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 mixed raw material into an extruder for melting at the feeding amount of 150-700kg / h, and carrying out extrusion, tape casting and cooling molding to obtain a cast sheet. And carrying out post-treatment on the casting sheet to form the diaphragm. Wherein the mixed raw materials comprise polyethylene powder and a pore-forming agent, the D50 of the polyethylene powder is 80-150 [mu] m, and the viscosity average molecular weight of the polyethylene powder is not lower than 1,800,000. The rotating speed of a screw of the extruder is 70-200r / min, and the extrusion temperature is 170-220 DEG C. The diaphragm prepared according to the method has relatively high specific needling strength.

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. It provides 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 strength to prevent impurities or burrs on the positive and negative electrodes from puncturing and causing a battery short circuit.

[0003] Currently, the manufacturing processes for diaphragms typically include wet and dry processes. Wet-process diaphragms typically offer uniform pore size distribution, good product consistency, a wide range of controllable porosity and permeability, and greater ease of thin-film production. Consequently, they have become the mainstream process in recent years. The wet process typically uses polyethylene powder as the raw material, which is then transported to an extrusion system for melt extrusion, followed by sheet casting and stretching.

[0004] However, the needle punch strength of the diaphragm produced by melt-extruding polyethylene powder using the current preparation 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] The mixed raw materials are fed into an extruder at a feed rate of 150-700 kg / h for melting. The mixture is then extruded, cast, and cooled to form a cast sheet. The cast sheet is then post-processed to form a diaphragm. The mixed raw materials include polyethylene powder and a pore-forming agent. The polyethylene powder has a D50 of 80-150 μm and a viscosity-average molecular weight of no less than 1.8 million. The extruder's screw speed is 70-200 rpm, and the extrusion temperature is 170-220°C.

[0009] In the above 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 base material of the diaphragm, and melt extrusion is performed at a feed rate of 150 to 700 kg / h and a screw speed of 70 to 200 r / min. This can improve the plasticizing effect, reduce the extrusion temperature and the extrusion pressure, and not only ensure the continuous extrusion of the melt, but also reduce the probability of defects such as white spots on the cast film, 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 D50 of the polyethylene powder is 90 to 150 μm, and the viscosity-average molecular weight is 1.8 million to 4 million.

[0011] Optionally, the polyethylene powder has a D50 of 90 to 110 μm and a viscosity-average molecular weight of 2 million to 2.4 million.

[0012] Optionally, the D50 of the polyethylene powder is 110 to 150 μm, and the viscosity-average molecular weight is 1.8 million to 2 million.

[0013] Optionally, the extrusion temperature is 185-195°C.

[0014] In the above implementation process, polyethylene powder with a D50 of 90 to 150 μm and a viscosity-average molecular weight of 1.8 million to 4 million is used as the base material of the diaphragm. During melt extrusion, it is matched with a specific feed amount and screw speed, and has a good plasticizing effect, which can reduce the melt temperature and extrusion temperature and the degradation rate of the polyethylene powder, so that the diaphragm prepared subsequently has a higher molecular weight and a higher specific needle punch strength.

[0015] When the viscosity-average molecular weight of polyethylene is between 2 million and 2.4 million, the particle size of the powder can be appropriately reduced, and polyethylene powder with a D50 of 90 to 110 μm can be selected for melt extrusion. With a specific feed rate and screw speed, it is possible to achieve a good plasticizing effect while reducing the extrusion pressure and extrusion temperature, reduce the degree of polyethylene degradation during the melt extrusion process, and improve the specific needle punch strength of the resulting diaphragm. If high-molecular-weight polyethylene is used as the main material of the diaphragm, and the particle size of the powder selected during melt extrusion is too large, the melt is prone to uneven plasticization under the extrusion conditions of the above-mentioned feed rate and screw speed, and it is necessary to increase the melt temperature and extrusion pressure, which not only easily causes polyethylene degradation, but also easily forms defects such as cast white spots during cast film. These defects easily lead to film rupture during post-processing such as stretching, resulting in a decrease in the specific needle punch strength of the diaphragm.

[0016] When the viscosity-average molecular weight of polyethylene is between 1.8 million and 2 million, the particle size of the powder can be appropriately increased, and polyethylene powder with a D50 of 110 to 150 μm can be selected for melt extrusion. With a specific feed rate and screw speed, the mixed raw materials can be melted and plasticized evenly under conditions of lower extrusion pressure and extrusion temperature. After subsequent casting, sheet casting and post-processing of the melt, a diaphragm with a higher specific needle punch strength can be obtained, and the raw material cost can also be reduced.

[0017] In combination with the first aspect, in an optional embodiment of the present application, the extrusion pressure is 70-150 bar, and / or the melt temperature is 206-270°C.

[0018] In the above-mentioned implementation process, when the polyethylene powder with a D50 of 90 to 150 μm and a viscosity-average molecular weight of not less than 1.8 million is melt-extruded, it is fed into the feeder at a feed rate of 150 to 700 kg / h, and the screw speed of the extruder is controlled to be 70 to 200 r / min. Under the conditions of a lower extrusion pressure of 70 to 150 bar and a lower melt temperature of 206 to 270°C, the melt can be plasticized uniformly and continuously extruded, which 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 of the prepared diaphragm.

[0019] In combination with the first aspect, in an optional embodiment of the present application, the ratio of the feed rate to the screw speed is 2.5 to 3.5 (kg / h) / (r / min).

[0020] In combination with the first aspect, in an optional embodiment of the present application, the feed rate is 350-490 kg / h, and the screw speed is 120-170 r / min.

[0021] In the above-mentioned implementation process, when the polyethylene powder with a D50 of 80 to 150 μm and a viscosity-average molecular weight of not less than 1.8 million is melt-extruded, the ratio of the feed rate to the screw speed is controlled to be 2.5 to 3.5 (kg / h) / (r / min). While ensuring continuous extrusion, the degree of plasticization uniformity of the melt can be improved while reducing the degree of degradation of the polyethylene powder during the melt extrusion process, thereby improving the specific needle puncture strength of the prepared diaphragm.

[0022] In combination with the first aspect, in an optional embodiment of the present application, the mixed 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.

[0023] Optionally, the mass ratio of the polyethylene powder to the low molecular weight polyethylene is 3 to 20:1.

[0024] In the above-mentioned implementation process, a certain proportion of low molecular weight polyethylene is added to the mixed raw materials. By matching high and low molecular weight, the screw speed during melt extrusion can be reduced, the melt pressure and melt temperature during melt extrusion can be reduced, and the degree of degradation of polyethylene powder during melt extrusion can be reduced. The quality and tensile quality of the cast sheet can be improved, and the specific needle punch strength of the diaphragm can be further improved.

[0025] In combination with the first aspect, in an optional embodiment of the present application, before the mixed raw materials are fed into the extruder, the mixed raw materials are premixed at a temperature of 90 to 130° C. for more than 40 minutes.

[0026] In the above implementation process, the mixed raw material containing polyethylene powder and pore-forming agent is premixed at a temperature of 90-130°C for more than 40 minutes, and then fed into the extruder for melt extrusion. This can form a uniform melt and continuously extrude at a lower screw speed, extrusion pressure and melt temperature, reduce the degree of polyethylene degradation during melt extrusion, reduce the probability of defects such as cast transparency during cast sheet, improve subsequent tensile quality, and increase the specific needle puncture 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 the extrusion pressure and extrusion temperature need to be increased, which will aggravate the degradation of polyethylene and reduce the specific needle puncture strength of the diaphragm. If the premixing temperature is too low or the premixing time is too short, the polyethylene powder is melt-extruded without sufficient swelling during melt extrusion, and a high-viscosity layer is easily formed on the surface of the polyethylene powder, which will inhibit the sufficient 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 puncture strength of the diaphragm.

[0027] In combination with the first aspect, in an optional embodiment of the present application, the melt is filtered before extrusion, and the filtration mesh size is 400 to 800 meshes.

[0028] In the above implementation process, before the melt in the extruder is extruded through the die head, the melt is filtered using a filter with a mesh size of 400 to 800. This can not only filter out the unplasticized powder or other impurities in the melt, and reduce the probability of these impurities mixing into the cast melt and affecting the quality of the casting, but also produce an appropriate shearing effect on the melt, further improve the physical and chemical properties of the melt, and can increase the specific needle punch strength of the diaphragm.

[0029] In conjunction with the first aspect, in an optional embodiment of the present application, the post-processing includes sequentially performing stretching, extraction, and heat setting. Alternatively, the post-processing includes sequentially performing longitudinal stretching, transverse stretching, extraction, and heat setting.

[0030] 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 89 gf / μm.

[0031] 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 a 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 base material of the diaphragm. The melt extrusion is carried out at a feed rate of 150 to 700 kg / h and a screw speed of 70 to 200 r / min. This can reduce the extrusion temperature and the extrusion pressure, reduce the degree of degradation of polyethylene during the melt extrusion process, improve the mixing uniformity of polyethylene and the pore-forming agent, improve the casting quality and tensile quality, and obtain a diaphragm with a needle punch strength of not less than 89 gf / μm. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] 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.

[0033] Figure 1 Flow chart of the preparation process of the diaphragm provided as an example in this application. DETAILED DESCRIPTION

[0034] 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.

[0035] 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.

[0036] 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.

[0037] At present, the diaphragm production materials 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 are divided into:

[0038] 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.

[0039] 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 solution. 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 a microporous structure between the polymer molecules. This allows the preparation of microporous membranes with excellent mechanical properties.

[0040] 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.

[0041] Wet-process diaphragms typically use ultra-high molecular weight polyethylene (UHMWPE) as the primary diaphragm material. However, due to inherent defects in UHMWPE and the unique characteristics of the wet-process diaphragm process, the mechanical properties of the product still need to be improved.

[0042] In the wet diaphragm process, ultra-high molecular weight polyethylene powder and a pore-forming agent need to be fed into an extruder for melt plasticization, so that the ultra-high molecular weight polyethylene and the pore-forming agent are mixed to form a homogeneous melt, which is then extruded through the die head of the extruder and formed into a cast sheet after casting and solidification. The inventor believes that since the molecular weight of ultra-high molecular weight polyethylene can usually reach 1 to 4 million or even higher, the higher the molecular weight of the polyethylene, the more crystallization points the chain has, the easier it is for the molecular chain to entangle, the slower the molecular thermal motion, the more difficult it is to plasticize evenly during screw extrusion, and the worse the melt fluidity. Therefore, in the melt extrusion process, in order to make the ultra-high molecular weight polyethylene plasticize evenly and form a uniformly mixed melt with the pore-forming agent for continuous extrusion, a higher plasticizing temperature, screw speed, extrusion temperature and extrusion pressure are usually required. However, due to the effects of high temperature and strong shear, polyethylene molecular chains are prone to thermal oxidation and mechanical degradation, causing the chemical bonds of the molecular chains to break into active free radicals and hydroperoxides. Hydroperoxides further decompose to produce hydrocarbon oxygen free radicals and hydroxyl free radicals, leading to a series of free radical chain reactions that cause fundamental changes in the structure and properties of polyethylene, which is not conducive to the subsequent cast film and stretching steps, and ultimately leads to low needle punch strength of the diaphragm.

[0043] 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.

[0044] See also Figure 1 The preparation method of the diaphragm provided in the embodiment of the present application includes:

[0045] S1. Feeding: Feed the mixed raw materials into the extruder at the set feed rate. The mixed raw materials include polyethylene powder and a pore-forming agent. The polyethylene powder has a D50 of 80-150 μm and a viscosity-average molecular weight of no less than 1.8 million. The feed rate is 150-700 kg / h.

[0046] S2. Melt extrusion: Melt and extrude the mixed raw materials. The screw speed of the extruder is 70-200 r / min and the extrusion temperature is 170-220°C.

[0047] S3, tape casting, casting sheet: the melt extruded from the extruder is tape cast, cooled and solidified to obtain a casting sheet.

[0048] S4. Post-processing: Post-process the casting to form a diaphragm.

[0049] According to the above method, a diaphragm is prepared, and 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 base material of the diaphragm. Melt extrusion is performed at a feed rate of 150 to 700 kg / h and a screw speed of 70 to 200 r / min. This can improve the plasticizing effect of the polyethylene powder, obtain a homogeneous melt of polyethylene and a pore-forming agent, and reduce the extrusion temperature and extrusion pressure during melt extrusion. 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.

[0050] 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, it will not only reduce the plasticizing effect, but also increase the extrusion pressure, affect the continuity of extrusion, reduce the quality of subsequent cast film and stretching steps, and reduce the specific needle punch strength of the diaphragm.

[0051] 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. This will also reduce the quality of subsequent steps such as cast film and stretching, and further reduce 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 it 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.

[0052] Under the above-mentioned raw material composition and screw speed conditions, if the feed rate is too large, the extrusion pressure will be too high, 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 strength of the diaphragm.

[0053] Under the above-mentioned raw material composition and feed rate conditions, if the screw speed is too fast, it will accelerate the degradation of polyethylene, leading to changes in the polyethylene structure and physical and chemical properties, affecting the cast film and stretching quality, and reducing the specific needle punch strength of the diaphragm. If the screw speed is too slow, it will be difficult to plasticize uniformly and extrude continuously, which will also affect the specific needle punch strength of the diaphragm.

[0054] The embodiment of the present application utilizes polyethylene powder with a specific particle size range and molecular weight, combined with a specific feed rate and a specific screw speed. It can reduce the extrusion pressure and extrusion temperature while improving the plasticizing effect to obtain a uniform melt, improve the subsequent cast film and stretching quality, and increase the specific needle punch strength of the diaphragm.

[0055] In step S1, the present application does not limit the specific particle size of the polyethylene powder. In some possible embodiments, the particle size of the polyethylene powder can 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.

[0056] The present application does not limit the molecular weight of the polyethylene powder. In some possible embodiments, the viscosity-average molecular weight of the polyethylene powder may be 1.8 million to 4 million.

[0057] As an example, the viscosity average molecular weight of the polyethylene powder can be 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.

[0058] Furthermore, in some possible embodiments, the polyethylene powder has a D50 of 90 to 110 μm and a viscosity-average molecular weight of 2 million to 2.4 million.

[0059] When the polyethylene has a large molecular weight of 2 million to 2.4 million, the particle size of the powder can be appropriately reduced, and polyethylene powder with a D50 of 90 to 110 μm can be selected for melt extrusion. Combined with the specific feed amount and screw speed of this application, it is possible to reduce the extrusion pressure and extrusion temperature while achieving a good plasticizing effect, reduce the degree of degradation of polyethylene during the melt extrusion process, and improve the specific needle puncture strength of the prepared diaphragm.

[0060] In some possible embodiments, the polyethylene powder has a D50 of 110 to 150 μm and a viscosity-average molecular weight of 1.8 million to 2 million.

[0061] When the viscosity-average molecular weight of polyethylene is between 1.8 million and 2 million, the particle size of the powder can be appropriately increased, and polyethylene powder with a D50 of 110 to 150 μm can be selected for melt extrusion. Combined with the specific feed rate and screw speed of this application, the mixed raw materials can be melted and plasticized evenly under lower extrusion pressure and extrusion temperature conditions. After subsequent casting, sheet casting and post-processing of the melt, a diaphragm with a higher specific needle punch strength can be obtained, and the raw material cost can also be reduced.

[0062] Furthermore, in some possible embodiments, in addition to polyethylene powder having a viscosity average molecular weight of not less than 1.8 million, the mixed raw material may also contain low molecular weight polyethylene having a viscosity average molecular weight of 500,000 to 1,000,000.

[0063] By adding a certain proportion of low molecular weight polyethylene to the mixed raw materials, the melt pressure and melt temperature during melt extrusion can be further reduced through the combination of high and low molecular weight, and the degree of degradation of polyethylene powder during melt extrusion can be reduced, which can improve the quality of cast sheet and tensile quality, and can further improve the specific needle punch strength of the diaphragm.

[0064] 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.

[0065] Furthermore, in the mixed raw materials, the mass ratio of polyethylene powder with a viscosity average molecular weight of 1.8 million or more to low molecular weight polyethylene is 3 to 20:1, which can further improve the specific needle punch strength of the separator. If the addition amount of low molecular weight polyethylene in the mixed raw materials is too high, the specific needle punch strength of the separator will be reduced.

[0066] As an example, in the mixed raw materials, the mass ratio of polyethylene powder with a viscosity average molecular weight of more than 1.8 million 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.

[0067] As an example, the mixed raw material includes 16% of polyethylene powder with a viscosity average molecular weight of 2.8 million and 2% of low molecular weight polyethylene with a viscosity average molecular weight of 900,000.

[0068] As an example, the mixed raw material includes 16% of polyethylene powder with a viscosity average molecular weight of 2.8 million and 3% of low molecular weight polyethylene with a viscosity average molecular weight of 900,000.

[0069] As an example, the mixed raw material includes 17% of polyethylene powder with a viscosity average molecular weight of 2.8 million and 1% of low molecular weight polyethylene with a viscosity average molecular weight of 900,000.

[0070] As an example, the mixed raw material includes 14% of polyethylene powder with a viscosity average molecular weight of 2.8 million and 4% of low molecular weight polyethylene with a viscosity average molecular weight of 900,000.

[0071] As an example, the mixed raw material includes 16% of polyethylene powder with a viscosity average molecular weight of 1.8 million and 2% of low molecular weight polyethylene with a viscosity average molecular weight of 500,000.

[0072] As an example, the mixed raw material includes 16% of polyethylene powder with a viscosity average molecular weight of 3 million and 2% of low molecular weight polyethylene with a viscosity average molecular weight of 700,000.

[0073] The present application does not limit the type of the pore former. In one possible embodiment, the pore former may include white oil.

[0074] Furthermore, in some possible embodiments, the dry material ratio of the mixed raw materials can be 15% to 19%, which can further improve the uniformity of the melt, improve the quality of the cast sheet, reduce the defects of stretching molding, and further improve the needle punch strength of the diaphragm.

[0075] The dry material ratio of the mixed raw materials refers to the mass proportion of polyethylene solid material in the mixed raw materials.

[0076] As an example, the dry material ratio of the mixed raw material may be one of 15%, 16%, 17%, 18% or 19%, or a range between any two of them.

[0077] Furthermore, in some possible embodiments, additives such as nucleating agents and antioxidants may be added to the mixed raw materials.

[0078] Furthermore, in some possible embodiments, before the mixed raw materials are fed into the extruder, the mixed raw materials are premixed at a temperature of 90 to 130° C. for more than 40 minutes.

[0079] The melt-plasticization process of polyethylene powder and pore-forming agent involves the pore-forming agent's action on the polyethylene, which gradually weakens the secondary bonds between macromolecular chains. Specifically, the interaction between the secondary bonds between functional groups on the macromolecular chains is replaced by the interaction between these functional groups and the small pore-forming agent (solvent). After the addition of the 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 the macromolecular chains are weak, and then penetrates into the crystalline regions, where secondary bonds between the 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's 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. Once the solvated macromolecular chains have gained a certain amount of energy, they 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 and affecting the homogeneity of the resulting melt.

[0080] 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 powder 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 mixed raw materials into the extruder, the mixed 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.

[0081] As an example, the temperature at which the mixed raw materials are premixed may be in the range of 90° C., 100° C., 110° C., 120° C., or 130° C., or any two thereof.

[0082] As an example, the temperature for premixing the mixed raw materials may be 90 to 120° C. Alternatively, the temperature for premixing the mixed raw materials may be 90 to 110° C. Alternatively, the temperature for premixing the mixed raw materials may be 90 to 100° C.

[0083] In step S1, the present application does not limit the specific feeding amount. In some possible embodiments, the feeding amount can be 150kg / h, 200kg / h, 250kg / h, 270kg / h, 300kg / h, 330kg / h, 350kg / h, 380kg / h, 400kg / h, 420kg / h, 450kg / h, 460kg / h, 490kg / h, 500kg / h, 550kg / h, 600kg / h, 650kg / h or 700kg / h, or a range between any two of them.

[0084] Furthermore, in step S2, 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.

[0085] Furthermore, in some possible embodiments, the ratio of the feed rate to the screw speed is 2.5 to 3.5 (kg / h) / (r / min), which can further increase the needle puncture strength of the diaphragm.

[0086] 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.

[0087] As an example, the ratio of feed rate to screw speed may be 3.2 (kg / h) / (r / min).

[0088] Furthermore, in some possible embodiments, the feed rate is 350-490 kg / h and the screw speed is 120-170 r / min, which can further improve the needle puncture strength of the diaphragm.

[0089] In some possible embodiments, during melt extrusion, the melt temperature may be 206-270°C.

[0090] 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.

[0091] Furthermore, in step S2, the extrusion temperature may be 170-220°C.

[0092] As an example, the extrusion temperature can be one of 170°C, 175°C, 180°C, 185°C, 186°C, 187°C, 188°C, 189°C, 190°C, 191°C, 192°C, 193°C, 194°C, 195°C, 200°C, 205°C, 210°C, 215°C or 220°C, or a range between any two of them.

[0093] As an example, the viscosity average molecular weight of the polyethylene powder is 1.8 to 2 million, and the extrusion temperature can be 185 to 195°C.

[0094] Typically, when melt-extruding polyethylene powder with a molecular weight of no less than 1.8 million, the extrusion temperature needs to be above 200°C. However, the preparation method provided in the embodiments of the present application can reduce the extrusion temperature to 185-195°C while ensuring uniform plasticization and continuous extrusion production. This can reduce the degree of shear degradation of the polyethylene during extrusion and improve the specific needle punch strength of the separator.

[0095] Furthermore, during melt extrusion, the extrusion pressure is 70 to 150 bar. According to the preparation method provided in the embodiments of the present application, the extrusion pressure can be reduced to 70 to 150 bar while ensuring uniform plasticization and continuous extrusion production. This can reduce the degree of shear degradation of polyethylene during extrusion and improve the specific needle puncture strength of the diaphragm.

[0096] 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.

[0097] Furthermore, in step S2, the melt may be filtered before extrusion, and the filtration mesh may be 400-800 mesh.

[0098] Before the melt in the extruder is extruded through the die head, the melt is filtered using a filter with a mesh size of 400 to 800. This can not only filter out the unplasticized powder or other impurities in the melt, reducing the chance of these impurities mixing into the cast melt and affecting the quality of the casting, but also produce an appropriate shearing effect on the melt, improve the physical and chemical properties of the melt, and increase the specific needle punch strength of the diaphragm.

[0099] If the filtration mesh is too large, the shear force on the melt will be too great, which will further aggravate the degradation of polyethylene and reduce the specific needle strength of the diaphragm. If the filtration mesh is too small, the melt cannot be properly sheared before extrusion to improve the physical and chemical properties of the melt, and the impurity content in the extruded melt is high, which will affect the casting and stretching quality and reduce the specific needle strength of the diaphragm.

[0100] As an example, the filtration mesh may be one of 400 mesh, 600 mesh and 800 mesh, or a range between any two thereof.

[0101] In step S3, the melt extruded from the die is cast onto a chilled roller for cooling and solidification to obtain a cast sheet.

[0102] The present application does not limit the cooling temperature, rotation speed and thickness of the chilled roller, which can be selected according to conventional cooling temperature and rotation speed in the art.

[0103] As an example, the cooling temperature of the chilled roller may be 5-10°C.

[0104] In some embodiments, post-processing includes stretching, extraction, and heat setting.

[0105] The present application does not limit the specific stretching, extraction and heat setting methods, which can be selected according to conventional methods in the art.

[0106] 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.

[0107] 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.

[0108] 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.

[0109] 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.

[0110] As an example, the temperature for longitudinal stretching may be 90 to 110°C.

[0111] Furthermore, the film material may be preheated before longitudinal stretching, for example, at a temperature of 60 to 90°C.

[0112] As an example, the temperature of the first transverse stretching may be 95 to 120°C.

[0113] Furthermore, before the first transverse stretching, the film material may be preheated. As an example, the preheating temperature may be 105-125°C.

[0114] As an example, after the second transverse stretching, the material may be retracted, and the retraction rate may be 12% to 20%.

[0115] As an example, the temperature of the second transverse stretching may be 120-135°C.

[0116] 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 89 gf / μm.

[0117] 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).

[0118] The diaphragm and its preparation method of the present application are further described in detail below with reference to the examples.

[0119] Example 1

[0120] Example 1 provides a diaphragm, the preparation method of which includes:

[0121] (1) A mixed raw material was fed into an extruder at a feed rate of 420 kg / h for melting, extrusion, casting, and cooling to obtain a cast sheet. The mixed raw material included polyethylene powder with a D50 of 90 μm and a viscosity-average molecular weight of 2 million and white oil, with the dry material ratio of polyethylene powder to white oil being 15%. The melt temperature was 210°C, the screw speed was 140 r / min, the extrusion pressure was 84.69 bar, and the extrusion temperature was 195°C.

[0122] (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 9.5 times. The first transverse stretching temperature was 120° C., and the first transverse stretching ratio was 13.5 times. The extraction temperature was 20° C., and the extractant was dichloromethane. The second transverse stretching ratio was 1.4 times, and the second transverse stretching temperature was 133° C. The heat setting temperature was 135° C., and the time was 27 seconds.

[0123] Example 2

[0124] Example 2 provides a diaphragm, which differs from Example 1 in that:

[0125] In step (1), the D50 of the polyethylene powder is 150 μm, the melt temperature is 244° C., and the extrusion pressure is 101.77 bar.

[0126] Example 3

[0127] Example 3 provides a diaphragm, which differs from Example 1 in that:

[0128] In step (1), the polyethylene powder has a D50 of 90 μm, a viscosity-average molecular weight of 1.8 million, a melt temperature of 206° C., and an extrusion pressure of 76.54 bar.

[0129] Comparative Example 1

[0130] Comparative Example 1 provides a diaphragm, which differs from Example 1 in that:

[0131] In step (1), the extrusion speed is 220 r / min. That is, the extrusion speed is greater than 200 r / min. The extrusion pressure is 98.78 bar.

[0132] Comparative Example 2

[0133] Comparative Example 2 provides a diaphragm, which differs from Example 1 in that:

[0134] In step (1), the feed rate is 750 kg / h. That is, the feed rate is >700 kg / h. The extrusion pressure is 135 bar.

[0135] Comparative Example 3

[0136] Comparative Example 3 provides a diaphragm, which differs from Example 1 in that:

[0137] In step (1), the polyethylene powder has a D50 of 170 μm, a viscosity-average molecular weight of 1.8 million, a melt temperature of 255° C., and an extrusion pressure of 115.33 bar.

[0138] Some process parameters of Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1.

[0139] Table 1

[0140]

[0141] Test Case

[0142] The diaphragms provided in Examples 1 to 3 and Comparative Examples 1 to 3 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.

[0143] Table 2

[0144] Group Diaphragm thickness μm Acupuncture intensity gf Specific needle puncture strength gf / μm Example 1 5 496 99.2 Example 2 5 445 89 Example 3 5 450 90 Comparative Example 1 5 98.78 19.756 Comparative Example 2 5 444 88.8 Comparative Example 3 5 439 87.8

[0145] Result analysis:

[0146] As can be seen from Table 2, the diaphragms provided in Examples 1 to 3 of the present application have a specific needle puncture strength of not less than 89 gf / μm, which is higher than the specific needle puncture strength of the diaphragms prepared in Comparative Examples 1 to 3.

[0147] In Table 2, by comparing Examples 1 and 2 with Comparative Example 3, it can be seen that, under the same viscosity-average molecular weight, when Examples 1 and 2 use powders with a D50 particle size of 90 to 150 μm for extrusion and melting, the melt temperature is 210 to 244°C, which is lower than the melt temperature of 255°C in Comparative Example 3. Furthermore, during extrusion, the extrusion temperature of Examples 1 and 2 is 195°C and the extrusion pressure is 84.69 bar and 101.77 bar, respectively, which are lower than the extrusion temperature of 200°C and the extrusion pressure of 115.33 bar in Comparative Example 3. Furthermore, the specific needle puncture strength of the diaphragms prepared in Examples 1 and 2 of the present application is not less than 89 gf / μm, which is higher than the specific needle puncture strength of 87.8 gf / μm in Comparative Example 3. This indicates that the preparation method provided by the examples of the present application can reduce the extrusion temperature and extrusion pressure during melt extrusion, thereby improving the needle puncture strength of the diaphragm.

[0148] Table 1 shows a comparison between Example 1 and Comparative Example 1. In Example 1, polyethylene powder with a viscosity-average molecular weight of 2,000,000 and a D50 of 90 μm was used as the separator substrate. Melt extrusion was performed at a feed rate of 420 kg / h, a screw speed of 140 r / min, and an extrusion pressure of 84.69 bar. The resulting separator exhibited a high specific needle punch strength of 99.2 gf / μm. In contrast, in Comparative Example 1, melt extrusion was performed at a screw speed of 240 r / min and an extrusion pressure of 220 bar. The resulting separator exhibited a specific needle punch strength of only 19.756 gf / μm, significantly lower than that of Example 1.

[0149] Comparison of Example 1 and Comparative Example 2 shows that Example 1 uses polyethylene powder with a viscosity-average molecular weight of 2,000,000 and a D50 of 90 μm as the separator substrate. Melt extrusion is performed at a feed rate of 420 kg / h, a screw speed of 140 r / min, and an extrusion pressure of 84.69 bar. The resulting separator exhibits a specific needle punch strength of 99.2 gf / μm. In Comparative Example 2, melt extrusion is performed at a feed rate of 750 kg / h and an extrusion pressure of 135 bar. The resulting separator exhibits a specific needle punch strength of 88.8 gf / μm, which is lower than that of Example 1.

[0150] The present embodiment utilizes polyethylene powder with a specific particle size range and molecular weight, along with a specific feed rate and screw speed. This allows for improved plasticization and a uniform melt to be achieved while reducing extrusion pressure and temperature. This improves subsequent cast film and stretching quality, and increases the specific needle punch strength of the diaphragm. Furthermore, the preparation method provided in the present embodiment enables the production of an ultra-thin, high-strength polyethylene diaphragm with a thickness of 5 μm.

[0151] 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 mixed raw materials are fed into an extruder at a feed rate of 150 to 700 kg / h for melting, and a casting sheet is obtained by extrusion, casting, and cooling; and the casting sheet is post-processed to form a diaphragm; The mixed raw material includes polyethylene powder and a pore-forming agent, the D50 of the polyethylene powder is 80-150 μm, and the viscosity-average molecular weight is not less than 1.8 million; the screw speed of the extruder is 70-200 r / min, and the extrusion temperature is 170-220°C.

2. The preparation method according to claim 1, characterized in that The polyethylene powder has a D50 of 90 to 150 μm and a viscosity-average molecular weight of 1.8 million to 4 million; Optionally, the polyethylene powder has a D50 of 90 to 110 μm and a viscosity-average molecular weight of 2 million to 2.4 million; Optionally, the polyethylene powder has a D50 of 110 to 150 μm and a viscosity-average molecular weight of 1.8 million to 2 million; Optionally, the extrusion temperature is 185-195°C.

3. The preparation method according to claim 2, characterized in that Extrusion pressure is 70-150 bar; And / or, the melt temperature is 206-270°C.

4. The preparation method according to claim 1, characterized in that The ratio of the feed rate to the screw speed is 2.5 to 3.5 (kg / h) / (r / min).

5. The preparation method according to claim 4, characterized in that The feeding amount is 350-490 kg / h, and the screw speed is 120-170 r / min.

6. The preparation method according to claim 1, characterized in that The mixed 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 powder to the low molecular weight polyethylene is 3 to 20:

1.

7. The preparation method according to claim 1, characterized in that Before feeding the mixed raw materials into the extruder, the mixed raw materials are premixed at a temperature of 90 to 130° C. for more than 40 minutes.

8. The preparation method according to claim 1, characterized in that Before the extrusion, the melt is filtered with a mesh size of 400 to 800 meshes.

9. The preparation method according to any one of claims 1 to 8, characterized in that The post-treatment includes stretching, extraction and heat setting performed in sequence; Alternatively, the post-treatment includes longitudinal stretching, transverse stretching, extraction and heat setting performed sequentially.

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 89 gf / μm.

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