Diaphragm and preparation method thereof

Abstract

A diaphragm and a preparation method thereof belong to the technical field of battery diaphragms. The preparation method of the diaphragm comprises the following steps: feeding a raw material containing polyethylene into an extruder for melting according to a set feeding amount, filtering, extruding by a die head, casting, cooling and forming to obtain a casting sheet. And carrying out post-treatment on the cast sheet to form the diaphragm. Wherein the viscosity average molecular weight of the polyethylene is not less than 1,800,000, the filtering mesh number is 400-800 meshes, and the ratio of the feeding amount to the rotating speed of a screw of an extruder is 2.5-3.5 (kg / h) / (r / min). According to the method for preparing the diaphragm, the specific needling strength of the diaphragm can be improved.

Description

Technical Field

[0001] The present application relates to the technical field of battery separators, and in particular to a separator and a preparation method thereof. Background Art

[0002] The separator is a crucial safety and functional component in lithium-ion batteries and other secondary batteries, providing both electronic insulation and ionic conductivity. The separator's insulation function refers to its position between the positive and negative electrodes, preventing them from shorting and failing. Therefore, the separator must possess high puncture strength to prevent impurities or burrs on the positive and negative electrodes from puncturing and causing a battery short circuit.

[0003] Currently, the manufacturing process of diaphragms generally includes wet film production and dry film production. Both wet and dry film production processes require polyolefin powder as raw material, which is fed into an extruder for melt extrusion, followed by sheet casting and stretching.

[0004] Among these, wet-process membranes typically feature uniform pore size distribution, good product consistency, a wide controllable range of porosity and permeability, and easier thin-film production. Consequently, they have become the mainstream process in recent years. The wet-process typically uses polyethylene powder as a raw material. However, the needle punch strength of membranes produced by melt-extruding polyethylene powder using the current wet-process is still relatively low and needs to be further improved. Summary of the Invention

[0005] Based on the above-mentioned deficiencies, the present application provides a diaphragm and a preparation method thereof, which can improve the specific needle puncture strength of the diaphragm.

[0006] This application is implemented as follows:

[0007] In a first aspect, an example of the present application provides a method for preparing a diaphragm, comprising:

[0008] A polyethylene raw material is fed into an extruder at a set feed rate, melted, filtered, extruded through a die, cast, and cooled to form a cast sheet. The cast sheet is then post-processed to form a diaphragm. The polyethylene has a viscosity-average molecular weight of no less than 1.8 million. The filtration mesh size is 400-800 mesh. The ratio of feed rate to extruder screw speed is 2.5-3.5 (kg / h) / (r / min).

[0009] In the above implementation process, polyethylene with a viscosity-average molecular weight of more than 1.8 million is used as the base material of the diaphragm, and the ratio of the feed amount to the screw speed of the extruder is limited to 2.5-3.5 (kg / h) / (r / min) for melt extrusion. The melt is filtered with a 400-800 mesh filter before extrusion, which can improve the plasticizing effect, improve the physical and chemical properties of the polyethylene melt, and improve the specific needle punch strength of the polyethylene diaphragm obtained after post-processing.

[0010] In combination with the first aspect, in an optional embodiment of the present application, the viscosity-average molecular weight of the polyethylene is 1.8 million to 4 million.

[0011] In the above implementation process, using polyethylene with a viscosity-average molecular weight of 1.8 million to 4 million as the base material of each diaphragm can improve the specific needle punch strength of the diaphragm.

[0012] In combination with the first aspect, in an optional embodiment of the present application, the viscosity-average molecular weight of the polyethylene is 2 million to 2.4 million, and the filtration mesh size is 400 to 600 meshes.

[0013] In the above implementation process, when the viscosity-average molecular weight of the polyethylene powder added during melt extrusion is 2 million to 2.4 million, the mesh size of the filter can be appropriately reduced, and a filter with a mesh size of 400 to 600 can be used to filter the melt. This can not only filter out the polyethylene powder that is not uniformly plasticized or other impurities in the melt, thereby improving the quality of the cast sheet, but also ensure that the polyethylene does not degrade to a large extent. It can also appropriately shear the melt, further improving the physical and chemical properties of the melt, improving the tensile quality during subsequent stretching, and further improving the specific needle punch strength of the diaphragm. If the molecular weight of the polyethylene is high and the mesh size of the filter is large, it will not only increase the degree of polyethylene degradation, but also increase the extrusion pressure, affecting the continuous extrusion of the melt, affecting the subsequent cast sheet quality and tensile quality, and affecting the specific needle punch strength of the diaphragm.

[0014] In combination with the first aspect, in an optional embodiment of the present application, the viscosity-average molecular weight of the polyethylene is 1.8 million to 2 million, and the filtration mesh size is 600 to 800 meshes.

[0015] In the above implementation process, when the viscosity-average molecular weight of the polyethylene powder added during melt extrusion is 1.8 to 2 million, the mesh size of the filter can be appropriately increased, and a filter with a mesh size of 600 to 800 can be used to filter the melt. This can not only filter out the polyethylene powder that is not uniformly plasticized or other impurities in the melt, thereby improving the quality of the cast sheet, but also ensure that the polyethylene does not degrade to a large extent. It can also appropriately shear the melt, further improving the physical and chemical properties of the melt, improving the stretching quality during subsequent stretching, and further increasing the specific needle punch strength of the diaphragm. If the viscosity-average molecular weight of the polyethylene is small and the mesh size of the filter is small, impurities cannot be effectively filtered before extrusion from the die head, and the melt cannot be appropriately sheared to improve its physical and chemical properties, resulting in a low specific needle punch strength of the resulting diaphragm.

[0016] In combination with the first aspect, in an optional embodiment of the present application, the feed rate is 250-500 kg / h, and the screw speed is 70-200 r / min.

[0017] Optionally, the ratio of feed rate to screw speed is 3 to 3.2 (kg / h) / (r / min).

[0018] Optionally, the feed rate is 390-450 kg / h and the screw speed is 130-140 r / min.

[0019] In the above implementation process, the raw materials are fed into the extruder at a feed rate of 250 to 500 kg / h for plasticization and melting, and the screw speed of the extruder is controlled to be 70 to 200 r / min, especially the ratio of the feed rate to the screw speed is controlled to be 2.5 to 3.5 (kg / h) / (r / min). This can improve the plasticization uniformity of the melt while ensuring continuous extrusion, reduce the degree of degradation of the polyethylene powder during the melt extrusion process, and improve the specific needle puncture strength of the prepared diaphragm. If the feed rate is too large, it will lead to excessive extrusion pressure, which will not only easily cause failure of the extruder equipment, affect the continuous production efficiency of the diaphragm, but also reduce the specific needle puncture strength of the diaphragm. If the screw speed is too fast, it will aggravate the degradation of polyethylene, cause changes in the structure and physical and chemical properties of polyethylene, affect the cast sheet and tensile quality, and reduce the specific needle puncture strength of the diaphragm. If the screw speed is too low, it is difficult to plasticize uniformly and extrude continuously, which will also affect the specific needle puncture strength of the diaphragm.

[0020] In combination with the first aspect, in an optional embodiment of the present application, the D50 of the polyethylene is 90 to 150 μm.

[0021] Optionally, the raw material further includes low molecular weight polyethylene, and the viscosity average molecular weight of the low molecular weight polyethylene is 500,000 to 1,000,000.

[0022] Optionally, the mass ratio of polyethylene to low molecular weight polyethylene is 3 to 20:1.

[0023] In the above-mentioned implementation process, polyethylene powder with a D50 of 80 to 150 μm and a viscosity-average molecular weight of more than 1.8 million is used as the raw material of the diaphragm. While improving the plasticizing effect of the polyethylene powder and obtaining a homogeneous melt of polyethylene and a pore-forming agent, it can also reduce the extrusion temperature and extrusion pressure during melt extrusion, and can be continuously filtered through a filter with a mesh size of 400 to 800 mesh, thereby improving the performance of the melt. In the subsequent processing process, the probability of defects such as white spots on the cast film can be reduced, the tensile quality can be improved, and the specific needle punch strength of the prepared polyethylene diaphragm can be increased.

[0024] Furthermore, adding a certain proportion of low-molecular-weight polyethylene to the raw materials, through the combination of high and low molecular weight, can further reduce the screw speed during melt extrusion, lower the melt pressure and melt temperature during melt extrusion, improve the physical and chemical properties of the melt when filtered through a filter with a mesh size of 400-800, reduce the degree of degradation of the polyethylene powder during the melt extrusion process, improve the quality of the cast sheet and the tensile quality, and further increase the specific needle punch strength of the diaphragm. Controlling the mass ratio of polyethylene to low-molecular-weight polyethylene to 3-20:1 can avoid the decrease in the specific needle punch strength of the diaphragm due to excessive addition of low-molecular-weight polyethylene.

[0025] In combination with the first aspect, in an optional embodiment of the present application, the extrusion temperature is 170-220°C, the extrusion pressure is 70-150 bar, and the melt temperature is 206-270°C.

[0026] Optionally, the extrusion temperature is 185-195°C.

[0027] In the above-mentioned implementation process, when the polyethylene powder with a viscosity-average molecular weight of not less than 1.8 million is melt-extruded, the melt is plasticized evenly at a feed rate of 250 to 500 kg / h under the conditions of a relatively low extrusion pressure of 70 to 105 bar and a relatively low melt temperature of 206 to 270°C. The melt can be continuously filtered through a filter with a mesh size of 400 to 800 meshes and then enter the die head for extrusion. This can reduce the degree of degradation of polyethylene during the melt extrusion process, reduce the probability of defects such as white spots in the cast film during the cast film casting process, reduce the probability of defects such as stretching film breakage during the post-processing stretching process, and improve the specific needle punch strength and yield of the prepared diaphragm.

[0028] In combination with the first aspect, in an optional embodiment of the present application, the raw material further includes a pore-forming agent.

[0029] Optionally, the pore former comprises white oil.

[0030] Optionally, before feeding the raw materials into the extruder, the raw materials are premixed at a temperature of 90 to 130° C. for more than 40 minutes.

[0031] In the above implementation process, the raw materials containing polyethylene and pore-forming agent are premixed at a temperature of 90 to 130° C. for more than 40 minutes, and then fed into the extruder for melt extrusion. A uniform melt can be formed at a lower screw speed, extrusion pressure and melt temperature, and can be continuously passed through a filter with a mesh size of 400 to 800 mesh and continuously extruded, thereby reducing the degree of polyethylene degradation during the melt extrusion process, improving the quality of the melt, reducing the probability of defects such as cast transparency during cast film, improving subsequent tensile quality, and increasing the specific needle punch strength of the diaphragm. If the premixing temperature is too high or the premixing time is too long, small-particle polyethylene powder is likely to aggregate, which will affect the plasticization quality and require an increase in extrusion pressure and extrusion temperature. When filtered through a filter with a mesh size of 400 to 800 mesh, the degradation of polyethylene will be aggravated, which will reduce the specific needle punch strength of the diaphragm. If the premixing temperature is too low or the premixing time is too short, the polyethylene powder will be melted and extruded without being fully swollen during melt extrusion, and a high viscosity layer will easily form on the surface of the polyethylene powder, which will inhibit the full contact and mixing of the pore-forming agent and polyethylene, reduce the quality of the cast sheet and the tensile quality, and affect the specific needle punch strength of the diaphragm.

[0032] In combination with the first aspect, in an optional embodiment of the present application, the post-processing includes stretching, extraction and heat setting performed in sequence.

[0033] In a second aspect, an example of the present application provides a diaphragm produced according to the production method provided in the first aspect, wherein the diaphragm has a specific needle punch strength of not less than 91.8 gf / μm.

[0034] In the above-mentioned implementation process, a diaphragm is prepared according to the preparation method provided in the first aspect of the present application, and polyethylene with a viscosity-average molecular weight of more than 1.8 million is used as the base material of the diaphragm. The ratio of the feed amount to the screw speed of the extruder is limited to 2.5~3.5 (kg / h) / (r / min) for melt extrusion, and the melt is filtered using a 400~800 mesh filter during extrusion. This can improve the plasticizing effect, improve the physical and chemical properties of the polyethylene melt, and prepare a polyethylene diaphragm with a higher specific needle punch strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art.

[0036] Figure 1 Flow chart of the preparation process of the diaphragm provided as an example in this application. DETAILED DESCRIPTION

[0037] The embodiments of the present application will be described in detail below with reference to the examples, but it will be understood by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if specific conditions are not specified, the conditions are carried out according to conventional conditions or manufacturer recommendations. The reagents or instruments used are not specified by the manufacturer and are conventional products that can be purchased commercially.

[0038] The separator is a core component of lithium-ion batteries and other secondary batteries, accounting for approximately 20-30% of the total battery cost. Its performance has a significant impact on the overall performance of the battery and is one of the key technologies restricting battery development.

[0039] As the application of secondary batteries continues to expand and the impact of lithium-ion products on people's lives continues to deepen, people's requirements for secondary battery performance are also getting higher and higher. To meet the development requirements of secondary batteries, the separator, as a key component of secondary batteries, should not only have good chemical stability and low manufacturing cost, but more importantly, high mechanical strength.

[0040] At present, the production materials of diaphragms for lithium-ion batteries and other secondary batteries are mainly polyolefins represented by polypropylene (PP) and polyethylene (PE). From the perspective of preparation process, they can be divided into:

[0041] 1. Dry-process separators—melt stretching, which can be divided into uniaxial and biaxial stretching—can be used to produce microporous PP and PE separators and has already been industrialized. However, due to inherent defects in polyolefins and the characteristics of existing processes, the mechanical properties of existing dry-process separators are insufficient. Lithium dendrites in lithium-ion batteries can puncture the separator, causing defects such as micro-short circuits.

[0042] 2. Wet-process membranes—thermally induced phase separation. This process works by mixing polymer macromolecules with a high-boiling-point, small-molecule pore-forming agent (also known as a diluent or solvent) at a certain temperature to form a homogeneous melt. The system temperature is then lowered to induce phase separation between the polymer macromolecules and the small-molecule pore-forming agent. After phase separation, the small-molecule pore-forming agent is dispersed within the polymer solid. Finally, the small-molecule pore-forming agent is extracted, leaving microporous structures between the polymer molecules. This allows the preparation of microporous membranes with excellent mechanical properties.

[0043] Wet-process diaphragms usually have the characteristics of uniform micropore size distribution, good product consistency, a wide controllable range of porosity and permeability, and easier thin-film standardization production. Therefore, they have developed into a mainstream process in recent years.

[0044] Wet-process diaphragms typically use ultra-high molecular weight polyethylene (UHMWPE) as the primary material. However, due to inherent defects in UHMWPE and the specific characteristics of the wet-process, the mechanical properties of current wet-process diaphragms still need to be improved, particularly needle punch strength.

[0045] Based on this, the present invention provides a method for preparing a diaphragm that can improve the problem of low needle punch strength of wet-process diaphragms. To make the purpose, technical solution, and advantages of the present invention more clear, the technical solution in the present invention will be clearly and completely described below in conjunction with the accompanying drawings.

[0046] See also Figure 1 , the preparation method provided in the embodiment of the present application includes:

[0047] S1. Feeding: Feed the raw material containing polyethylene into the extruder at a set feeding rate. The viscosity average molecular weight of the polyethylene is not less than 1.8 million.

[0048] S2. Melt extrusion: melt, filter, and extrude the raw materials. The mesh size of the filter is 400-800 mesh, and the ratio of the feed rate to the screw speed of the extruder is 2.5-3.5 (kg / h) / (r / min).

[0049] S3, tape casting, casting sheet: the melt extruded from the extruder is tape cast, cooled and solidified to obtain a casting sheet.

[0050] S4. Post-processing: Post-process the casting to form a diaphragm.

[0051] Polyethylene with a viscosity-average molecular weight of more than 1.8 million is used as the base material of the diaphragm, and melt extrusion is carried out with a specific feed amount and screw speed of the extruder. The melt is filtered with a 400-800 mesh filter before extrusion, which can improve the plasticizing effect and properly shear the melt. It can not only improve the fluidity of the melt and reduce the extrusion pressure and temperature during extrusion, but also filter and remove impurities in the melt, improve the physical and chemical properties of the polyethylene melt, improve the subsequent cast sheet quality and tensile quality, and improve the specific needle punch strength of the polyethylene diaphragm obtained after post-processing.

[0052] If the melt is not filtered before extrusion, or the filtration mesh is too low, the melt is likely to contain unevenly plasticized polyethylene particles or other impurities. If these impurities enter the die head of the extruder for extrusion, it will not only easily clog the die head, thereby causing uneven shape or thickness of the casting, but also these impurities will remain in the casting after extrusion. During subsequent post-processing such as stretching, these impurities will become weak connection points, which are prone to defects such as film breakage, affecting the specific needle punch strength and yield of the diaphragm.

[0053] If, under the extrusion conditions of the above-mentioned feed rate and screw speed, the mesh size of the melt is too high when it is filtered before extrusion, for example, a polyethylene melt containing a viscosity-average molecular weight of not less than 1.8 million is filtered with a filter larger than 800 mesh, it will not only produce a large shear on the melt, but also aggravate the degradation of polyethylene, fail to effectively improve the physical and chemical properties of the melt, affect the specific needle puncture strength of the diaphragm, but also increase the extrusion pressure, resulting in discontinuous extrusion and inability to produce normally.

[0054] The embodiment of the present application utilizes polyethylene of a specific molecular weight as the substrate of the diaphragm, and combines it with a specific feed rate, screw speed, and filter mesh size, which can improve the quality of the melt, the quality of extrusion, cast film, and stretching, and thereby improve the specific needle punch strength of the diaphragm.

[0055] In step S1, the present application does not limit the viscosity average molecular weight of the polyethylene. In some possible embodiments, the viscosity average molecular weight of the polyethylene may be 1.8 million to 4 million.

[0056] As an example, the viscosity average molecular weight of the polyethylene can be one of 1.8 million, 1.9 million, 2 million, 2.1 million, 2.2 million, 2.3 million, 2.4 million, 2.5 million, 2.6 million, 2.7 million, 2.8 million, 2.9 million, 3 million, 3.1 million, 3.2 million, 3.3 million, 3.4 million, 3.5 million, 3.6 million, 3.7 million, 3.8 million, 3.9 million, or 4 million, or a range between any two of them.

[0057] As an example, the viscosity average molecular weight of polyethylene may be 1.8 million to 3 million.

[0058] As an example, the viscosity average molecular weight of polyethylene may be 1.8 million to 2.4 million.

[0059] Furthermore, in some possible embodiments, in addition to ultra-high molecular weight polyethylene with a viscosity-average molecular weight of not less than 1.8 million, the raw materials may also contain low molecular weight polyethylene with a viscosity-average molecular weight of 500,000 to 1,000,000.

[0060] By adding a certain proportion of low molecular weight polyethylene to the raw materials, the melt pressure and melt temperature during melt extrusion can be further reduced through the combination of high and low molecular weight. Filtering through a filter with a mesh size of 4 to 8 million before extrusion can improve the fluidity of the melt, reduce the degree of polyethylene degradation, improve the quality of cast sheet and subsequent stretching quality, and further improve the specific needle punch strength of the diaphragm.

[0061] As an example, the viscosity average molecular weight of the low molecular weight polyethylene may be 500,000, 600,000, 700,000, 800,000, 900,000 or 1,000,000, or a range between any two of them.

[0062] Furthermore, the mass ratio of ultra-high molecular weight polyethylene (UHMWPE) with a viscosity-average molecular weight of 1.8 million or more to low molecular weight polyethylene (LMPE) in the raw materials can be 3 to 20:1, which can further improve the specific needle punch strength of the separator. If the amount of LMWPE added to the raw materials is too high, the specific needle punch strength of the separator will be reduced.

[0063] As an example, in the raw materials, the mass ratio of ultra-high molecular weight polyethylene with a viscosity average molecular weight of 1.8 million or more and low molecular weight polyethylene can be 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1 or 20:1 or a range between any two of them.

[0064] As an example, the raw material includes 16 wt % of ultra-high molecular weight polyethylene with a viscosity average molecular weight of 2.8 million and 2 wt % of low molecular weight polyethylene with a viscosity average molecular weight of 900,000.

[0065] As an example, the raw material includes 16 wt % of ultra-high molecular weight polyethylene with a viscosity average molecular weight of 2.8 million and 3 wt % of low molecular weight polyethylene with a viscosity average molecular weight of 900,000.

[0066] As an example, the raw material includes 17 wt % of ultra-high molecular weight polyethylene with a viscosity average molecular weight of 2.8 million and 1 wt % of low molecular weight polyethylene with a viscosity average molecular weight of 900,000.

[0067] As an example, the raw material includes 14 wt % of ultra-high molecular weight polyethylene with a viscosity average molecular weight of 2.8 million and 4 wt % of low molecular weight polyethylene with a viscosity average molecular weight of 900,000.

[0068] As an example, the raw material includes 16 wt % of ultra-high molecular weight polyethylene with a viscosity average molecular weight of 1.8 million and 2 wt % of low molecular weight polyethylene with a viscosity average molecular weight of 500,000.

[0069] As an example, the raw material includes 16 wt % of ultra-high molecular weight polyethylene with a viscosity average molecular weight of 3 million and 2 wt % of low molecular weight polyethylene with a viscosity average molecular weight of 700,000.

[0070] In order to further improve the melt quality, in some possible embodiments, polyethylene powder with a median particle size D50 of 80 to 150 μm may be selected as the raw material for melt extrusion.

[0071] According to the preparation method provided in the embodiment of the present application, polyethylene with a D50 of 80 to 150 μm and a viscosity-average molecular weight of 1.8 million or more is used as a raw material for melt extrusion, and melt extrusion is performed at a specific feed rate and a specific screw speed. This can improve the plasticizing effect of the polyethylene powder and obtain a homogeneous melt of polyethylene and a pore-forming agent. At the same time, when filtering through a filter with a mesh size of 400 to 800 mesh, the melt can be properly sheared, the physical and chemical properties of the melt can be improved, and the extrusion temperature and extrusion pressure during melt extrusion can be reduced. In the subsequent processing process, the probability of defects such as white spots in the casting can be reduced, the tensile quality can be improved, and the specific needle punch strength of the prepared polyethylene diaphragm can be improved.

[0072] If the particle size of the polyethylene powder is too large, when melt extrusion is carried out under the above-mentioned feed rate and screw speed conditions, not only will the plasticizing effect be reduced, but the shear force will increase during filtration, the extrusion pressure will also be increased, and the continuity of extrusion will be affected, reducing the quality of subsequent cast film and stretching steps, and the specific needle strength of the diaphragm will be reduced.

[0073] If the polyethylene powder particle size is too small and the powder surface energy is high, when melt extrusion is carried out under the aforementioned feed rate and screw speed conditions, the polyethylene powder is in an unstable state in the early stage, and the particles are prone to coagulation and agglomeration, which may form secondary particles. This will worsen the plasticization effect and require an increase in extrusion temperature and pressure. After filtration, this will affect the physical and chemical properties of the melt, reducing the quality of subsequent steps such as cast film and stretching, and further reducing the needle punch strength of the diaphragm. In addition, because ultra-high molecular weight polyethylene exhibits low fluidity even in a fully molten state, it is difficult to mold into small-sized pellets. The cost of preparing ultra-small-sized polyethylene powder is high, which will increase the production cost of the diaphragm and reduce its competitive advantage.

[0074] As an example, the particle size of the polyethylene powder may be one of 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm, or a range between any two of them.

[0075] As an example, the particle size of the polyethylene powder may be 90 to 150 μm.

[0076] In the wet film production process, it is necessary to mix the polymer macromolecules with a high-boiling-point small molecule pore-forming agent (also known as a diluent or solvent) at a certain temperature to form a homogeneous melt. Therefore, in step S1, the raw materials also include a pore-forming agent.

[0077] The type of pore-forming agent affects the phase separation process and is a key factor in controlling the structure and size of the membrane's micropores. Consequently, it significantly impacts the membrane's pore size distribution, porosity, and permeability. This application is not limited to a specific type of pore-forming agent; in one possible embodiment, the pore-forming agent is selected from white oil.

[0078] White oil has a high boiling point and is less susceptible to loss during melt extrusion, sheet casting, and stretching, improving the quality of stretched membranes. After extraction and removal of the white oil, a membrane with a suitable pore size distribution can be obtained. White oil also has an appropriate viscosity, which, when filtered through a filter, further improves the mixing uniformity of the white oil and polyethylene melt, improving the physical and chemical properties of the melt and increasing the specific needle punch strength of the membrane.

[0079] In addition, the addition ratio of the pore-forming agent will also affect the microstructure and mechanical properties of the separator. In some possible embodiments, the dry material ratio of the raw materials can be 15% to 19%.

[0080] The dry material ratio refers to the mass proportion of polyethylene solid material in the raw material.

[0081] An appropriate dry-to-material ratio can further improve melt uniformity, enhance the quality of cast sheets, reduce defects during stretch forming, and further enhance the membrane's needle punch strength. If the dry-to-material ratio is too high, the membrane's porosity and average pore size will decrease. If the dry-to-material ratio is too low, mechanical properties such as the membrane's specific needle punch strength will be affected.

[0082] As an example, the dry material ratio of the raw material may be one of 15%, 16%, 17%, 18% or 19%, or a range between any two of them.

[0083] Furthermore, in some possible embodiments, additives such as nucleating agents and antioxidants may be added to the raw materials.

[0084] Furthermore, in some possible embodiments, before the raw materials are fed into the extruder for melting, they may be premixed at a temperature of 90 to 130° C. for more than 40 minutes.

[0085] The melt plasticization process of polyethylene and a pore-forming agent involves the pore-forming agent gradually weakening the secondary bonds between macromolecular chains. Specifically, the secondary bonds between functional groups on the macromolecular chains are replaced by interactions between these functional groups and the small pore-forming agent. After the addition of a small pore-forming agent, the polymer generally swells first and then dissolves. The swelling process primarily manifests as the small pore-forming agent permeating the polymer, causing the polymer to expand in volume. This permeation first occurs in the amorphous regions, where secondary bonds between macromolecular chains are weak, and then penetrates into the crystalline regions, where secondary bonds between macromolecular chains are stronger. The rate of penetration of the small pore-forming agent into the polymer is governed by factors such as the pore-forming agent, the polymer properties, and temperature, and requires a certain amount of time to complete. The swollen polymer weakens the secondary bonds between the functional groups on the macromolecular chains due to solvation. After gaining a certain amount of energy, the solvated macromolecular chains can easily disentangle themselves from their original entangled state and disperse into the pore-forming agent, completing the dissolution and mixing process and producing a homogeneous melt. Once the macromolecular chains enter the pore-forming agent, the melt viscosity increases dramatically. If the polymer is dissolved before sufficient swelling, a high-viscosity layer will easily form at the polymer interface, inhibiting the penetration of the pore-forming agent into the polymer interior. This will affect the uniformity of the resulting melt, and during subsequent filtration and extrusion, the shear effect will be enhanced, increasing the extrusion pressure and affecting the specific needle punch strength of the separator.

[0086] The swelling process of ultra-high molecular weight polyethylene resin is a slow endothermic process, and swelling requires a certain amount of time. If the swelling temperature is too low and the swelling time is too short, the polyethylene cannot be fully swelled. If the temperature during swelling is too high, it may cause secondary agglomeration of small-particle polyethylene powder with high surface energy, which will reduce the plasticization uniformity. Therefore, in the embodiment of the present application, before feeding the raw materials into the extruder, the raw materials are premixed at a temperature of 90 to 130°C for more than 40 minutes, which can further improve the mixing uniformity of the melt and further improve the specific needle puncture strength of the diaphragm.

[0087] As an example, the temperature at which the raw materials are premixed may be one of 90° C., 100° C., 110° C., 120° C., or 130° C., or a range between any two thereof.

[0088] As an example, the temperature for premixing the raw materials may be 90 to 120° C. Alternatively, the temperature for premixing the raw materials may be 90 to 110° C. Alternatively, the temperature for premixing the raw materials may be 90 to 100° C.

[0089] The embodiment of the present application adopts a specific feed rate and screw speed to melt-extrude the raw materials, which can improve the specific needle puncture strength of the diaphragm.

[0090] If the ratio of the feed rate to the screw speed of the extruder exceeds 2.5-3.5 (kg / h) / (r / min) when melt-extruding polyethylene with a viscosity-average molecular weight of not less than 180, the feed rate is too large. When filtering through a 400-800 mesh filter, the shear pressure is too large, which cannot effectively improve the physical and chemical properties of the melt and will increase the extrusion pressure, which will not only easily cause malfunction of the extruder equipment and affect the continuous production efficiency of the diaphragm, but also reduce the specific needle strength of the diaphragm.

[0091] If the ratio of the feed rate to the screw speed of the extruder is less than 2.5-3.5 (kg / h) / (r / min) when melt-extruding polyethylene with a viscosity-average molecular weight of not less than 180, the screw speed is too fast, which will aggravate the degradation of the polyethylene. Even if it is filtered through a filter with a mesh size of 400-800, the structure and physical and chemical properties of the polyethylene cannot be effectively improved, which will affect the quality of the cast sheet and stretching and reduce the specific needle strength of the diaphragm.

[0092] As an example, the ratio of feed rate to screw speed can be 2.5 (kg / h) / (r / min), 2.6 (kg / h) / (r / min), 2.7 (kg / h) / (r / min), 2.8 (kg / h) / (r / min), 2.9 (kg / h) / (r / min), 3.0 (kg / h) / (r / min), 3.1 (kg / h) / (r / min), 3.2 (kg / h) / (r / min), 3.3 (kg / h) / (r / min), 3.4 (kg / h) / (r / min) or 3.5 (kg / h) / (r / min) or the range between any two of them.

[0093] As an example, the ratio of feed rate to screw speed may be 2.7 to 3.4 (kg / h) / (r / min). Alternatively, the ratio of feed rate to screw speed may be 2.8 to 3.3 (kg / h) / (r / min). Alternatively, the ratio of feed rate to screw speed may be 2.9 to 3.2 (kg / h) / (r / min).

[0094] The present application does not limit the specific feeding amount. In some possible embodiments, the feeding amount may be 250-500 kg / h.

[0095] As an example, the feed rate can be one of 250 kg / h, 270 kg / h, 300 kg / h, 330 kg / h, 350 kg / h, 380 kg / h, 400 kg / h, 420 kg / h, 450 kg / h, 460 kg / h, 490 kg / h or 500 kg / h, or a range between any two of them.

[0096] Furthermore, the present application does not limit the specific screw speed. In some possible embodiments, the screw speed can be one of 70r / min, 80r / min, 90r / min, 100r / min, 110r / min, 120r / min, 130r / min, 140r / min, 150r / min, 160r / min, 170r / min, 180r / min, 190r / min or 200r / min, or a range between any two of them.

[0097] Furthermore, in some possible embodiments, the feed rate is 390-450 kg / h and the screw speed is 130-140 r / min, which can further improve the needle puncture strength of the diaphragm.

[0098] Furthermore, in some possible embodiments, when the raw materials are fed into the extruder for melting, the melt temperature is 206-270°C.

[0099] As an example, during melt extrusion, the melt temperature may be one of 206°C, 210°C, 215°C, 220°C, 230°C, 235°C, 240°C, 244°C, or 270°C, or a range between any two thereof.

[0100] Before the melt is extruded through the die, filtering the melt can further improve the physical and chemical properties of the melt and increase the specific needle punch strength of the diaphragm. The present application does not limit the specific mesh number of the filter, and can be adjusted accordingly within the range of 400 to 800 mesh.

[0101] As an example, the filtration mesh size may be one of 400 mesh, 600 mesh, or 800 mesh, or a range between any two thereof.

[0102] Furthermore, in some possible embodiments, when the viscosity-average molecular weight of polyethylene is 2 million to 2.4 million, a filter with a mesh size of 400 to 600 can be used to filter the melt.

[0103] In some possible embodiments, when the viscosity-average molecular weight of polyethylene is 1.8 million to 2 million, a filter with a mesh size of 600 to 800 can be used to filter the melt.

[0104] In some embodiments, the extrusion temperature may be 170-220°C.

[0105] Furthermore, in step S2, during melt extrusion, the extrusion temperature is 185-195° C., and the extrusion pressure is 70-150 bar.

[0106] As an example, the extrusion temperature may be one of 185°C, 186°C, 187°C, 188°C, 189°C, 190°C, 191°C, 192°C, 193°C, 194°C, or 195°C, or a range between any two thereof.

[0107] As an example, the extrusion pressure may be one of 70 bar, 75 bar, 76 bar, 80 bar, 84 bar, 85 bar, 90 bar, 95 bar, 98 bar, 100 bar, 105 bar or 150 bar, or a range between any two of these.

[0108] In the wet-process diaphragm manufacturing process, after the polymer macromolecules and the high-boiling-point small pore-forming agent are mixed to form a homogeneous melt, the melt extruded from the die is cast onto chilled rollers to cool, allowing the polymer macromolecules and the small pore-forming agent molecules to phase separate and solidify into a cast sheet. After phase separation, the small pore-forming agent molecules are dispersed within the polymer solid state.

[0109] The present application does not limit the cooling temperature, rotation speed and thickness of the chilled roller, and they can be selected according to conventional cooling temperature, rotation speed and thickness of the cast sheet in the art.

[0110] As an example, the cooling temperature of the chill roller can be 5-10° C., and the ratio of the linear velocity of the chill roller to the flow velocity of the melt cast sheet can be greater than 0.8. The unit of the linear velocity is m / min, and the unit of the flow velocity is m / min.

[0111] Appropriate cooling temperature and speed can improve the situation where a large amount of pore-forming agent in the melt is precipitated due to excessive temperature difference during cooling, resulting in poor adhesion of the flaky melt to the roller, uneven shrinkage, and a situation where the middle is thick and the sides are thin. It can also avoid the situation where the temperature is too high, less pore-forming agent is precipitated, and the friction between the melt and the roller is large. Under the action of this friction, the melt is stretched, resulting in a situation where the middle is thin and the sides are thick, thereby improving the thickness uniformity of the casting.

[0112] In the wet-process diaphragm production process, after the cast sheet is formed, it is usually necessary to perform post-processing such as stretching and extraction. The stretching process is a key step in the production of wet-process polyethylene diaphragms. Typically, the diaphragm needs to be biaxially stretched. Under the action of biaxial stretching, the diaphragm molecular chains are oriented in the longitudinal and transverse directions, giving the diaphragm high tensile strength in both the longitudinal and transverse directions, while also making the micropores evenly distributed between the molecular chains, so that the diaphragm has a certain pore size distribution and porosity.

[0113] Because stretching causes the molecular chains of the diaphragm to undergo a certain degree of deorientation, the stretching process, after preheating and stretching, also involves setting and cooling. The heat setting process further stretches the diaphragm transversely to release internal stress, improve its thermal stability, and refine the micropore morphology. The principle of heat setting is to relax and reconstruct certain interchain connections, transforming unstable structures into stable ones. During the heat setting process, a higher heat setting temperature promotes molecular thermal motion, relaxes internal stresses, and transforms highly elastic deformation into plastic deformation.

[0114] As an example, the cast sheet can be subjected to biaxial stretching. The biaxial stretching can be synchronous biaxial stretching or asynchronous biaxial stretching. The stretching can be performed once or multiple times.

[0115] As an example, the cast sheet can be sequentially stretched 3 times in the longitudinal direction, 3 times in the transverse direction, and 3 times in the bidirectional simultaneous stretching.

[0116] As an example, the cast sheet can be sequentially stretched 3 times in the longitudinal direction, 3 times in the transverse direction, 3 times in the longitudinal direction, and 3 times in the transverse direction.

[0117] As an example, the cast sheet can be sequentially subjected to longitudinal stretching of 7 to 16 times, first transverse stretching of 7 to 16 times, extraction, and second transverse stretching of 1.2 to 1.8 times.

[0118] As an example, the temperature for longitudinal stretching may be 90 to 110°C.

[0119] Furthermore, the film material may be preheated before longitudinal stretching, for example, at a temperature of 60 to 90°C.

[0120] As an example, the temperature of the first transverse stretching may be 95 to 120°C.

[0121] Furthermore, before the first transverse stretching, the film material may be preheated. As an example, the preheating temperature may be 105-125°C.

[0122] As an example, dichloromethane can be used as the extractant.

[0123] As an example, after the second transverse stretching, the material may be retracted, and the retraction rate may be 12% to 20%.

[0124] As an example, the heat setting temperature may be 120-145°C.

[0125] As an example, the heat setting temperature may be 120 to 135° C. Alternatively, the heat setting temperature may be 135 to 145° C.

[0126] Different stretch ratios result in different degrees of membrane orientation and accumulated internal stress. Different biaxial stretching temperatures result in different degrees of membrane orientation and deorientation. Different heat setting temperatures result in different degrees of stress release. Using these biaxial stretch ratios, stretching temperatures, and heat setting temperatures can improve membrane thickness uniformity, tensile strength, and needle punch strength, adjust pore size distribution, and enhance membrane performance.

[0127] Furthermore, an embodiment of the present application also provides a diaphragm prepared according to the above method, wherein the diaphragm has a specific needle punch strength of not less than 91.8 gf / μm.

[0128] In the present application, the specific needle puncture strength refers to the ratio of the needle puncture strength (unit: gf) to the thickness of the separator (unit: μm).

[0129] The diaphragm and its preparation method of the present application are further described in detail below with reference to the examples.

[0130] Example 1

[0131] Example 1 provides a diaphragm, the preparation method of which includes:

[0132] (1) Raw materials were fed into an extruder at a specific feed rate for melting, filtration, die extrusion, casting, and cooling to obtain a cast sheet. The raw materials included white oil and polyethylene with a viscosity-average molecular weight of 2 million, with a dry material ratio of 15%. The feed rate to screw speed ratio (Q / Ns) was 3.2 (kg / h) / (r / min), the filter mesh size was 400 mesh, and the extrusion pressure was 71.47 bar.

[0133] (2) The cast sheet was sequentially subjected to longitudinal stretching, first transverse stretching, extraction, and second transverse stretching to obtain a polyethylene diaphragm. The longitudinal stretching temperature was 110° C., and the longitudinal stretching ratio was 9 times. The first transverse stretching temperature was 120° C., and the first transverse stretching ratio was 13 times. The extraction temperature was 20° C., and the extractant was dichloromethane. The second transverse stretching ratio was 1.3 times, and the second transverse stretching temperature was 133° C. The heat setting temperature was 135° C., and the time was 27 seconds.

[0134] Example 2

[0135] Example 2 provides a diaphragm, which differs from Example 1 in that:

[0136] In step (1), the filtration mesh size is 800 mesh and the extrusion pressure is 89.34 bar.

[0137] Example 3

[0138] Example 3 provides a diaphragm, which differs from Example 1 in that:

[0139] In step (1), the filtration mesh size is 600 mesh and the extrusion pressure is 80.26 bar.

[0140] Comparative Example 1

[0141] Comparative Example 1 provides a diaphragm, which differs from Example 2 in that:

[0142] In step (1), Q / Ns was 2.0 and the extrusion pressure was 71.4 bar.

[0143] Comparative Example 2

[0144] Comparative Example 2 provides a diaphragm, which differs from Example 2 in that:

[0145] In step (1), the viscosity-average molecular weight of the polyethylene raw material is 1.2 million, the filtration mesh size is 800 mesh, and the extrusion pressure is 50.1 bar.

[0146] Comparative Example 3

[0147] Comparative Example 3 provides a diaphragm, which differs from Example 1 in that:

[0148] In step (1), the viscosity-average molecular weight of the polyethylene raw material is 1.2 million, the filtration mesh size is 400 mesh, and the extrusion pressure is 41.45 bar.

[0149] Comparative Example 4

[0150] Comparative Example 4 provides a diaphragm, which differs from Example 1 in that:

[0151] In step (1), the filtration mesh size is 200 mesh and the extrusion pressure is 41.45 bar.

[0152] Comparative Example 5

[0153] Comparative Example 5 provides a diaphragm, which differs from Example 1 in that:

[0154] In step (1), the filtration mesh size is 1000 mesh, and the extrusion pressure is too high, so normal extrusion cannot be performed.

[0155] Some of the process conditions of Examples 1 to 3 and Comparative Examples 1 to 5 are shown in Table 1.

[0156] Table 1

[0157]

[0158] Test Case

[0159] The diaphragms provided in Examples 1 to 3 and Comparative Examples 1 to 4 were subjected to a needle puncture strength test, and the test results are shown in Table 2. The test method for needle puncture strength is in accordance with GB / T 36363-2018.

[0160] Table 2

[0161]

[0162]

[0163] In Table 2, the specific needle puncture strength refers to the ratio of needle puncture strength to thickness.

[0164] Result analysis:

[0165] As shown in Table 2, for high molecular weight polyethylene (HMWPE) above 2,000,000, when melt-extruded using a 400-800 mesh filter and controlling the Q / Ns ratio to 2.5-3.2, the resulting polyethylene membranes exhibited needle punch strengths of no less than 459 gf and specific needle punch strengths of no less than 91.8 gf / μm. The membranes produced in Comparative Examples 1-4 exhibited specific needle punch strengths of no more than 86 gf / μm, significantly lower than those in Examples 1-3.

[0166] Comparison of Example 2 and Comparative Example 1 shows that although both examples used an 800-mesh filter to filter the melt of a polyethylene raw material with a viscosity-average molecular weight of 2 million, the Q / Ns ratio in Comparative Example 1 was too small, less than 2.5, and the resulting polyethylene membrane had a specific needle punch strength of 71.4 gf / μm, which was lower than the 89.34 gf / μm needle punch strength of the polyethylene membrane produced in Example 2. This indicates that the present examples, by using a 400-800 mesh filter combined with a specific Q / Ns ratio to melt-extrude a polyethylene raw material with a viscosity-average molecular weight of 1.8 million or more, can improve the specific needle punch strength of the polyethylene membrane.

[0167] By comparing Examples 1 to 2 and Comparative Examples 2 to 3, it can be seen that for Comparative Examples 2 and 3, when the viscosity-average molecular weight of the polyethylene raw material is too low, less than 1.8 million, and the melt is filtered using 400-mesh and 800-mesh filters, respectively, the needle puncture strength of the obtained polyethylene diaphragm is basically unchanged, and the specific needle puncture strength of the polyethylene diaphragm is much lower than that of Example 1 and Example 2.

[0168] As can be seen from Examples 1, 2, and 3, for high-molecular-weight polyethylene raw materials, the specific needle punch strength of the polyethylene membrane increases as the filter mesh size decreases. However, when the filter mesh size is too low, such as in Comparative Example 4, where a 200-mesh filter is used to filter a 2-million-molecular-weight polyethylene melt, the resulting polyethylene membrane contains a large amount of impurities and defects, rendering the polyethylene membrane unusable. When the filter mesh size is too large, such as in Comparative Example 5, where a 1000-mesh filter is used to filter a 2-million-molecular-weight polyethylene melt, normal extrusion is impossible, making continuous production impossible.

[0169] In summary, the present embodiment feeds a polyethylene raw material with a viscosity-average molecular weight of not less than 1.8 million into an extruder for melting, controls the ratio of feed rate to screw speed of the extruder to 2.5 to 3.5 (kg / h) / (r / min), filters the material through a 400-800 mesh filter, and then extrudes it through a die. Casting and cooling form a cast sheet, which is then post-processed to produce a diaphragm with high specific needle punch strength. Furthermore, according to the preparation method provided in the present embodiment, an ultra-thin, high-strength diaphragm with an average thickness of 5 μm can be produced.

[0170] The above description is merely a preferred embodiment of the present application and is not intended to limit the present application. Various modifications and variations are possible for those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A method for preparing a diaphragm, characterized in that: include: The raw material containing polyethylene is fed into the extruder at a set feed rate for melting, and then filtered, extruded from a die, cast, and cooled to form a cast sheet; performing post-processing on the casting sheet to form a diaphragm; The viscosity-average molecular weight of the polyethylene is not less than 1.8 million; the mesh size of the filtration is 400-800 mesh; and the ratio of the feed rate to the screw speed of the extruder is 2.5-3.5 (kg / h) / (r / min).

2. The preparation method according to claim 1, characterized in that The viscosity average molecular weight of the polyethylene is 1.8 million to 4 million.

3. The preparation method according to claim 2, characterized in that The viscosity average molecular weight of the polyethylene is 2 million to 2.4 million, and the mesh size of the filtration is 400 to 600 meshes.

4. The preparation method according to claim 2, characterized in that The viscosity average molecular weight of the polyethylene is 1.8 million to 2 million, and the mesh size of the filtration is 600 to 800 meshes.

5. The preparation method according to claim 1, characterized in that The feeding rate is 250-500 kg / h, and the screw speed is 70-200 r / min; Optionally, the ratio of the feed rate to the screw speed is 3 to 3.2 (kg / h) / (r / min); Optionally, the feeding rate is 390-450 kg / h, and the screw speed is 130-140 r / min.

6. The preparation method according to claim 5, characterized in that The D50 of the polyethylene is 90 to 150 μm; Optionally, the raw material further comprises low molecular weight polyethylene, and the viscosity average molecular weight of the low molecular weight polyethylene is 500,000 to 1,000,000; Optionally, the mass ratio of the polyethylene to the low molecular weight polyethylene is 3 to 20:

1.

7. The preparation method according to claim 6, characterized in that The extrusion temperature is 170-220°C, the extrusion pressure is 70-150 bar, and the melt temperature is 206-270°C; Optionally, the extrusion temperature is 185-195°C.

8. The preparation method according to claim 1 or 7, characterized in that The raw materials also include a pore-forming agent; Optionally, the pore former includes white oil; Optionally, before feeding the raw materials into the extruder, the raw materials are premixed at a temperature of 90 to 130° C. for more than 40 minutes.

9. The preparation method according to claim 1, characterized in that The post-treatment includes stretching, extraction and heat setting performed in sequence.

10. A separator prepared according to the method according to any one of claims 1 to 9, wherein the separator has a specific needle punch strength of not less than 91.8 gf / μm.

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