porosity and pore size while having low Gurley value and tortuosity. Therefore, the characteristics of the polyethylene porous membrane, such as the pore size, thickness and Gurley value, are jointly determined by the stretching rate, stretching ratio, shrinkage ratio and other process parameters during the stretching process. As a result, the polyethylene porous membrane has a low Gurley value and tortuosity, and the permeability is improved.

[0020] In some embodiments, when the intermediate is subjected to a first longitudinal stretching and a first transverse stretching in sequence to form a microporous oil film, the control parameters satisfy at least one of the following:

[0021] (a) the stretching rate of the first longitudinal stretching is 70 to 100 mm / min, preferably 80 to 90 mm / min;

[0022] (b) the stretching ratio of the first longitudinal stretching is 4 to 9 times, preferably 5 to 7 times;

[0023] (c) the stretching rate of the first transverse stretching is 120 to 160 mm / min, preferably 130 to 150 mm / min;

[0024] (d) The stretching ratio of the first transverse stretching is 4 to 9 times, preferably 5 to 7 times.

[0025] In some embodiments, when the film is subjected to a second transverse stretching to form a microporous membrane, the stretching ratio of the second transverse stretching is 1 to 2 times, preferably 1.6 to 2 times.

[0026] In some embodiments, when the microporous membrane is heat-set to form a porous membrane, the control parameters satisfy at least one of the following:

[0027] (A) Control the retraction ratio to 0-10%;

[0028] (B) The temperature of heat setting is 120-135°C;

[0029] (C) The heat setting time is 1 to 2 minutes.

[0030] In some embodiments, when the intermediate is subjected to a first longitudinal stretching and a first transverse stretching in sequence to form a microporous oil film, the control parameters satisfy at least one of the following:

[0031] (i) The total stretching ratio is 20 to 80 times;

[0032] (ii) The stretching temperature is 85 to 150°C.

[0033] In some embodiments, when the film is subjected to a second transverse stretching to form a microporous membrane, the control parameters satisfy at least one of the following:

[0034] (1) The stretching rate is 90 to 130 mm / min;

[0035] (2) The stretching temperature is 110 to 145°C.

[0036] In some embodiments, the polyethylene resin and the porogen are heat treated to form a melt, and at least one of the following is satisfied:

[0037] (I) The weight average molecular weight of the polyethylene resin is 400,000 to 600,000;

[0038] (II) the mass percentage of the polyethylene resin to the porogen is (20-35):(65-80);

[0039] (III) the porogen includes at least one of hydrocarbons, alcohols and esters;

[0040] (IV) The temperature of the heat treatment is 200 to 230°C.

[0041] Compared with the prior art, the present invention has the following beneficial technical effects:

[0042] (1) The polyethylene porous membrane of the present invention has a Gurley value of ≤25s / 100mL and a tortuosity of <1.5. Therefore, the polyolefin porous membrane of the present invention has low permeability and tortuosity, and can meet the permeability requirements in the fields of separation and filtration.

[0043] (2) The porous membrane of the present invention adopts the preparation process of thermally induced phase separation (TIPS). After longitudinal and transverse stretching and extraction, a second transverse stretching is performed, which can further increase the porosity of the porous membrane and reduce the permeability of the porous membrane. The preparation process is continuous, thereby improving production efficiency.

[0044] Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. DETAILED DESCRIPTION

[0045] 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 understood as limiting the present invention.

[0046] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features. Further, in the description of the present invention, unless otherwise specified, the meaning of "plurality" is two or more.

[0047] The endpoints and any values ​​of the ranges disclosed in this article 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 each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0048] In this document, the terms “include” or “comprising” are open expressions, that is, including the contents specified in the present invention but not excluding other contents.

[0049] In a first aspect of the embodiments of the present invention, the present invention provides a polyolefin porous membrane, wherein the polyolefin porous membrane has a Gurley value of ≤25s / 100mL and a tortuosity of <1.5.

[0050] Gurley value is an important parameter for measuring the permeability of porous membranes. The lower the Gurley value, the better the permeability of the porous membrane. When the Gurley value is ≤25s / 100mL, the permeability of the porous membrane is good, and gas or liquid can pass through the membrane pores relatively smoothly, which enables it to quickly separate and screen substances during the filtration process. For example, in water treatment, water molecules can quickly pass through the membrane pores, while impurities in the water are trapped, thereby achieving the purpose of purifying water. Tortuousness <1.5 indicates that the path of the membrane pores is relatively straight, and the flow resistance of the fluid in the membrane is small, further improving the filtration efficiency. Due to good permeability and low flow resistance, the energy consumption required in the process of filtration, separation, etc. is low. For example, in the process of gas separation, no excessive pressure is required to separate the gas through the membrane, reducing energy consumption and equipment operating costs. Therefore, the polyolefin porous membrane of the present invention has excellent permeability and can meet the permeability requirements in the fields of separation and filtration.

[0051] In some embodiments of the present invention, the polyolefin porous membrane has a Gurley value of 10 to 25 s / 100 mL and a tortuosity of 1 to 1.5. Thus, the polyolefin porous membrane has excellent permeability and tortuosity, and can further meet the permeability requirements in the fields of separation and filtration.

[0052] Illustratively, the Gurley value is 10 s / 100 mL, 12 s / 100 mL, 14 s / 100 mL, 16 s / 100 mL, 18 s / 100 mL, 20 s / 100 mL, 22 s / 100 mL, 24 s / 100 mL, 25 s / 100 mL, and the like.

[0053] Exemplarily, the tortuosity is 1, 1.2, 1.3, 1.4, 1.5, etc.

[0054] In some embodiments of the present invention, the polyolefin porous membrane satisfies at least one of the following:

[0055] (1) The thickness of the polyolefin porous membrane is 5 to 100 μm;

[0056] (2) The porosity of the polyolefin porous membrane is 65 to 80%;

[0057] (3) The pore size of the polyolefin porous membrane is 50 to 200 nm.

[0058] Therefore, the polyolefin porous membrane of the present invention has excellent performance and can meet the permeability requirements in the fields of separation and filtration.

[0059] Illustratively, the thickness of the porous membrane is 5 μm, 15 μm, 25 μm, 35 μm, 45 μm, 55 μm, 65 μm, 75 μm, 85 μm, 95 μm, 100 μm, or the like.

[0060] Illustratively, the porosity of the porous membrane is 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, etc.

[0061] Illustratively, 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.

[0062] In a second aspect of the embodiment of the present invention, the present invention provides a method for preparing a polyolefin porous membrane, comprising the following steps:

[0063] The polyethylene resin and the porogen are subjected to heat treatment to form a melt;

[0064] The melt solidifies to form an intermediate;

[0065] The intermediate is sequentially subjected to a first longitudinal stretching and a first transverse stretching to form a microporous oil film;

[0066] Remove the porogen in the microporous oil film to form a thin film;

[0067] The film is stretched transversely for a second time to form a microporous membrane;

[0068] The microporous membrane is heat-set to form a porous membrane.

[0069] The polyethylene resin and the porogen are fully mixed and melted to obtain a melt. This step completes the mixing of the raw materials. The melt is extruded through the die head of the equipment, falls on the cooling roller, and the temperature is quickly reduced. It is cooled and solidified to obtain an intermediate. The intermediate is sequentially subjected to the first longitudinal stretching and the first transverse stretching to form a microporous oil film. Stretching can orient the molecular chains in the microporous oil film in a specific direction, thereby forming a membrane material with a specific pore structure and thickness. During the stretching process, the pore size is regulated by adjusting the stretching rate and the stretching ratio. For example, at a larger stretching ratio, because there are a large number of porogens in the microporous oil film, during the stretching process, the small droplets of the porogen gather to form large droplets in order to reduce the free energy between molecules, so that larger micropores are formed after the first stretching is completed and the porogen is removed; at the same time, a lower stretching rate is controlled, the molecular chain segments can move for a longer time, and the small droplets of the porogen between the spherulites gather to form large droplets. After removing the porogen, the pore size of the film is also larger. After the first stretching is completed, the porogen in the microporous oil film is removed to produce a larger microporous structure in the microporous oil film to form a film. The film is then stretched transversely for a second time to form a microporous membrane, which not only expands the overall stretching ratio of the film, so that the pores between the internal spherulites are stretched transversely, but also further enlarges the coarse fiber structure of the film, reduces the Gurley value, and further improves the permeability of the membrane. During the heat setting process of the microporous membrane, by reducing the shrinkage ratio, the change of the pore structure during the heat setting process can be reduced, ensuring that the obtained polyethylene porous membrane has excellent porosity and pore size while having low Gurley value and tortuosity. Therefore, the characteristics of the polyethylene porous membrane, such as the pore size, thickness and Gurley value, are jointly determined by the stretching rate, stretching ratio, shrinkage ratio and other process parameters during the stretching process. As a result, the polyethylene porous membrane has a low Gurley value and tortuosity, and the permeability is improved.

[0070] In some embodiments of the present invention, when the intermediate is subjected to a first longitudinal stretching and a first transverse stretching in sequence to form a microporous oil film, the control parameters satisfy at least one of the following:

[0071] (a) the stretching rate of the first longitudinal stretching is 70 to 100 mm / min, preferably 80 to 90 mm / min;

[0072] (b) the stretching ratio of the first longitudinal stretching is 4 to 9 times, preferably 5 to 7 times;

[0073] (c) the stretching rate of the first transverse stretching is 120 to 160 mm / min, preferably 130 to 150 mm / min;

[0074] (d) The stretching ratio of the first transverse stretching is 4 to 9 times, preferably 5 to 7 times.

[0075] The stretching rate directly affects the pore structure, crystallization behavior, thermal stability and morphology of the porous membrane, thus determining the final application performance of the porous membrane. During the stretching process, micropores between molecular chains and crystal nuclei are not easy to form quickly, but gradually form during the stretching process. Therefore, the micropores formed first become macropores after being stretched. When the stretching rate of the first longitudinal stretching is 70-100 mm / min, preferably 80-90 mm / min, it will neither slow down the regularity of the molecular chain arrangement and relax the molecular chain, nor prevent the molecular chain from being unable to keep up with the stretching and causing the sample to break.

[0076] Illustratively, the stretching rate of the first longitudinal stretching is 70 mm / min, 75 mm / min, 80 mm / min, 85 mm / min, 90 mm / min, 95 mm / min, 100 mm / min, etc.

[0077] The first longitudinal stretching ratio is 4 to 9 times, preferably 5 to 7 times, which can avoid the increase in the amount of molecular segments in the membrane that are arranged into the lattice under stress, which causes an increase in crystallinity and adversely affects the transverse stretching process, thereby making the thickness of the membrane more uniform. In addition, a coarse fiber bundle structure is generated after longitudinal stretching, and a large transverse stretching ratio can orient the coarse fiber bundle structure along the direction of the external force, thereby improving the air permeability of the porous membrane and reducing the tortuosity.

[0078] Illustratively, the stretching ratio of the first longitudinal stretching is 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, etc.

[0079] Since the longitudinal stretching first will cause a part of the solvent small molecule porogen in the membrane to be squeezed out on the one hand, and on the other hand it will increase the orientation and crystallinity of the diaphragm, when the transverse stretching is performed again, the stretching rate of the first transverse stretching is controlled to be 120-160 mm / min, preferably 130-150 mm / min. The mobility of the molecular chain is weakened, so the stretching rate of the first transverse stretching needs to be greater than the stretching rate of the first longitudinal stretching.

[0080] Illustratively, the stretching rate of the first transverse stretching 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.

[0081] When the stretching ratio of the first transverse stretching is 4 to 9 times, preferably 5 to 7 times, the film thickness can be made uniform without causing the film to rupture, thereby ensuring the excellent performance of the polyethylene porous membrane.

[0082] Illustratively, the stretching ratio of the first transverse stretching is 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, etc.

[0083] In some embodiments of the present invention, when the film is subjected to a second transverse stretching to form a microporous membrane, the stretching ratio of the second transverse stretching is 1 to 2 times, preferably 1.6 to 2 times.

[0084] When the stretching ratio is 1 to 2 times, preferably 1.6 to 2 times, the film can have good stretching properties while allowing the micropores to reach a suitable pore size, thereby reducing the Gurley value, further improving the air permeability of the porous film and reducing the tortuosity.

[0085] Illustratively, the stretching ratio of the second transverse stretching is 1 times, 1.1 times, 1.2 times, 1.3 times, 1.4 times, 1.5 times, 16 times, 1.7 times, 1.8 times, 1.9 times, 2 times, etc.

[0086] In some embodiments of the present invention, when the microporous membrane is heat-set to form a porous membrane, the control parameters satisfy at least one of the following:

[0087] (A) Control the retraction ratio to 0-10%;

[0088] (B) The temperature of heat setting is 120-135°C;

[0089] (C) The heat setting time is 1 to 2 minutes.

[0090] Controlling the shrinkage ratio to 0-10% can reduce the displacement of fibers in the microporous membrane during the shrinkage process, thereby reducing the change in the pore structure during the heat setting process, ensuring that the microporous membrane has a lower Gurley value and tortuosity, and improving permeability.

[0091] The microporous membrane is heat-set at 120-135°C for 1-2 minutes, which can relax the molecular chains after stretching and orientation, further improve the crystals, release the internal stress, and improve the thermal stability of the microporous membrane.

[0092] Exemplarily, the retraction ratio is 0, 2%, 4%, 6%, 8%, 10%, etc.

[0093] Illustratively, the temperature of heat setting is 120°C, 125°C, 130°C, 135°C, etc.

[0094] Exemplarily, the heat setting time is 1 min, 1.2 min, 1.4 min, 1.6 min, 1.8 min, 2 min, etc.

[0095] In some embodiments of the present invention, when the intermediate is subjected to a first longitudinal stretching and a first transverse stretching in sequence to form a microporous oil film, the control parameters satisfy at least one of the following:

[0096] (i) the total stretching ratio is 20 to 80 times, preferably 25 to 48 times;

[0097] (ii) The stretching temperature is 85 to 150°C.

[0098] When the total stretching ratio is 20 to 80 times, preferably 25 to 48 times, the obtained porous membrane can have sufficient strength, and at the same time, the membrane can be prevented from breaking during the stretching process and a higher productivity can be achieved.

[0099] Illustratively, 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.

[0100] Stretching at a temperature range of 85 to 150°C can promote the softening of polyethylene, thereby making it easier to form a microporous structure, which helps to increase the porosity of the porous membrane and thus enhance its air permeability.

[0101] Illustratively, the stretching temperature is 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, 150°C, etc.

[0102] In some embodiments of the present invention, when the film is subjected to a second transverse stretching to form a microporous film, the control parameters satisfy at least one of the following:

[0103] (i) the stretching rate of the second transverse stretching is 90 to 130 mm / min, preferably 100 to 120 mm / min;

[0104] (iii) The stretching temperature of the second transverse stretching is 110 to 145°C.

[0105] The second transverse stretching has a lower degree of stretching than the first transverse stretching, but the overall stretching ratio is increased, and the pores between the internal spherulites are stretched laterally, which is conducive to further improving the permeability of the microporous membrane. When the stretching rate of the second transverse stretching is 90 to 130 mm / min, preferably 100 to 120 mm / min, the pore size and permeability of the membrane can be further regulated. Thus, the air permeability of the porous membrane can be further improved and the tortuosity can be reduced.

[0106] Illustratively, 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.

[0107] The stretching temperature of the second transverse stretching is 110-145°C, which can ensure good stretching properties of the film while allowing the micropores to reach a suitable pore size, thereby reducing the Gurley value and further improving the air permeability of the film.

[0108] Illustratively, the stretching temperature is 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, etc.

[0109] In some embodiments of the present invention, the polyethylene resin and the porogen are heat treated to form a melt, and at least one of the following is satisfied:

[0110] (I) The weight average molecular weight of the polyethylene resin is 400,000 to 600,000;

[0111] (II) The mass percentage of polyethylene resin to porogen is (20-35):(65-80);

[0112] (III) the porogen includes at least one of hydrocarbons, alcohols and esters;

[0113] (IV) The temperature of the heat treatment is 200 to 230°C.

[0114] The total mass percentage of polyethylene resin and porogen is 100%, wherein 20-35% of polymer concentration is conducive to maintaining appropriate viscosity of the mixed solution, and the coarsening movement of the droplet phase proceeds normally, ensuring the normal size and porosity of the pore size on the porous membrane surface.

[0115] The porogen includes at least one of hydrocarbons, alcohols and esters. These substances are non-volatile solvents with good compatibility with polyethylene. When used as porogens, their melting points are above the melting point of polyethylene, which is conducive to the separation of the two after cooling.

[0116] Illustratively, the hydrocarbons include, but are not limited to, at least one of paraffin oil, white oil, and mineral oil.

[0117] Illustratively, the alcohols include, but are not limited to, at least one of oleyl alcohol and stearyl alcohol.

[0118] Illustratively, the esters include, but are not limited to, at least one of dioctyl phthalate and dibutyl phthalate.

[0119] The heat treatment temperature is 200-230°C, which is conducive to the uniform melting of polyethylene into the porogen.

[0120] Illustratively, the heat treatment temperature is 200°C, 205°C, 210°C, 220°C, 225°C, 230°C, etc.

[0121] The scheme of the present invention will be explained below in conjunction with the embodiments. It will be appreciated by those skilled in the art that the following embodiments are only used to illustrate the present invention and should not be considered as limiting the scope of the present invention. Where specific techniques or conditions are not indicated in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used are not indicated by the manufacturer and are all conventional products that can be obtained commercially.

[0122] Embodiment 1:

[0123] (1) Pour 92 parts of polyethylene (weight average molecular weight of 400,000 to 600,000) into the extruder silo, and add 308 parts of paraffin oil, i.e., solid content of 23%. Mix thoroughly to form a melt with a hot melt temperature of 205°C.

[0124] (2) The molten material is delivered to the extrusion die by a metering pump at a rate of 214 kg / h, extruded and dropped onto a cooling roller for cooling and solidification to form an intermediate.

[0125] (3) The intermediate is subjected to the first longitudinal stretching by multi-point stretching, with a stretching rate of 80 mm / min, a stretching ratio of 5.12 times, and a stretching temperature of 102°C; the two sides of the stretched film are fixed with clamps and subjected to the first transverse stretching, with a stretching rate of 130 mm / min, a stretching ratio of 5.12 times, and a stretching temperature of 126.8°C, to obtain a microporous oil film.

[0126] (4) The microporous oil film is extracted with dichloromethane and dried to obtain an extracted film.

[0127] (5) The extracted film was 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° C., and a shrinkage ratio of 10% to obtain a microporous membrane.

[0128] (6) Finally, the microporous membrane was heat-set at 125.3°C for 2 min to obtain a polyethylene porous membrane.

[0129] Embodiment 2:

[0130] The difference between Example 2 and Example 1 is: the solid content of Example 2 is 25%, and the extrusion volume is 200kg / h; the stretching rate of the first longitudinal stretching is 90mm / min, and the stretching ratio is 6.85 times; the stretching rate of the first transverse stretching is 150mm / min, and the stretching ratio is 6.73 times; the stretching ratio of the second transverse stretching is 1.7 times, and the retraction ratio is 0.

[0131] Embodiment 3:

[0132] The difference between Example 3 and Example 1 is: the solid content of Example 3 is 28%, and the extrusion volume is 70kg / h; the stretching rate of the first longitudinal stretching is 85mm / min, and the stretching ratio is 6.63 times; the stretching rate of the first transverse stretching is 145mm / min, and the stretching ratio is 6.73 times; the stretching ratio of the second transverse stretching is 1.7 times, and the retraction ratio is 0.

[0133] Comparative Example 1:

[0134] The difference between Comparative Example 1 and Example 1 is as follows: the solid content of Comparative Example 1 is 28%, and the extrusion volume 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 retraction ratio is 20%.

[0135] Performance Testing

[0136] 1. Test Method

[0137] 1.Thickness test:

[0138] Use a thickness tester with a resolution of not less than 1μm and follow GB / T 6672-2001.

[0139] 2. Average pore size test:

[0140] The pore size distribution analyzer of the filter membrane using the 3H-2000PB bubble pressure method and the BSD16 infiltration solution were used. The test was carried out in accordance with the national standard GB / T 32361-2005 separation membrane pore size test method.

[0141] 3. Air permeability test:

[0142] Use a manual or automatic air permeability tester to test and record according to GB / T 458-2008.

[0143] 4. Porosity test:

[0144] The test was carried out using the weighing method and calculated according to the formula: p = [1-m / (s×d×ρ0)]×100%, where: p is the porosity of the porous membrane, expressed in %; m is the mass of the porous membrane, in g; s is the area of ​​the porous membrane, in cm 2 ; d is the thickness of the porous membrane, in cm; ρ0 is the density of the porous membrane, in g / cm 3 ;

[0145] 5. Tortuosity test:

[0146] According to the formula, we can calculate: t = 5.18 × 10 -3 (τ2 ·L) / (d · ε), where: t is the air permeability of the porous membrane, expressed in s / 100mL; L is the thickness of the porous membrane, expressed in μm; d is the pore size of the porous membrane, expressed in nm; ε is the porosity of the porous membrane, expressed in %; τ is the tortuosity of the porous membrane.

[0147] 6. Water flux test:

[0148] Since the polyethylene porous membrane is hydrophobic, the membrane was soaked in IPA for 5 minutes and tested according to GB / T 32360-2015, and the following formula was used to calculate:

[0149] P = V / St, where P is the pure water permeability (L / m 2 ·h), V is the pure water permeation (L), S is the effective membrane filtration area (m 2 ), t is time (h).

[0150] 2. Test Results

[0151] The preparation parameters and performance test results of the above embodiments and comparative examples are shown in Table 1.

[0152] Table 1 Preparation parameters and performance test results of examples and comparative examples

[0153]

[0154] As can be seen from Table 1, the Gurley values ​​and tortuosity values ​​of the polyethylene porous membranes of Examples 1 to 3 are significantly smaller than those of the comparative example, indicating that the polyethylene porous membrane of the present invention has excellent permeability.

[0155] The thickness range and pore size range of the polyethylene porous membrane in Examples 1 to 3 are relatively large, indicating that during the preparation process, by adjusting process parameters such as stretching ratio, stretching rate and retraction ratio, performance parameters such as pore size and thickness of the polyethylene porous membrane can be flexibly adjusted to meet the use of the polyethylene porous membrane in different application scenarios such as separation and filtration.

[0156] The polyethylene porous membranes of Examples 1 to 3 have relatively high water flux values, indicating that they have excellent filtration efficiency, which is crucial for precise separation in the fields of water treatment, medicine, food processing, etc.

[0157] This indicates that the polyolefin porous membrane of the present invention has excellent permeability and can meet its application in the fields of separation and filtration.

[0158] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0159] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A polyolefin porous membrane, characterized in that The polyolefin porous membrane has a Gurley value of ≤25s / 100mL and a tortuosity of <1.

5.

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

5.

3. The polyolefin porous membrane 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 to 100 μm; (2) The porosity of the polyolefin porous membrane is 65 to 80%; (3) The pore size of the polyolefin porous membrane is 50 to 200 nm.

4. A method for preparing a polyolefin porous membrane, characterized in that: The following steps are involved: The polyethylene resin and the porogen are subjected to heat treatment to form a melt; The molten body solidifies to form an intermediate body; The intermediate is sequentially subjected to a first longitudinal stretching and a first transverse stretching to form a microporous oil film; removing the porogen in the microporous oil film to form a thin film; The film is subjected to a second transverse stretching to form a microporous membrane; The microporous membrane is heat-set to form a polyethylene porous membrane.

5. The preparation method according to claim 4, characterized in that: In the step 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 70 to 100 mm / min, preferably 80 to 90 mm / min; (b) the stretching ratio of the first longitudinal stretching is 4 to 9 times, preferably 5 to 7 times; (c) the stretching rate of the first transverse stretching is 120 to 160 mm / min, preferably 130 to 150 mm / min; (d) The stretching ratio of the first transverse stretching is 4 to 9 times, preferably 5 to 7 times.

6. The preparation method according to claim 4, characterized in that: In the step of performing a second transverse stretching on the film to form a microporous membrane, the stretching ratio of the second transverse stretching is 1 to 2 times, preferably 1.6 to 2 times.

7. The preparation method according to claim 4, characterized in that: When the microporous membrane is heat-set to form a porous membrane, the control parameters satisfy at least one of the following: (A) Control the retraction ratio to 0-10%; (B) the heat setting temperature is 120 to 135° C.; (C) The heat setting time is 1 to 2 minutes.

8. The preparation method according to claim 4, characterized in that: In the step 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: (i) The total stretching ratio is 20 to 80 times; (ii) The stretching temperature is 85 to 150°C.

9. The preparation method according to claim 4, characterized in that: In the step of performing a second transverse stretching on the film to form a microporous film, the control parameters satisfy at least one of the following: (1) The stretching rate is 90 to 130 mm / min; (2) The stretching temperature is 110 to 145°C.

10. The preparation method according to claim 4, characterized in that: In the step of heat treating the polyethylene resin and the porogen to form a melt, at least one of the following conditions is satisfied: (I) The weight average molecular weight of the polyethylene resin is 400,000 to 600,000; (II) the mass percentage of the polyethylene resin to the porogen is (20-35):(65-80); (III) the porogen comprises at least one of hydrocarbons, alcohols and esters; (IV) The temperature of the heat treatment is 200-230°C.

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