Method for improving problem of poor ventilation consistency of wet-process diaphragm

Abstract

The invention provides a method for improving the problem of poor ventilation consistency of a wet-process diaphragm. The method comprises the following steps: preparing polyolefin resin; the preparation method comprises the following steps: adding polyolefin resin, a nucleating agent accounting for 0.1-0.5% of the mass of the polyolefin resin and a plasticizer accounting for 25-35% of the mass of the polyolefin resin into a twin-screw extruder; carrying out high-speed gradient shearing and extrusion in a double-screw extruder; and carrying out stretching and wet extraction on the obtained product to obtain the wet-process diaphragm. According to the invention, the ventilation consistency of the diaphragm is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery separator preparation, and specifically relates to a method for improving the poor air permeability consistency of wet separators. Background Art

[0002] Lithium-ion batteries, as an important energy storage device, are widely used in fields such as electric vehicles and consumer electronics. As one of the key components of lithium-ion batteries, the separator has a crucial impact on the performance of the battery; its main functions are to isolate the positive and negative electrodes, prevent short circuits, and at the same time provide a channel for the migration of lithium ions. Among the many performance indicators of the separator, air permeability consistency is a key parameter, which directly affects the internal resistance, rate performance, and cycle life of the battery, etc.

[0003] At present, wet separators are one of the main types of lithium-ion battery separators. Due to their advantages such as small and uniform pore size, excellent mechanical properties, and thin thickness, they are widely used in ternary batteries that emphasize energy density. However, wet separators have the problem of poor air permeability consistency during the production process. From the perspective of the preparation process, wet separators utilize the principle of thermally induced phase separation, mixing a plasticizer with a polyolefin resin, undergoing phase separation during the cooling process, pressing the film sheet and then heating and stretching it, and finally extracting the plasticizer with a volatile solvent to obtain a microporous membrane. In this process, parameter fluctuations in each link may affect the air permeability consistency of the separator. For example, the non-uniformity of the cooling roller temperature during the casting sheet cooling will cause differences in the crystallization degree of different parts of the film sheet, which will in turn affect the formation and distribution of micropores during the subsequent stretching process, and ultimately result in poor air permeability consistency. In the stretching process, the stability of the stretching rate, the uniformity of the stretching ratio at different positions on the film surface, etc., will all affect the microporous structure of the separator. If there is local over-stretching or under-stretching during the stretching process, it will cause inconsistencies in air permeability.

[0004] In practical applications, wet separators with poor air permeability consistency will bring many adverse effects to lithium-ion batteries. When the air permeability consistency of the separator is not good, the migration rates of lithium ions in different regions inside the battery will show differences, resulting in uneven distribution of the internal resistance of the battery. During high-current charge and discharge processes, regions with larger internal resistance will generate more heat, leading to local overheating of the battery. This will not only accelerate the aging of the battery, reduce the cycle life of the battery, but also may cause safety hazards. The uneven air permeability will also reduce the rate performance of the battery and cannot meet some application scenarios with high requirements for fast charge and discharge. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for improving the poor air permeability consistency of wet separators. By adding a nucleating agent and a plasticizer to the polyolefin resin, and adopting processes such as gradient shear extrusion and gradient wet extraction, the problem of air permeability consistency of the separator is greatly improved.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for improving the poor air permeability consistency of a wet diaphragm, comprising the following steps:

[0008] S1: Prepare a polyolefin resin;

[0009] S2: Take the polyolefin resin in step S1, a nucleating agent accounting for 0.1 - 0.5% by mass of the polyolefin resin, and 25 - 35% of a plasticizer and add them to a twin-screw extruder;

[0010] S3: High-speed gradient shear and extrude in the twin-screw extruder;

[0011] S4: The product obtained in step S3 is subjected to wet extraction to obtain the wet diaphragm.

[0012] Further, the polyolefin resin in step S1 is composed of 70 - 80% ultra-high molecular weight polyethylene (UHMWPE) and 20 - 30% high-density polyethylene (HDPE) blended; or composed of 90 - 95% high-density polyethylene (HDPE) and 10 - 5% linear low-density polyethylene (LLDPE) blended; or made of polypropylene.

[0013] Further, the ultra-high molecular weight polyethylene (UHMWPE) has a molecular weight of 2 million - 6 million, a molecular weight distribution (PDI) of 6 - 8, a melt flow rate (MFR) of 0.1 - 0.2 g / 10 min at 90 °C / 21.6 kg; the high-density polyethylene (HDPE) has a molecular weight of 0.5 million - 1 million, a molecular weight distribution (PDI) of 4 - 6, a melt flow rate (MFR) of 0.3 - 0.5 g / 10 min at 190 °C / 2.16 kg; the high-density polyethylene (HDPE) has a molecular weight of 0.8 million - 1.2 million, a molecular weight distribution (PDI) of 3 - 5, a melt flow rate fluctuation (MFR) of 0.1 - 0.5 g / 10 min at 190 °C / 2.16 kg; the polypropylene (PP); the PP has a molecular weight of 0.3 million - 0.5 million, a molecular weight distribution (PDI) of 2 - 3, a melt flow rate fluctuation (MFR) of 1 - 3 g / 10 min at 230 °C / 2.16 kg, and a stereoregularity ≥ 95%. Resins with a narrower molecular weight distribution are preferably selected because this can make the molecular chains behave more uniformly during processing, reduce the non-uniformity of the microporous structure caused by molecular chain differences, and lay a foundation for subsequent uniform pore formation.

[0014] Further, the nucleating agent described in step S2 is an organic phosphate nucleating agent with a particle size of 0.5 - 1.5 μm; precisely controlling the nucleating agent content between 0.3 - 0.5% can effectively promote homogeneous nucleation, refine the grain size, and enable the diaphragm to form a more uniform microporous structure during the stretching process; pore density: the pore density is increased to 10^8 - 10^9 pores / cm2 (the traditional process is 10^7 - 10^8 pores / cm2); crystal morphology: inducing the resin to form an α crystal form (polypropylene system) or an orthorhombic crystal form (polyethylene system), and the grain size is reduced from 50 - 100 μm to 5 - 10 μm.

[0015] The plasticizer is paraffin oil with a molecular weight of 300 - 500 g / mol, a viscosity of 50 - 80 mPa·s at 40°C, and a flash point ≥ 220°C. It is precisely formulated within the range of 15 - 25% to ensure that it can reduce the resin melting point for phase separation while not overly affecting the mechanical properties and gas permeability consistency of the membrane.

[0016] Further, in step S3, the screw speed is 100 - 400 rpm, the mixing temperature is 160 - 210°C, the residence time is 3 - 5 min, and the vacuum degassing is -0.08 to -0.1 MPa.

[0017] Further, in step S3, control the screw speed at 100 - 200 rpm to pre-disperse the nucleating agent with low shear; then break the plasticizer droplets to the sub-micron level at a speed of 400 rpm.

[0018] Further, the extraction process for the PE system in step S4 is as follows: first, use dichloromethane with a mass fraction of 20 - 30% to pre-extract for 2 - 3 h at a temperature of 40 - 45°C, 20 kHz, and 50 W; then use dichloromethane with a mass fraction of 50 - 60% to react with stirring at a rate of 200 - 300 rpm for 8 - 10 h at a temperature of 50 - 55°C; subsequently, use deionized water to rinse countercurrently at a flow rate of 1 - 2 m / s for 5 - 7 h at a temperature of 25 - 30°C.

[0019] Further, the extraction process for the PP system in step S4 is as follows: first, use trichloroethylene with a mass fraction of 30 - 40% to pre-extract for 1 - 2 h at a temperature of 55 - 60°C; then use a mixture of white oil with a mass fraction of 30 - 40% and n-heptane with a mass fraction of 60 - 70% to react at a pulsed pressure of 0.1 - 0.3 MPa for 8 - 10 h at a temperature of 65 - 70°C; subsequently, use a mixture of ethanol and water at a ratio of 1:1 to vacuum suction at -0.05 MPa for 4 - 5 h at a temperature of 35 - 40°C.

[0020] Further, in step S4, the stretching rate ratio of MD / TD rate is maintained at 2.5:1 to 3:1 in the main stretching stage; acceleration control: adopt S-shaped curve acceleration; total stretching ratio: in the PE system, MD×TD = 10 - 12, and in the PP system, it is 8 - 10.

[0021] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0022] The present invention blends ultra-high molecular weight polyethylene (UHMWPE) and HDPE. On the basis of providing high strength, high puncture resistance, and melt uniformity, etc., through their synergistic effect, the overall PDI after blending is reduced to less than 5, and the MFR is increased to 0.1 - 0.3 g / 10 min (190 °C / 21.6 kg), achieving uniform film formation and a stable pore structure. By blending HDPE and LLDPE, short branches are introduced to adjust the crystallization rate and further optimize the pore size distribution. By precisely controlling the nucleating agent content between 0.3% - 0.5%, homogeneous nucleation can be effectively promoted, the grain size can be refined, and a more uniform microporous structure can be formed during the stretching process of the separator; the plasticizer needs to be precisely formulated within the range of 15% - 25% according to the resin characteristics and the target micropore size to ensure that it can not only reduce the resin melting point for phase separation but also not overly affect the mechanical properties and gas permeability consistency of the film. At the same time, advanced stirring and dispersion equipment, such as a high-shear mixer, is used to ensure the uniform dispersion of additives in the resin and avoid performance non-uniformity caused by local additive concentration differences. Precisely controlling the nucleating agent content between 0.3% - 0.5% can effectively promote homogeneous nucleation, refine the grain size, and form a more uniform microporous structure during the stretching process of the separator; the plasticizer needs to be precisely formulated within the range of 15% - 25% according to the resin characteristics and the target micropore size to ensure that it can not only reduce the resin melting point for phase separation but also not overly affect the mechanical properties and gas permeability consistency of the film. At the same time, advanced stirring and dispersion equipment, such as a high-shear mixer, is used to ensure the uniform dispersion of additives in the resin and avoid performance non-uniformity caused by local additive concentration differences. The melt viscosity of the UHMWPE / HDPE blend system is reduced (the viscosity drops by 40 - 60%), promoting the regulation of molecular chain entanglement and phase separation: forming continuous oil-phase channels during the extraction stage (such as soaking in dichloromethane) to induce a microporous through-hole structure; the synergistic effect of paraffin oil and the nucleating agent is proposed: the nucleating agent inhibits the formation of large grains, and the plasticizer regulates the phase separation rate, reducing the pore size fluctuation range from ±40% to ±15% after extraction. The "gradient shear" process is proposed: in the front area of the mixing section, the nucleating agent is pre-dispersed with low shear (100 - 200 rpm), and in the rear area, the plasticizer droplets are broken to the sub-micron level with high shear (400 rpm), finally achieving a dispersed phase size distribution D90 / D10 < 3. The extraction time and temperature are precisely controlled, and a continuous countercurrent extraction process is adopted to improve the extraction efficiency and uniformity. The extraction temperature fluctuation is controlled within ±1 °C, and the extraction time error is controlled within ±5 s to ensure the uniform removal of the plasticizer from the membrane sheet and avoid microporous structure differences caused by uneven extraction. Detailed implementation mode

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0024] The characteristics of the raw materials used in the present invention are as follows:

[0025] UHMWPE characteristics: molecular weight is 2 million - 6 million, molecular weight distribution PDI is 6 - 8, melt flow rate (MFR) is 0 g / 10 min at 190 °C / 21.6 kg. HDPE characteristics: molecular weight is 500,000 - 1 million, molecular weight distribution PDI is 4 - 6, MFR is 0.3 - 0.5 g / 10 min (190 °C / 2.16 kg).

[0026] Example 1

[0027] This example provides a method for improving the poor air permeability consistency of the wet diaphragm, including the following steps:

[0028] S1: Blend 70% ultra-high molecular weight polyethylene UHMWPE and 30% high density polyethylene HDPE to make polyolefin resin;

[0029] S2: Take the polyolefin resin in step S1, 0.1% nucleating agent sodium bis(4-tert-butylphenyl) phosphate by mass percentage of the polyolefin resin, and 25% plasticizer paraffin oil and add them to a twin-screw extruder;

[0030] S3: Perform high-speed gradient shearing and extrusion in the twin-screw extruder; control the screw speed at 100 rpm, the mixing temperature at 160 °C, the residence time at 3 min, and the vacuum degassing at -0.08 MPa;

[0031] S4: The product obtained in step S3 is subjected to wet extraction. First, use dichloromethane with a mass fraction of 20% at a temperature of 40 °C, 20 kHz, and 50 W for pre-extraction for 2 h; then use dichloromethane with a mass fraction of 50% at a temperature of 50 °C and stir and react at a rate of 200 rpm for 8 h; subsequently, use deionized water at a temperature of 25 °C and rinse countercurrently at a flow rate of 1 m / s for 5 h; then the wet diaphragm can be obtained.

[0032] Example 2

[0033] This example provides a method for improving the poor air permeability consistency of the wet diaphragm, including the following steps:

[0034] S1: Blend 80% ultra-high molecular weight polyethylene UHMWPE and 20% high density polyethylene HDPE to make polyolefin resin;

[0035] S2: Take the polyolefin resin, 0.5% by mass of the nucleating agent sodium bis(4-tert-butylphenyl) phosphate, and 35% of the plasticizer paraffin oil in step S1 and add them to a twin-screw extruder;

[0036] S3: Perform high-speed gradient shearing and extrusion in the twin-screw extruder; control the screw speed at 300 rpm, the mixing temperature at 180 °C, the residence time at 5 min, and the vacuum degassing at -0.1 MPa;

[0037] S4: The product obtained in step S3 is subjected to wet extraction. First, use dichloromethane with a mass fraction of 30% and pre-extract for 3 h at a temperature of 45 °C, 20 kHz, and 50 W; then use dichloromethane with a mass fraction of 60% and stir and react at a rate of 300 rpm for 10 h at a temperature of 55 °C; subsequently, use deionized water and perform countercurrent rinsing at a flow rate of 2 m / s for 7 h at a temperature of 30 °C; thus, the wet separator can be obtained.

[0038] Example 3

[0039] This example provides a method for improving the poor air permeability consistency problem of the wet separator, including the following steps:

[0040] S1: Blend 90% high-density polyethylene HDPE and 10% linear low-density polyethylene LLDPE to make a polyolefin resin;

[0041] S2: Take the polyolefin resin, 0.5% by mass of the nucleating agent sodium bis(4-tert-butylphenyl) phosphate, and 35% of the plasticizer paraffin oil in step S1 and add them to a twin-screw extruder;

[0042] S3: Perform high-speed gradient shearing and extrusion in the twin-screw extruder; control the screw speed at 250 rpm, the mixing temperature at 170 °C, the residence time at 5 min, and the vacuum degassing at -0.1 MPa;

[0043] S4: The product obtained in step S3 is subjected to wet extraction. First, use dichloromethane with a mass fraction of 25% and pre-extract for 2.5 h at a temperature of 42 °C, 20 kHz, and 50 W; then use dichloromethane with a mass fraction of 60% and stir and react at a rate of 300 rpm for 9 h at a temperature of 55 °C; subsequently, use deionized water and perform countercurrent rinsing at a flow rate of 2 m / s for 6 h at a temperature of 30 °C; thus, the wet separator can be obtained.

[0044] Example 4

[0045] This example provides a method for improving the poor air permeability consistency problem of the wet separator, including the following steps:

[0046] S1: Blend 95% high-density polyethylene HDPE and 5% linear low-density polyethylene LLDPE to make a polyolefin resin;

[0047] S2: Take the polyolefin resin, 0.5% by mass of the polyolefin resin of the nucleating agent sodium bis(4-tert-butylphenyl) phosphate, and 35% of the plasticizer paraffin oil in Step S1 and add them to a twin-screw extruder;

[0048] S3: Perform high-speed gradient shearing and extrusion in the twin-screw extruder; control the screw speed at 300 rpm, the mixing temperature at 175 °C, the residence time at 5 min, and the vacuum degassing at -0.9 MPa;

[0049] S4: The product obtained in Step S3 is subjected to wet extraction. First, use dichloromethane with a mass fraction of 30% to pre-extract for 3 h at a temperature of 42 °C, 20 kHz, and 50 W; then use dichloromethane with a mass fraction of 60% to stir and react for 9 h at a temperature of 55 °C at a rate of 300 rpm; subsequently, use deionized water to perform countercurrent rinsing for 7 h at a temperature of 30 °C at a flow rate of 2 m / s; thus, the wet separator can be obtained.

[0050] Example 5

[0051] This example provides a method for improving the poor air permeability consistency of the wet separator, including the following steps:

[0052] S1: Select polypropylene resin;

[0053] S2: Take the polypropylene resin, 1% by mass of the polyolefin resin of the nucleating agent sodium bis(4-tert-butylphenyl) phosphate, and 35% of the plasticizer paraffin oil in Step S1 and add them to a twin-screw extruder;

[0054] S3: Perform high-speed gradient shearing and extrusion in the twin-screw extruder; control the screw speed at 300 rpm, the mixing temperature at 185 °C, the residence time at 5 min, and the vacuum degassing at -0.09 MPa;

[0055] S4: The product obtained in Step S3 is subjected to wet extraction. First, use trichloroethylene with a mass fraction of 30% to pre-extract for 1 h at a temperature of 55 °C; then use a mixture of 30% white oil and 60% n-heptane to react for 8 h at a temperature of 65 °C under a pulsed pressure of 0.1 MPa; subsequently, use a mixture of ethanol and water at a ratio of 1:1 to perform vacuum suction for 4 h at a temperature of 35 °C and -0.05 MPa; thus, the wet separator can be obtained.

[0056] Example 6

[0057] This example provides a method for improving the poor air permeability consistency of the wet separator, including the following steps:

[0058] S1: Blend 95% high-density polyethylene HDPE and 5% linear low-density polyethylene LLDPE to make a polyolefin resin;

[0059] S2: Add the polyolefin resin, 0.5% nucleating agent sodium bis(4-tert-butylphenyl) phosphate by mass percentage of the polyolefin resin, and 35% plasticizer paraffin oil in step S1 into a twin-screw extruder;

[0060] S3: Conduct high-speed gradient shearing and extrusion in the twin-screw extruder; control the screw speed at 300 rpm, the mixing temperature at 190 °C, the residence time at 5 min, and the vacuum degassing at -0.08 MPa;

[0061] S4: The product obtained in step S3 is subjected to wet extraction. First, pre-extract with trichloroethylene with a mass fraction of 40% at a temperature of 60 °C for 2 h; then use a mixture of white oil with a mass fraction of 40% and n-heptane with a mass fraction of 70%, react at a temperature of 70 °C under a pulsed pressure of 0.3 MPa for 10 h; subsequently, use a 1:1 mixture of ethanol and water, under a vacuum suction of -0.05 MPa at a temperature of 40 °C for 5 h, and the wet diaphragm can be obtained.

[0062] Air permeability consistency detection method:

[0063]

[0064] Data statistics and consistency evaluation

[0065] Coefficient of variation (CV value) calculation:

[0066] \[CV(\%)=\frac{\text{Standard deviation}}{\text{Mean}}\times100\]

[0067] Qualified judgment criteria:

[0068] Gurley value CV ≤ 5% (requirements for power battery level);

[0069] Pore size distribution CV ≤ 8%;

[0070] Thickness CV ≤ 1.5%.

[0071] Proof conclusion and example data

[0072] Performance comparison between the optimized process and the control group

[0073]

[0074] Microstructural characterization

[0075] SEM image analysis:

[0076] The pore channels in the experimental group show a uniform honeycomb structure, and the standard deviation of the pore size ≤ 2 nm (the pore size of the control group varies significantly, and there are local closed pores);

[0077] The micropore penetration rate ≥ 98% (≤ 85% for the control group).

[0078] AFM surface roughness:

[0079] The Ra value is reduced from 35 ± 8 nm of the traditional process to 18 ± 3 nm, reducing the air permeability fluctuations caused by surface defects.

[0080] Verification of actual application performance

[0081]

[0082] The above are the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for improving the poor air permeability consistency of wet diaphragms, characterized in that, It includes the following steps: S1: Prepare polyolefin resin; S2: Take the polyolefin resin in step S1, a nucleating agent accounting for 0.1 - 0.5% of the mass of the polyolefin resin, and 25 - 35% of a plasticizer and add them to a twin-screw extruder; S3: High-speed gradient shear and extrude in the twin-screw extruder; S4: The product obtained in step S3 is stretched and wet-extracted to obtain the wet separator.

2. A method for improving the poor air permeability consistency of a wet diaphragm according to claim 1, characterized in that, The polyolefin resin in step S1 is composed of 70 - 80% ultra-high molecular weight polyethylene (UHMWPE) and 20 - 30% high-density polyethylene (HDPE) blended; or composed of 90 - 95% high-density polyethylene (HDPE) and 10 - 5% linear low-density polyethylene (LLDPE) blended; or made from polypropylene.

3. A method for improving the poor air permeability consistency of a wet diaphragm according to claim 2, characterized in that, The molecular weight of the ultra-high molecular weight polyethylene (UHMWPE) is 2 - 6 million, the molecular weight distribution (PDI) is 6 - 8, the melt flow rate (MFR) is 0.1 - 0.2 g / 10min at 90 °C / 21.6 kg; the molecular weight of the high-density polyethylene (HDPE) is 0.5 - 1 million, the molecular weight distribution (PDI) is 4 - 6, the melt flow rate (MFR) is 0.3 - 0.5 g / 10min at 190 °C / 2.16 kg; the molecular weight of the high-density polyethylene (HDPE) is 0.8 - 1.2 million, the molecular weight distribution (PDI) is 3 - 5, the melt flow fluctuation (MFR) is 0.1 - 0.5 g / 10min at 190 °C / 2.16 kg; the polypropylene (PP); the molecular weight of the PP is 0.3 - 0.5 million, the molecular weight distribution (PDI) is 2 - 3, the melt flow fluctuation (MFR) is 1 - 3 g / 10min at 230 °C / 2.16 kg, and the stereoregularity is ≥95%.

4. A method for improving the poor air permeability consistency of a wet diaphragm according to claim 1, characterized in that, The nucleating agent in step S2 is an organic phosphate nucleating agent with a particle size of 0.5 - 1.5 μm; the plasticizer is paraffin oil with a molecular weight of 300 - 500 g / mol, a viscosity of 50 - 80 mPa·s at 40 °C, and a flash point ≥220 °C.

5. A method for improving the poor air permeability consistency of a wet diaphragm according to claim 1, characterized in that, In step S3, the screw speed is 100 - 400 rpm, the mixing temperature is 160 - 210 °C, the residence time is 3 - 5 min, and the vacuum degassing is -0.08 to -0.1 MPa.

6. A method for improving the poor air permeability consistency of a wet diaphragm according to claim 5, characterized in that, In step S3, control the screw speed at 100 - 200 rpm for low-shear pre-dispersion of the nucleating agent; then break the plasticizer droplets to the sub-micron level at a speed of 400 rpm.

7. A method for improving the poor air permeability consistency of a wet diaphragm according to claim 1, characterized in that The extraction process for the PE system in step S4 is: First, pre-extract with dichloromethane with a mass fraction of 20 - 30% at a temperature of 40 - 45 °C, 20 kHz, and 50 W for 2 - 3 h; then react with dichloromethane with a mass fraction of 50 - 60% at a temperature of 50 - 55 °C with a stirring rate of 200 - 300 rpm for 8 - 10 h; subsequently, rinse with deionized water at a temperature of 25 - 30 °C with a flow rate of 1 - 2 m / s in a countercurrent manner for 5 - 7 h.

8. A method for improving the poor air permeability consistency of a wet diaphragm according to claim 1, characterized in that The extraction process of the PP system in step S4 is as follows: First, use trichloroethylene with a mass fraction of 30-40% to pre-extract for 1-2 h at a temperature of 55-60 °C; then use a mixture of white oil with a mass fraction of 30-40% and n-heptane with a mass fraction of 60-70% to react at a temperature of 65-70 °C under a pulsed pressure of 0.1-0.3 MPa for 8-10 h; subsequently, use a 1:1 mixture of ethanol and water to perform vacuum suction at a temperature of 35-40 °C under -0.05 MPa for 4-5 h.

9. A method for improving the poor air permeability consistency of a wet diaphragm according to claim 1, characterized in that, In step S4, the stretching rate ratio of MD / TD rate, during the main stretching stage, is maintained at 2.5:1 to 3:1; acceleration control: adopt S-shaped curve acceleration; total stretching ratio: in the PE system, MD×TD = 10-12, and in the PP system, it is 8-10.

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