Polyethylene diaphragm and preparation method thereof

Abstract

A polyethylene diaphragm and a preparation method thereof belong to the technical field of diaphragms. A test sample with the size of 1m * 1m is taken from the middle position along the transverse direction of the polyethylene diaphragm; respectively testing a group of first SEM pictures with magnification times of 1 thousand times and a group of second SEM pictures with magnification times of 5 thousand times at four corner positions of the test sample by using a scanning electron microscope, counting the number of crude fibers with the diameter not less than 500nm in the fibers extending along the longitudinal direction in the first SEM pictures, and counting the average diameter of the fibers extending along the longitudinal direction in the second SEM pictures. In the four first SEM pictures, the number of crude fibers in at least three first SEM pictures is not less than 15. The average diameter is not less than 200nm. The polyethylene diaphragm is provided with the coarse fibers which are proper in number and diameter and extend in the longitudinal direction, so that the polyethylene diaphragm has certain friction performance in the transverse direction, and the polyethylene diaphragm is not prone to slipping, dislocation and wrinkling in the transverse direction during rolling.

Description

Technical Field

[0001] The present application relates to the field of diaphragm technology, and in particular to a polyethylene diaphragm and a preparation method thereof. Background Art

[0002] The separator is a crucial safety and functional component in lithium batteries, which has the dual functions of electronic insulation and ion conductivity.

[0003] At present, according to the preparation process of diaphragms, they can be divided into wet-process diaphragms and dry-process diaphragms. Among them, wet-process diaphragms usually have the characteristics of uniform micropore size distribution, good product consistency, a large controllable range of porosity and air permeability, and easier thin-film standardization production. Therefore, it has developed into a mainstream process in recent years. The preparation process of wet-process diaphragms mainly includes five steps: phase separation, stretching, extraction, heat setting and winding. However, the diaphragms obtained by the current process are prone to misalignment during winding, resulting in wrinkles in the diaphragm, affecting the performance of the diaphragm. Summary of the Invention

[0004] Based on the above-mentioned deficiencies, the present application provides a polyethylene diaphragm and a preparation method thereof to improve the problem that the diaphragm is prone to wrinkles due to slippage and dislocation during winding.

[0005] This application is implemented as follows:

[0006] In the first aspect, the example of the present application provides a polyethylene diaphragm. The polyethylene diaphragm has a transverse direction and a longitudinal direction. In the transverse direction, a test sample with a size of 1 meter by 1 meter is taken in the middle position of the polyethylene diaphragm. A scanning electron microscope is used to test the four corners of the test sample, and a set of first SEM pictures with a magnification of 1,000 times and a second SEM picture with a magnification of 5,000 times are respectively tested. The number of coarse fibers with a diameter of not less than 500 nm in the fibers extending longitudinally in the first SEM picture is counted, and the average diameter of the fibers extending longitudinally in the second SEM picture is counted. Among the 4 first SEM pictures, the number of coarse fibers in at least 3 first SEM pictures is not less than 15. The average diameter is not less than 200 nm.

[0007] In the above implementation process, the polyethylene separator has a suitable number and diameter of coarse fibers extending longitudinally, which makes the polyethylene separator have a certain friction in the transverse direction. When the polyethylene separator is wound, it is not easy to slip in the transverse direction and wrinkle. When the polyethylene separator is used after unwinding, the overall performance of the polyethylene separator can be improved. If the number of coarse fibers in the polyethylene separator is too large, the average diameter of the fibers is too large, and the roughness of the polyethylene separator is too high, the mechanical properties such as strength of the polyethylene separator will be affected. When used in batteries, it will affect the electrochemical properties such as current efficiency and cycle stability of the battery.

[0008] In combination with the first aspect, in an optional embodiment of the present application, in the first SEM image, the number of coarse fibers is 10 to 50.

[0009] In the above implementation process, the polyethylene diaphragm has coarse fibers with a suitable distribution density, which can improve the friction of the diaphragm in the lateral direction without basically affecting the electrochemical properties of the polyethylene diaphragm, reduce the probability of lateral slippage and dislocation of the polyethylene diaphragm during the winding process, and thus reduce the probability of wrinkles in the polyethylene diaphragm, thereby further improving the comprehensive performance of the polyethylene diaphragm.

[0010] In combination with the first aspect, in an optional embodiment of the present application, when counting the average diameter of the fibers extending longitudinally in the second SEM image, the number of statistical samples for each second SEM image is not less than 100.

[0011] In the above implementation process, when counting the diameters of fibers extending longitudinally in the SEM image, the number of statistical samples is not less than 100, which can more accurately reflect the true average diameter of the fibers in the polyethylene diaphragm and reduce measurement errors.

[0012] In combination with the first aspect, in an optional embodiment of the present application, the average diameter is 200 to 400 nm.

[0013] In the above implementation process, the polyethylene diaphragm provided in the embodiment of the present application has coarse fibers of appropriate quantity and size, and also has an appropriate average diameter. While having certain friction properties, it also has diaphragm properties such as good mechanical properties and pore structure. It can improve the problem of wrinkles generated when the polyethylene diaphragm is rolled up, and improve the comprehensive performance of the diaphragm.

[0014] In combination with the first aspect, in an optional embodiment of the present application, the tensile strength of the polyethylene diaphragm in the longitudinal direction is greater than the tensile strength in the direction.

[0015] In combination with the first aspect, in an optional embodiment of the present application, the difference between the tensile strength of the polyethylene diaphragm in the longitudinal direction and the tensile strength in the transverse direction is not less than 1000 gf / cm 2 .

[0016] In the above implementation process, the tensile strength of the polyethylene diaphragm provided in the embodiment of the present application in the longitudinal direction is greater than the tensile strength of the polyethylene diaphragm in the transverse direction, especially the difference between the tensile strength of the polyethylene diaphragm in the longitudinal direction and the tensile strength in the transverse direction is not less than 1000 gf / cm 2 It can reduce the probability of polyethylene diaphragm breaking or snapping due to longitudinal tension during winding, and further improve the winding quality of polyethylene diaphragm.

[0017] In a second aspect, an embodiment of the present application provides a method for preparing a polyethylene diaphragm, comprising:

[0018] A casting sheet containing polyethylene and a pore-forming agent is obtained. The casting sheet is stretched and post-treated to obtain a polyethylene separator. In the stretching step, the total stretching ratio of the casting sheet in the longitudinal direction is not less than 9 times.

[0019] In the above-mentioned implementation process, the casting sheet containing polyethylene and pore-forming agent is stretched, and the total stretching ratio in the longitudinal direction is not less than 9 times, which can enable the polyethylene diaphragm to produce coarse fibers of appropriate diameter and quantity extending in the longitudinal direction, and can improve the roughness of the polyethylene diaphragm in the transverse direction. When the polyethylene diaphragm is wound, the probability of the polyethylene diaphragm being dislocated and slipping in the transverse direction can be reduced, thereby reducing the probability of the polyethylene diaphragm having defects such as wrinkles, thereby improving the quality of the polyethylene diaphragm.

[0020] In combination with the second aspect, in an optional embodiment of the present application, the total stretching ratio of the polyethylene diaphragm in the longitudinal direction is 9 to 11.5 times.

[0021] Optionally, the total stretching ratio of the polyethylene separator in the transverse direction is lower than the total stretching ratio in the longitudinal direction.

[0022] In the above implementation process, the polyethylene membrane is stretched longitudinally by a total stretching ratio of 9 to 11.5 times, which can not only make the polyethylene membrane have good tensile strength, pore distribution and other properties, but also have suitable friction properties, which can further improve the quality of the polyethylene membrane.

[0023] In conjunction with the second aspect, in an optional embodiment of the present application, stretching includes longitudinal stretching and transverse stretching performed in sequence. Alternatively, stretching includes transverse stretching, longitudinal unidirectional stretching and bidirectional synchronous stretching performed in sequence. Alternatively, stretching includes transverse stretching, bidirectional synchronous stretching and longitudinal stretching performed in sequence. Alternatively, stretching includes longitudinal stretching, transverse stretching and bidirectional synchronous stretching performed in sequence. Alternatively, stretching includes bidirectional synchronous stretching, transverse stretching and longitudinal stretching performed in sequence. Alternatively, stretching includes bidirectional synchronous stretching, longitudinal stretching and transverse stretching performed in sequence. Alternatively, stretching includes longitudinal stretching, bidirectional synchronous stretching and transverse stretching performed in sequence. Alternatively, stretching includes transverse stretching, longitudinal stretching, transverse stretching and longitudinal stretching performed in sequence. Alternatively, stretching includes longitudinal stretching, transverse stretching, longitudinal stretching and transverse stretching performed in sequence. Alternatively, stretching includes longitudinal stretching, transverse stretching, longitudinal stretching and transverse stretching performed in sequence.

[0024] In the above implementation process, when the polyethylene membrane is stretched, the polyethylene membrane can be biaxially stretched, so that the polyethylene membrane has good tensile strength, pore distribution and other properties in both the transverse and longitudinal directions, thereby improving the overall performance of the polyethylene membrane. In addition, when biaxially stretching, a multiple mixed stretching method can be used, which can help achieve a large stretching ratio of the membrane material, produce coarse fibers with suitable distribution density and diameter, improve the friction of the polyethylene membrane in the transverse direction, and further improve the stretching quality of the polyethylene membrane, thereby improving the mechanical properties such as needle punch strength and tensile strength and membrane properties such as pore distribution of the polyethylene membrane.

[0025] In conjunction with the second aspect, in an optional embodiment of the present application, the polyethylene includes high molecular weight polyethylene having a viscosity average molecular weight of not less than 1.8 million and low molecular weight polyethylene having a viscosity average molecular weight of not more than 1 million. The high molecular weight polyethylene accounts for 10% to 20% by weight of the cast sheet, and the low molecular weight polyethylene accounts for 0% to 8% by weight of the cast sheet.

[0026] In the above implementation process, a mixture of high molecular weight polyethylene with a viscosity average molecular weight of not less than 1.8 million and low molecular weight polyethylene with a viscosity average molecular weight of not more than 1 million is used, which can further improve the tensile quality, and can make the polyethylene diaphragm have suitable friction properties while also having good tensile strength, needle puncture strength, pore distribution and other properties, thereby improving the comprehensive performance of the polyethylene diaphragm. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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.

[0028] Figure 1 This is a set of SEM images of the test sample provided in Example 1 of the present application, with magnifications of 1000x and 5000x respectively;

[0029] Figure 2 This is a set of SEM images of the test sample provided in Example 2 of the present application, with magnifications of 1000x and 5000x respectively;

[0030] Figure 3 This is a set of SEM images of the test sample provided in Example 3 of the present application at magnifications of 1000x and 5000x respectively;

[0031] Figure 4 This is a set of SEM images of the test sample provided in Example 4 of the present application at magnifications of 1000x and 5000x respectively;

[0032] Figure 5This is a set of SEM images of the test sample provided in Comparative Example 1 of the present application, with magnifications of 1000x and 5000x respectively;

[0033] Figure 6 This is a set of SEM images of the test sample provided in Comparative Example 2 of the present application, with magnifications of 1000x and 5000x respectively;

[0034] Figure 7 This is a set of SEM images of the test sample provided in Comparative Example 3 of the present application, with magnifications of 1000x and 5000x respectively;

[0035] Figure 8 This is a set of SEM images of the test sample provided in Comparative Example 4 of the present application, with magnifications of 1000x and 5000x respectively;

[0036] Figure 9 This is a histogram of fiber diameter distribution of the test sample provided in Example 1 of the present application;

[0037] Figure 10 This is a histogram of fiber diameter distribution of the test sample provided in Example 2 of the present application;

[0038] Figure 11 This is a histogram of fiber diameter distribution of the test sample provided in Example 3 of the present application;

[0039] Figure 12 This is a histogram of the fiber diameter distribution of the test sample provided in Example 4 of the present application;

[0040] Figure 13 This is a histogram of fiber diameter distribution of the test sample provided in Comparative Example 1 of this application;

[0041] Figure 14 This is a histogram of fiber diameter distribution of the test sample provided in Comparative Example 2 of this application;

[0042] Figure 15 This is a histogram of fiber diameter distribution of the test sample provided in Comparative Example 3 of this application;

[0043] Figure 16 This is a bar graph of the fiber diameter distribution of the test sample provided in Comparative Example 4 of the present application. DETAILED DESCRIPTION

[0044] 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.

[0045] 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.

[0046] As the application fields of secondary batteries continue to expand and the impact of lithium-ion products in people's lives continues to deepen, people's requirements for the performance of secondary batteries are also getting higher and higher.

[0047] 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:

[0048] 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.

[0049] 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.

[0050] After wet-process and dry-process diaphragms are manufactured, they typically undergo slitting and winding operations. Winding involves rolling the slit, finished diaphragms that meet dimensional specifications onto a reel for storage.

[0051] 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 standard production. Therefore, it has developed into a mainstream process in recent years. The material of wet-process diaphragms is mainly polyethylene. Its process mainly includes phase separation, stretching, extraction, heat setting, slitting and winding. It mainly melts and blends the polymer with a high-boiling point small molecule solvent at a specific temperature to form a homogeneous melt. After extrusion, it is stretched, pulled and cooled by rollers to form a cast sheet. The cast sheet is then stretched into a film using a stretching device, and the solvent in the film is extracted. Finally, it is heat-set to give the film good dimensional stability. The diaphragm is then cut into target size specifications and then wound.

[0052] However, the diaphragm prepared by the current wet process is prone to dislocation and slippage along the lateral direction of the diaphragm when it is rolled up, causing wrinkles in the diaphragm, which will affect the electrochemical performance of the secondary battery.

[0053] Based on this, an embodiment of the present application provides a polyethylene diaphragm that can improve the problem of wet-process diaphragms being prone to dislocation and slipping during winding.

[0054] The polyethylene separator provided in the embodiment of the present application has a transverse direction (also called TD direction) and a longitudinal direction (also called MD direction). The longitudinal direction refers to the length direction of the membrane material, and the transverse direction refers to the width direction of the membrane material.

[0055] Take a 1m x 1m test sample from the center of the polyethylene separator along the transverse direction. Using a scanning electron microscope, take a first SEM image at a magnification of 1000x and a second SEM image at a magnification of 5000x at each of the four corners of the sample. Count the number of coarse fibers with a diameter of at least 500nm in the longitudinal direction of the fibers in the first SEM image. Count the average diameter of the longitudinal fibers in the second SEM image. In at least three of the four first SEM images, the number of coarse fibers must be at least 15. The average diameter must be at least 200nm.

[0056] The polyethylene diaphragm provided in the embodiment of the present application has coarse fibers extending longitudinally with suitable distribution density and diameter, so that the polyethylene diaphragm has certain friction in the transverse direction. When winding, the polyethylene diaphragm is not easy to slip or dislocate in the transverse direction, and is not easy to wrinkle, which can improve the comprehensive performance of the polyethylene diaphragm.

[0057] If there are too many coarse fibers in the polyethylene membrane, the average diameter of the fibers is too large, and the roughness of the polyethylene membrane is too high, the strength and other mechanical properties of the polyethylene membrane will be reduced, and the uniformity of the pore distribution will be affected. When used in batteries, it will affect the electrochemical properties of the battery, such as current efficiency and cycle stability.

[0058] The polyethylene diaphragm has a relatively wide width before being rolled up, and the width is usually greater than 1m. For example, the width of the polyethylene diaphragm in the transverse direction can be 5.6m. In the embodiment of the present application, when measuring the crude fiber distribution density and the average fiber diameter of the polyethylene diaphragm, a test sample with a size of 1m×1m is selected from the middle of the polyethylene diaphragm. The position of the test sample is relatively fixed, which is convenient for positioning and sampling. When performing microscopic morphology analysis on the test sample, a group of SEM images with a magnification of 1000 times and a magnification of 5000 times are taken from the four corners of the test sample respectively. The number of crude fibers and the average diameter of the fibers in multiple groups of SEM images are counted, which can reduce statistical errors and more accurately reflect the actual distribution of crude fibers in the polyethylene diaphragm and the actual average fiber diameter.

[0059] When a set of SEM images with a magnification of 1000 times and a magnification of 5000 times are taken from the four corners of the test sample, since the test sample is a 1m×1m square, the square can be divided into four quadrants, and a set of SEM images with a magnification of 1000 times and a magnification of 5000 times are randomly counted in each quadrant.

[0060] Furthermore, in some possible embodiments, in a set of SEM images in the same quadrant, the sampling of the first SEM image and the second SEM image may be different, that is, the sampling of the first SEM image and the second SEM image are independent of each other.

[0061] When counting the number of coarse fibers extending longitudinally in a polyethylene separator, selecting the first SEM image at a magnification of 1,000x for statistical analysis facilitates statistical analysis and further improves statistical accuracy. If the magnification of the first SEM image is too high, such as 5,000x, the number of fibers within the first SEM image is limited, and the actual statistical area is too small, resulting in a smaller statistical result and a larger statistical error. For example, in the first SEM image, due to the small statistical area, there may be no coarse fibers with a diameter of not less than 500 nm, resulting in a smaller statistical result.

[0062] If the magnification of the first SEM image is too low, for example, 500x, even though the actual detection area in the first SEM image is larger, coarse fibers with a diameter of not less than 500 nm in the first SEM image cannot be clearly observed during statistics, resulting in a smaller statistical result and a larger statistical error. For example, many coarse fibers with a diameter between 500 and 1500 nm may be missed, resulting in a smaller statistical result.

[0063] Since the number of coarse fibers in the polyethylene membrane accounts for a small proportion of the total number of fibers, most of the polyethylene membrane is still fine fibers with a diameter not exceeding 500 nm. Therefore, when counting the average diameter of the fibers extending longitudinally in the polyethylene membrane, the second SEM image with a magnification of 5,000 times is selected as the statistical object, which can clearly observe the morphology of more fine fibers, facilitate the calculation of the average diameter of the fibers, and reduce errors.

[0064] If the magnification of the second SEM image is too high, for example, 10,000 times, the number of fibers in the image is limited, the statistical sample is small, and the error is large. If the magnification of the second SEM image is too low, for example, 1,000 times, although the statistical area is large, the morphology of fine fibers cannot be clearly observed, resulting in an inflated statistical result.

[0065] In this application, "fibers extending longitudinally" do not necessarily mean they extend along an absolute mathematically straight line, such as horizontal or vertical. If a polyethylene membrane is placed vertically, with its length facing downward, fibers extending longitudinally do not necessarily mean they are absolutely vertical. A fiber is considered longitudinally stretched if its angle with the vertical direction is less than its angle with the horizontal direction. Furthermore, because fibers often exhibit irregular curvature, the angle between a fiber and the vertical direction can be used to determine the angle between the line connecting the fiber's starting and ending points and the vertical direction.

[0066] Furthermore, in some embodiments, when calculating the average diameter of the fibers extending longitudinally in the second SEM image, the number of statistical samples for each second SEM image is no less than 100. A larger number of statistical samples can more accurately reflect the actual average diameter of the fibers.

[0067] In some implementations, the number of statistical samples may be 100 to 300.

[0068] In some implementations, the number of statistical samples may be 100 to 200.

[0069] As an example, the number of statistical samples may be one of 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, or 200, or a range between any two of them.

[0070] Furthermore, in some embodiments, the average diameter of the polyethylene membrane provided in the embodiments of the present application is 200 to 400 nm.

[0071] In some embodiments, the polyethylene membrane provided by the embodiments of the present application has coarse fibers extending in the longitudinal direction, and the tensile strength of the polyethylene membrane in the longitudinal direction is greater than its tensile strength in the transverse direction.

[0072] Furthermore, in some embodiments, the difference between the tensile strength of the polyethylene membrane in the longitudinal direction and the tensile strength in the transverse direction is not less than 1000 gf / cm 2 .

[0073] Furthermore, the present invention also provides a method for preparing a polyethylene diaphragm, comprising:

[0074] S1. Obtain a casting sheet containing polyethylene and a pore-forming agent.

[0075] S2. Stretching the cast sheet. In the stretching step, the total stretching ratio of the cast sheet in the longitudinal direction is not less than 9 times.

[0076] S3, post-processing to obtain a polyethylene diaphragm.

[0077] Stretching the cast sheet containing polyethylene and a pore-forming agent, with the total stretching ratio in the longitudinal direction being not less than 9 times, can enable the polyethylene diaphragm to produce coarse fibers extending in the longitudinal direction with an appropriate diameter and quantity, can improve the roughness of the polyethylene diaphragm in the transverse direction, and when the polyethylene diaphragm is subsequently wound up, can reduce the probability of the polyethylene diaphragm being dislocated and slipping in the transverse direction, thereby reducing the probability of the polyethylene diaphragm having defects such as wrinkles, thereby improving the quality of the polyethylene diaphragm.

[0078] Furthermore, the present application does not limit how the cast sheet is obtained. In some possible embodiments, the present application also provides a method for preparing the cast sheet, including:

[0079] S101, feeding: feeding the mixed raw material containing polyethylene and pore-forming agent into the extruder.

[0080] S102, melt extrusion: melt and extrude the mixed raw materials.

[0081] S103, tape casting, sheet casting: the melt extruded from the extruder is tape cast, cooled and solidified to obtain a sheet.

[0082] In some possible embodiments, in order to improve the quality of the casting and the stretching quality, in some possible embodiments, the viscosity-average molecular weight of the polyethylene is not less than 1.8 million.

[0083] Furthermore, in some possible embodiments, the viscosity-average molecular weight of polyethylene may be 1.8 million to 4 million.

[0084] 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.

[0085] As an example, the viscosity average molecular weight of polyethylene may be 1.8 million to 3 million.

[0086] As an example, the viscosity average molecular weight of polyethylene may be 1.8 million to 2.4 million.

[0087] 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 mixed raw material may also contain low molecular weight polyethylene with a viscosity average molecular weight of 500,000 to 1,000,000.

[0088] By adding a certain proportion of low molecular weight polyethylene to the mixed raw materials, the quality of the casting can be improved through the combination of high and low molecular weight. In the subsequent stretching process, the stretching state can be improved. While forming coarse fibers extending in the longitudinal direction to improve the friction of the polyethylene diaphragm in the transverse direction, it can also reduce the probability of defects or poor performance such as excessive pore size and film breakage in the membrane material, which can further improve the overall performance of the polyethylene diaphragm.

[0089] 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.

[0090] Furthermore, in the mixed raw materials, the mass proportion of high molecular weight polyethylene can be 10% to 20%, and the mass proportion of low molecular weight polyethylene can be 0 to 8%.

[0091] As an example, the mixed 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.

[0092] As an example, the mixed 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.

[0093] As an example, the mixed 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.

[0094] As an example, the mixed 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.

[0095] As an example, the mixed 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.

[0096] As an example, the mixed 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.

[0097] To further improve the quality of the cast sheet and enhance the tensile strength, some possible embodiments can reduce the degree of polyethylene degradation during the melt extrusion process and increase the molecular weight of the polyethylene in the cast sheet. For example, polyethylene powder with a median particle size D50 of 80 to 150 μm can be used as the raw material for melt extrusion.

[0098] The type of pore-forming agent affects the phase separation process and is a key factor in controlling the structure and size of the pores in the polyethylene membrane. Consequently, it significantly impacts the pore size distribution, porosity, air permeability, and tensile quality of the polyethylene membrane. This application does not limit the specific type of pore-forming agent; the pore-forming agent may be selected based on conventional pore-forming agents in the art.

[0099] In one possible embodiment, the pore former is selected from white oil.

[0100] 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 polyethylene membrane with a suitable pore size distribution can be obtained. White oil also acts as a lubricant during the stretching process, improving stretching quality and facilitating high-ratio stretching. It can form coarse fibers that improve the polyethylene membrane's transverse friction and impart excellent membrane properties, such as a good pore size distribution.

[0101] In addition, the addition ratio of the pore-forming agent will also affect the microstructure and mechanical properties of the polyethylene membrane. In some possible embodiments, the dry material ratio of the mixed raw materials can be 15% to 19%.

[0102] The dry material ratio refers to the mass proportion of polyethylene solid material in the mixed raw materials.

[0103] An appropriate dry-to-material ratio 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 comprehensive performance of the polyethylene diaphragm.

[0104] 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.

[0105] Furthermore, in step S101, the present application does not limit the feeding rate of the mixed raw material into the extruder. In some possible embodiments, the feeding rate may be 150 to 700 kg / h.

[0106] As an example, the feed rate may be 250-500 kg / h.

[0107] In step S102, the mixed raw materials are plasticized and melted to form a homogeneous melt of the polyethylene and the pore-forming agent. This application does not limit the specific melt extrusion parameters. In some possible embodiments, during melt extrusion, the screw speed can be 70 to 200 r / min, the melt temperature can be 206 to 270°C, the extrusion temperature can be 170 to 220°C, and the extrusion pressure can be 70 to 150 bar.

[0108] In step S103, the present application does not limit the cooling temperature of the chilled roller, and the cooling temperature can be selected according to conventional cooling temperatures in the art. As an example, the cooling temperature of the chilled roller can be 5-10°C.

[0109] In step S2, the present application does not limit the specific stretching method. In some possible embodiments, the cast sheet can be biaxially stretched.

[0110] As an example, the bidirectional stretching method may be asynchronous bidirectional stretching or bidirectional synchronous stretching.

[0111] In one embodiment, when performing asynchronous biaxial stretching, the cast sheet may be stretched longitudinally and transversely sequentially.

[0112] Alternatively, in some embodiments, multiple stretching methods may be employed, for example, the multiple stretching methods comprising sequentially performing transverse stretching, longitudinal stretching, and bidirectional simultaneous stretching.

[0113] As an example, the multiple stretching processes include sequentially performing transverse stretching, bidirectional simultaneous stretching, and longitudinal stretching.

[0114] As an example, the multiple stretching processes include sequential longitudinal stretching, transverse stretching, and bidirectional simultaneous stretching.

[0115] As an example, the multiple stretching processes include bidirectional simultaneous stretching, transverse stretching, and longitudinal stretching performed sequentially.

[0116] As an example, the multiple stretching processes include bidirectional simultaneous stretching, longitudinal stretching, and transverse stretching performed sequentially.

[0117] As an example, the multiple stretching processes include longitudinal stretching, bidirectional simultaneous stretching, and transverse stretching performed sequentially.

[0118] As an example, the multiple stretching steps include sequentially performing transverse stretching, longitudinal stretching, transverse stretching, and longitudinal stretching.

[0119] As an example, the multiple stretching steps include longitudinal stretching, transverse stretching, longitudinal stretching, and transverse stretching performed sequentially.

[0120] Furthermore, the present application does not limit the specific stretching ratio of the longitudinal stretching. As an example, the total stretching ratio of the longitudinal stretching can be 9 to 11.5 times.

[0121] As an example, the total ratio of longitudinal stretching may be 9 times, 9.5 times, 10 times, 10.5 times, 11 times or 11 times, or a range between any two of them.

[0122] Furthermore, the present application does not limit the specific temperature of longitudinal stretching. In some embodiments, during longitudinal stretching, the preheating temperature may be 60-90°C, and the stretching temperature of longitudinal stretching may be 90-110°C.

[0123] Furthermore, the present application does not limit the total ratio during transverse stretching. In some possible embodiments, the total ratio during transverse stretching may be 7 to 16 times.

[0124] In some possible embodiments, the total ratio during transverse stretching is not higher than the total ratio during longitudinal stretching.

[0125] Furthermore, the present application does not limit the specific temperature during transverse stretching. In some possible embodiments, during transverse stretching, the preheating temperature is 105-125°C, the stretching temperature is 95-120°C, and the stretching temperature is not higher than the preheating temperature.

[0126] Furthermore, after transverse stretching, the film material can be shrunk by 5% to 8%.

[0127] In step S3, during post-processing, the stretched membrane material may be extracted to remove the pore-forming agent in the membrane material. For example, dichloromethane may be used as the extractant.

[0128] Furthermore, after extraction, the film material can be heat-set.

[0129] For example, during heat setting, the film can be subjected to a second transverse stretching, with a ratio of 1.2 to 1.8 times, a shrinkage of 12% to 20%, a heat setting temperature of 120 to 145°C, and a heat setting time of 20 to 30 seconds.

[0130] As an example, the heat setting temperature may be 135-145°C.

[0131] Furthermore, the polyethylene membrane prepared by the above method can be slit and then rolled up. Since the polyethylene membrane prepared by the above method has coarse fibers extending longitudinally with a certain distribution density and size, the polyethylene membrane has a certain degree of friction in the transverse direction. This can reduce the probability of the polyethylene membrane being displaced and slipping in the transverse direction during rolling, and can also reduce the probability of the polyethylene membrane being wrinkled, thereby improving the overall performance of the polyethylene membrane.

[0132] The polyethylene diaphragm and its preparation method of the present application are further described in detail below with reference to the examples.

[0133] Example 1

[0134] Example 1 provides a polyethylene diaphragm, the preparation method of which is as follows:

[0135] (1) Polyethylene powder with a viscosity-average molecular weight of 2 million and white oil were mixed at a dry material ratio of 15%. The mixed raw materials were fed into an extruder at a feed rate of 420 kg / h for melt extrusion. The melt extrusion parameters included: melt temperature of 210°C, screw speed of 140 r / min, and extrusion temperature of 195°C.

[0136] (2) The melt is extruded from the extruder and cast onto a chilled roller, which is then cooled to room temperature to obtain a cast sheet.

[0137] (3) The cast sheet was sequentially subjected to longitudinal stretching, transverse stretching, extraction, and heat setting to obtain a polyethylene diaphragm. The preheating temperature for longitudinal stretching was 80°C, the stretching temperature was 110°C, and the stretching ratio was 9 times. The preheating temperature for transverse stretching was 118°C, the stretching temperature was 115°C, and the stretching ratio was 7 times. The extraction temperature was 20°C, and the extractant was dichloromethane. During heat setting, the film was subjected to a second transverse stretching of 1.4 times at a stretching temperature of 133°C, a setting temperature of 135°C, and a time of 27 seconds.

[0138] Example 2

[0139] Example 2 provides a polyethylene diaphragm, which differs from Example 1 in that:

[0140] In step (3), the ratio of longitudinal stretching is 9.5 times.

[0141] Example 3

[0142] Example 3 provides a polyethylene diaphragm, which differs from Example 1 in that:

[0143] In step (3), the ratio of longitudinal stretching is 10 times.

[0144] Example 4

[0145] Example 4 provides a polyethylene diaphragm, which differs from Example 1 in that:

[0146] In step (3), the ratio of longitudinal stretching is 11.5 times.

[0147] Comparative Example 1

[0148] Comparative Example 1 provides a polyethylene diaphragm, which differs from Example 1 in that:

[0149] In step (3), the ratio of longitudinal stretching is 8.5 times.

[0150] Comparative Example 2

[0151] Comparative Example 2 provides a polyethylene diaphragm, which differs from Example 1 in that:

[0152] In step (3), the ratio of longitudinal stretching is 8.2 times.

[0153] Comparative Example 3

[0154] Comparative Example 3 provides a polyethylene diaphragm, which differs from Example 1 in that:

[0155] In step (3), the ratio of longitudinal stretching is 7.8 times.

[0156] Comparative Example 4

[0157] Comparative Example 4 provides a polyethylene diaphragm, which differs from Example 1 in that:

[0158] In step (3), the ratio of longitudinal stretching is 7.6 times.

[0159] Test Case

[0160] The crude fiber count and average fiber diameter of the polyethylene diaphragms provided in Examples 1 to 4 and Comparative Examples 1 to 4 were statistically analyzed using the following statistical method:

[0161] A 1m x 1m test sample was taken from the center of the polyethylene separator along the transverse direction (TD). A scanning electron microscope (SEM) image was taken at each of the four corners of the sample, with a first SEM image at a magnification of 1000x and a second SEM image at a magnification of 5000x. The number of coarse fibers with a diameter of 500nm or greater in the longitudinal direction of the first SEM image was counted, and the average diameter of the longitudinal fibers in the second SEM image was calculated. The statistical software used was Nano Measurer.

[0162] Among them, one set of SEM images of the test sample provided in Example 1 with magnifications of 1000 times and 5000 times are as follows: Figure 1 As shown, a set of SEM images of the test sample provided in Example 2 with magnifications of 1000 times and 5000 times are shown in FIG. Figure 2 As shown, a set of SEM images of the test sample provided in Example 3 with magnifications of 1000 times and 5000 times are shown in FIG. Figure 3 As shown, a set of SEM images of the test sample provided in Example 4 with magnifications of 1000 times and 5000 times are shown in FIG. Figure 4 As shown, a set of SEM images of the test sample provided in Comparative Example 1 with magnifications of 1000 times and 5000 times are shown in FIG. Figure 5 As shown, a set of SEM images of the test sample provided in Comparative Example 2 with magnifications of 1,000 times and 5,000 times are shown in FIG. Figure 6 As shown, a set of SEM images of the test sample provided in Comparative Example 3 with magnifications of 1,000 times and 5,000 times are shown in FIG. Figure 7 As shown, a set of SEM images of the test sample provided in Comparative Example 4 with magnifications of 1,000 times and 5,000 times are shown in FIG. Figure 8 shown. Figures 1 to 8 In the figure, the magnification of the left image is 1000 times, and the magnification of the right image is 5000 times.

[0163] The statistical results of the crude fiber number are shown in Table 1, and the statistical results of the average fiber diameter are shown in Table 2. Figures 9 to 16 The fiber diameter distribution histograms provided for Examples 1 to 4 and Comparative Examples 1 to 4 are respectively.

[0164] Table 1

[0165]

[0166]

[0167] Table 2

[0168]

[0169] Result analysis:

[0170] It can be seen from Table 1 and Table 2 that in the polyethylene diaphragm provided in the embodiment of the present application, in at least 3 of the 4 first SEM images, the number of crude fibers with a diameter of not less than 500 nm is not less than 15. The average fiber diameter of the polyethylene diaphragm is not less than 200 nm. In Comparative Examples 1 to 4, the number of crude fibers in the first SEM image is less than 10, the average diameter is less than 200 nm, the roughness along the transverse direction is low, and the friction is small. The polyethylene diaphragm provided in the embodiment of the present application has suitable friction along the transverse direction, and when winding, it can reduce the probability of the polyethylene diaphragm slipping and dislocation along the transverse direction, and can improve the problem of the polyethylene diaphragm easily generating wrinkles after winding, thereby improving the overall quality of the diaphragm.

[0171] 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 polyethylene diaphragm, characterized in that: The polyethylene diaphragm has a transverse and longitudinal direction; along the transverse direction, a test sample with a size of 1 meter × 1 meter is taken in the middle position of the polyethylene diaphragm; a scanning electron microscope is used to test the four corner positions of the test sample, and a set of first SEM pictures with a magnification of 1 thousand times and a second SEM picture with a magnification of 5 thousand times are respectively tested, and the number of coarse fibers with a diameter of not less than 500 nm in the fibers extending along the longitudinal direction in the first SEM picture is counted, and the average diameter of the fibers extending along the longitudinal direction in the second SEM picture is counted; among the four first SEM pictures, the number of coarse fibers in at least three of the first SEM pictures is not less than 15; the average diameter is not less than 200 nm.

2. The polyethylene diaphragm according to claim 1, characterized in that In the first SEM image, the number of the coarse fibers is 10 to 50.

3. The polyethylene diaphragm according to claim 1, characterized in that When counting the average diameter of the fibers extending in the longitudinal direction in the second SEM image, the number of statistical samples for each second SEM image is not less than 100.

4. The polyethylene diaphragm according to claim 3, characterized in that The average diameter is 200-400 nm.

5. The polyethylene diaphragm according to any one of claims 1 to 4, characterized in that: The polyethylene diaphragm has a tensile strength in the longitudinal direction greater than that in the transverse direction.

6. The polyethylene diaphragm according to claim 5, characterized in that The difference between the tensile strength of the polyethylene membrane in the longitudinal direction and the tensile strength in the transverse direction is not less than 1000 gf / cm 2 .

7. A method for preparing a polyethylene diaphragm according to any one of claims 1 to 6, characterized in that: include: obtaining a cast sheet comprising polyethylene and a pore-forming agent; stretching and post-processing the cast sheet to obtain the polyethylene diaphragm; In the stretching step, the total stretching ratio of the cast sheet along the longitudinal direction is not less than 9 times.

8. The preparation method according to claim 7, characterized in that The total stretching ratio of the polyethylene diaphragm in the longitudinal direction is 9 to 11.5 times; Optionally, the total stretching ratio of the polyethylene membrane along the transverse direction is lower than the total stretching ratio along the longitudinal direction.

9. The preparation method according to claim 8, characterized in that The stretching includes longitudinal stretching and transverse stretching performed sequentially; Alternatively, the stretching comprises sequentially performing transverse stretching, longitudinal uniaxial stretching and biaxial simultaneous stretching; Alternatively, the stretching comprises sequentially performing transverse stretching, bidirectional simultaneous stretching, and longitudinal stretching; Alternatively, the stretching comprises longitudinal stretching, transverse stretching and bidirectional simultaneous stretching performed sequentially; Alternatively, the stretching comprises bidirectional simultaneous stretching, transverse stretching and longitudinal stretching performed sequentially; Alternatively, the stretching comprises bidirectional simultaneous stretching, longitudinal stretching and transverse stretching performed sequentially; Alternatively, the stretching comprises longitudinal stretching, bidirectional simultaneous stretching and transverse stretching performed sequentially; Alternatively, the stretching comprises sequentially performing transverse stretching, longitudinal stretching, transverse stretching, and longitudinal stretching; Alternatively, the stretching includes longitudinal stretching, transverse stretching, longitudinal stretching and transverse stretching performed sequentially.

10. The preparation method according to claim 7, characterized in that The polyethylene includes high molecular weight polyethylene with a viscosity average molecular weight of not less than 1.8 million and low molecular weight polyethylene with a viscosity average molecular weight of not more than 1 million; the mass proportion of the high molecular weight polyethylene in the cast sheet is 10% to 20%, and the mass proportion of the low molecular weight polyethylene in the cast sheet is 0 to 8%.

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