An ultra-thin polyethylene separator based on ultra-high molecular weight polyethylene modification and a preparation method thereof

Abstract

The application discloses the technical field of polyethylene diaphragm preparation of lithium battery, in particular to an ultrathin polyethylene diaphragm based on ultrahigh molecular weight polyethylene modification and a preparation method thereof, which comprises the following steps: step one, modifying ultrahigh molecular weight polyethylene with high weight average molecular weight by a mechanical method to obtain ultrahigh molecular weight polyethylene modified material; step two, blending the ultrahigh molecular weight polyethylene modified material with ultrahigh molecular weight polyethylene with low weight average molecular weight to obtain formula material; step three, melting and plasticizing the formula material with a diluent to obtain a base film; and step four, obtaining the ultrathin polyethylene diaphragm by stretching the base film by a super uniform method. The application makes the final prepared ultrathin polyethylene diaphragm plasticize uniformly, and there are no plasticized resin particles, thereby improving the consistency of the product and reducing energy consumption.

Description

Technical Field

[0001] This application relates to the field of lithium battery polyethylene separator preparation technology, specifically to an ultra-thin polyethylene separator based on ultra-high molecular weight polyethylene modification and its preparation method. Background Technology

[0002] Lithium-ion batteries are widely used in electric vehicles, energy storage systems, military equipment, and aerospace due to their high energy density and long cycle life. The lithium-ion battery separator is one of the core components of a lithium battery, directly affecting its capacity, cycle life, and safety performance. A high-performance separator plays a crucial role in improving the overall performance of the battery.

[0003] Currently, industrial production mainly employs dry and wet processes to prepare lithium-ion battery separators. Wet-process separators are widely used due to their high strength and uniform pore size. As the energy density requirements of lithium batteries increase, ultra-thin separators are a major trend. To ensure strength, ultra-thin separators are typically produced using raw materials with higher molecular weights, achieving high strength through excessive stretching. However, as the molecular weight of the raw materials increases, their plasticizing properties deteriorate, leading to uneven plasticization and the formation of undissolved substances in localized areas. This results in membrane breakage or uneven stretching during production, affecting production efficiency and product consistency.

[0004] Chinese patent CN116417751A discloses a plasticizing process in which high-density polyethylene or a polyethylene-polypropylene blend, additives, and white oil are mixed and then subjected to twin-screw plasticizing melt extrusion followed by shear plasticizing at low temperatures. CN102757577B first mixes ultra-high molecular weight polyethylene with a first solvent to obtain a treated material, which is then mixed with high-density polyethylene and a second solvent for dissolution plasticizing, thus solving the problem of difficult plasticization. Both methods employ a mixed system reacting with chemical agents to achieve shear plasticizing or dissolution plasticizing; however, neither modifies the ultra-high molecular weight polyethylene itself to improve its plasticizing properties. Summary of the Invention

[0005] The purpose of this invention is to provide an ultra-thin polyethylene diaphragm based on ultra-high molecular weight polyethylene (UHMWPE) modification and its preparation method. High weight-average molecular weight UHMWPE is mechanically modified to increase its specific surface area. Different weight parts of modified UHMWPE are then mixed with low weight-average UHMWPE to obtain a formulation. The formulation is then melt-plasticized with a diluent, repeatedly stretched, and high-temperature shaped to obtain the finished film. This invention solves the aforementioned problems existing in the prior art.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following solution:

[0007] A method for preparing an ultrathin polyethylene separator based on ultra-high molecular weight polyethylene modification includes the following steps:

[0008] Step 1: Modify high weight-average molecular weight ultra-high molecular weight polyethylene using mechanical methods to obtain modified ultra-high molecular weight polyethylene material;

[0009] Step 2: Blend the modified ultra-high molecular weight polyethylene with ultra-high molecular weight polyethylene with low weight average molecular weight to obtain the formulation.

[0010] Step 3: Melt and plasticize the formulated materials with the diluent to obtain the base film;

[0011] Step 4: The base film is stretched uniformly to obtain an ultra-thin polyethylene diaphragm.

[0012] Since the higher the molecular weight of ultra-high molecular weight polyethylene (UHMWPE), the worse its plasticizing ability, the purpose of this invention is to modify UHMWPE with high weight-average molecular weight by changing the spherical UHMWPE particles to oblate spheroids, thereby increasing the specific surface area of ​​the particles. This increases the wettability of small molecules in the diluent to UHMWPE, allowing the small molecules in the diluent to enter the macromolecular chains of UHMWPE more quickly, breaking down the interaction forces between the macromolecular chains, thus improving the plasticizing ability of UHMWPE. As a result, the final ultra-thin polyethylene separator is free of unplasticized resin particles, thereby improving the consistency of the final product membrane.

[0013] This invention simultaneously blends modified high weight-average molecular weight ultra-high molecular weight polyethylene with low weight-average molecular weight ultra-high molecular weight polyethylene in a certain proportion to obtain a specific formulation. The formulation is then mixed with a diluent to prepare an ultra-thin polyethylene membrane. The raw materials for preparation are ultra-high molecular weight polyethylene and a diluent, which effectively reduces the cost of raw materials.

[0014] Specifically, Figure 1 and Figure 2 It can be seen that the external shape of the modified ultra-high molecular weight polyethylene particles has changed, becoming oblate spheroidal particles. Figure 3 As the modification time increased, the dissolution temperature of ultra-high molecular weight polyethylene in the diluent continuously decreased. The dissolution temperature before modification was 135℃, the dissolution temperature after 2 minutes of modification was 131℃, the dissolution temperature after 5 minutes of modification was 128℃, the dissolution temperature after 10 minutes of modification was 125℃, and the dissolution temperature after 15 minutes of modification was 122℃. This indicates that as the temperature increases, the modified ultra-high molecular weight polyethylene macromolecular chains begin to dissolve in the diluent, which is reflected in the increase of melt viscosity. Therefore, the lower the temperature, the better the dissolution ability of the modified ultra-high molecular weight polyethylene particles in the diluent, and the stronger its plasticizing ability.

[0015] Preferably, the mechanical method in step one is mechanical ball milling, and the ball milling time is 30s-30min.

[0016] The method employed is mechanical ball milling, such as roller milling or vibratory milling. Through the application of mechanical force, the high weight-average molecular weight ultra-high molecular weight polyethylene is milled for an extended period, thereby altering the particle morphology and increasing the specific surface area.

[0017] Preferably, the weight-average molecular weight of the ultra-high molecular weight polyethylene in step one is 1.5-4 million.

[0018] Preferably, the weight-average molecular weight of the ultra-high molecular weight polyethylene in step two is 200,000 to 1,200,000.

[0019] Preferably, the modified ultra-high molecular weight polyethylene material is 10-80 parts by weight, and the ultra-high molecular weight polyethylene with a weight average molecular weight of 200,000-1,200,000 is 20-90 parts by weight.

[0020] Preferably, in step three, the diluent is at least one of paraffin oil, mineral oil, soybean oil, phthalate, and fatty acid ester.

[0021] Preferably, 10-50 parts by weight of the formulation material and 50-90 parts by weight of the diluent are injected into a twin-screw extruder for melt plasticization, and the extrusion processing temperature is 170-230℃ to obtain a base film with a thickness of 0.8-4mm.

[0022] Preferably, the base film is subjected to longitudinal stretching, a first transverse stretching, and a second transverse stretching in sequence;

[0023] In the longitudinal stretching, the stretching temperature is 90-120℃ and the stretching ratio is 10-15 times;

[0024] In the first transverse stretching, the stretching temperature is 100-130℃ and the stretching ratio is 18-25 times.

[0025] In the secondary transverse stretching, the stretching temperature is 110-140℃, and the stretching ratio is 1-3 times.

[0026] Preferably, extraction is performed after the first transverse stretching, and the extractant is dichloromethane.

[0027] An ultrathin polyethylene separator is prepared using the above-mentioned method for preparing an ultrathin polyethylene separator based on ultra-high molecular weight polyethylene modification, wherein the thickness of the ultrathin polyethylene separator is 4.0–5.5 μm.

[0028] The beneficial effects of this invention are as follows: This invention specifically modifies ultra-high molecular weight polyethylene (UHMWPE) with high weight average molecular weight by mechanical ball milling, which changes the microstructure of the modified UHMWPE from spherical to oblate, increases the specific surface area, thereby increasing the swelling performance of the diluent on UHMWPE particles and improving plasticizing ability.

[0029] Furthermore, by designing specific components for modified high weight-average molecular weight ultra-high molecular weight polyethylene and low weight-average molecular weight ultra-high molecular weight polyethylene, and by super-uniform stretching, the processing and mechanical properties are balanced, so that the final ultra-thin polyethylene diaphragm is uniformly plasticized and free of plasticized resin particles, thereby improving product consistency and reducing energy consumption. Attached Figure Description

[0030] Figure 1 Electron microscope image of ultra-high molecular weight polyethylene particles, bar = 50 μm;

[0031] Figure 2 Electron microscope image of mechanically modified ultra-high molecular weight polyethylene particles, bar = 50 μm;

[0032] Figure 3 A comparison chart of the initial dissolution temperatures of ultra-high molecular weight polyethylene before and after modification;

[0033] Figure 4 This is an electron microscope image of the ultrathin polyethylene diaphragm obtained in Example 1 of the present invention, bar = 10 μm;

[0034] Figure 5 This is an electron microscope image of the ultrathin polyethylene diaphragm obtained in Comparative Example 1 of the present invention, bar = 10 μm. Detailed Implementation

[0035] To more clearly demonstrate the purpose, technical solution, and advantages of this invention, the following description will be provided in conjunction with embodiments.

[0036] This invention relates to a method for preparing an ultrathin polyethylene separator based on ultra-high molecular weight polyethylene modification, comprising the following steps:

[0037] Step 1: Preparation of modified material. High weight-average molecular weight ultra-high molecular weight polyethylene is modified by mechanical means to obtain ultra-high molecular weight polyethylene modified material.

[0038] The mechanical method is mechanical ball milling, and the milling time is 30 seconds to 30 minutes.

[0039] The weight-average molecular weight of ultra-high molecular weight polyethylene is 1.5-4 million.

[0040] Step 2: Blend the modified ultra-high molecular weight polyethylene with ultra-high molecular weight polyethylene with low weight average molecular weight to obtain the formulation.

[0041] The weight average molecular weight of ultra-high molecular weight polyethylene with low weight average molecular weight is 200,000 to 1,200,000.

[0042] When blending, the weight parts of the modified ultra-high molecular weight polyethylene are 10-80 parts, and the weight parts of the ultra-high molecular weight polyethylene with a weight average molecular weight of 200,000-1,200,000 are 20-90 parts.

[0043] Step 3: Melt and plasticize the formulated materials with the diluent to obtain the base film;

[0044] 10-50 parts by weight of the formulation material and 50-90 parts by weight of the diluent are injected into a twin-screw extruder for melt plasticization. The extrusion processing temperature is 170-230℃ to obtain a base film with a thickness of 0.8-4mm.

[0045] Step 4: The base film is stretched uniformly to obtain an ultra-thin polyethylene diaphragm.

[0046] The base film is first subjected to longitudinal stretching at a temperature of 90-120℃ and a stretching ratio of 10-15 times.

[0047] Perform another transverse stretching at a temperature of 100-130℃ and a stretching ratio of 18-25 times.

[0048] After a single transverse stretching, the film was extracted with dichloromethane to obtain a microporous membrane.

[0049] The microporous membrane is then subjected to a second transverse stretching at a temperature of 110-140℃ and a stretching ratio of 1-3 times. After high-temperature setting, the finished membrane is obtained. The finished membrane is then rolled up, cut, and stored. The finished membrane is the ultra-thin polyethylene separator.

[0050] Example 1

[0051] 1) The weight-average molecular weight is 150 × 10⁻⁶. 4 Ultra-high molecular weight polyethylene was mechanically ball-milled for 5 minutes to obtain modified ultra-high molecular weight polyethylene material.

[0052] 2) Mix 70 parts of ultra-high molecular weight polyethylene modified material and 30 parts of material with a weight average molecular weight of 60×10⁻⁶. 4 The ultra-high molecular weight polyethylene was blended for 5 minutes to obtain the formulation.

[0053] 3) Mix 25 parts of the formulated material and 75 parts of white oil as a diluent. Feed the above raw materials separately into a twin-screw extruder for melt mixing. Set the extrusion temperature to 185-215℃ and the screw torque to 55%. The resulting film is cast to obtain a base film with a thickness of 2.5 mm.

[0054] 4) The base film is stretched longitudinally at a ratio of 10 times and a stretching temperature of 80-110℃;

[0055] 5) The longitudinally stretched film is subjected to a transverse stretching with a stretching ratio of 18 times and a stretching temperature of 95-115℃.

[0056] 6) The film after one transverse stretching is extracted with dichloromethane to obtain a microporous membrane;

[0057] 7) The microporous membrane is subjected to a second transverse stretching, with a stretching ratio of 1.3 times and a temperature of 125-135℃;

[0058] 8) The finished film (ultra-thin polyethylene film) obtained after high-temperature setting is wound and slit. The thickness of the ultra-thin polyethylene film is 5.5μm. Figure 4 As shown.

[0059] Example 2

[0060] 1) The weight-average molecular weight is 240 × 10⁻⁶. 4 Ultra-high molecular weight polyethylene was mechanically ball-milled for 10 minutes to obtain modified ultra-high molecular weight polyethylene material.

[0061] 2) Mix 60 parts of ultra-high molecular weight polyethylene modified material and 40 parts of material with a weight average molecular weight of 60×10⁻⁶. 4 The ultra-high molecular weight polyethylene was blended for 5 minutes to obtain the formulation.

[0062] 3) Mix 23 parts of the formulated material and 77 parts of white oil as a diluent. Feed the above raw materials separately into a twin-screw extruder for melt mixing. Set the extrusion temperature to 185-215℃ and the screw torque to 62%. The resulting film is cast to obtain a base film with a thickness of 2.0 mm.

[0063] 4) The base film is stretched longitudinally at a ratio of 12 times and a stretching temperature of 90-115℃.

[0064] 5) The longitudinally stretched film is subjected to a transverse stretching with a stretching ratio of 22 times and a stretching temperature of 95-120℃.

[0065] 6) The film after one transverse stretching is extracted with dichloromethane to obtain a microporous membrane;

[0066] 7) The microporous membrane is subjected to a second transverse stretching, with a stretching ratio of 1.3 times and a temperature of 125-135℃;

[0067] 8) The finished film (ultra-thin polyethylene film) obtained after high-temperature setting is wound up and slit. The thickness of the ultra-thin polyethylene film is 5.0 μm.

[0068] Example 3

[0069] 1) The weight-average molecular weight is 400×10 4 Ultra-high molecular weight polyethylene was mechanically ball-milled for 15 minutes to obtain modified ultra-high molecular weight polyethylene material.

[0070] 2) Take 50 parts of ultra-high molecular weight polyethylene modified material, 50 parts of which have a weight average molecular weight of 60×10⁻⁶. 4 The ultra-high molecular weight polyethylene was blended for 5 minutes to obtain the formulation.

[0071] 3) Mix 20 parts of the formulated material and 80 parts of white oil as a diluent. Feed the above raw materials separately into a twin-screw extruder for melt mixing. Set the extrusion temperature to 185-215℃ and the screw torque to 65%. The resulting film is cast to obtain a base film with a thickness of 1.8 mm.

[0072] 4) The base film is stretched longitudinally at a ratio of 15 times and a stretching temperature of 90-115℃.

[0073] 5) The longitudinally stretched film is subjected to a transverse stretching of 25 times at a temperature of 95-120℃.

[0074] 6) The film after one transverse stretching is extracted with dichloromethane to obtain a microporous membrane;

[0075] 7) The microporous membrane is subjected to a second transverse stretching, with a stretching ratio of 1.3 times and a temperature of 125-135℃;

[0076] 8) The finished film (ultra-thin polyethylene film) obtained after high-temperature setting is wound up and slit. The thickness of the ultra-thin polyethylene film is 4.0μm.

[0077] Comparative Example 1

[0078] 1) Take 30 portions with a weight-average molecular weight of 60 × 10⁻⁶ 4 Ultra-high molecular weight polyethylene, with a weight average molecular weight of 150×10⁻⁶ parts per 70 parts. 4 The ultra-high molecular weight polyethylene was blended for 5 minutes to obtain the formulation.

[0079] 2) Mix 25 parts of the formulated material and 75 parts of white oil as a diluent. Feed the above raw materials separately into a twin-screw extruder for melt mixing. Set the extrusion temperature to 185-215℃ and the screw torque to 70%. The resulting film is cast to obtain a base film with a thickness of 2.5 mm.

[0080] 3) The base film is stretched longitudinally at a ratio of 10 times and a stretching temperature of 80-110℃;

[0081] 4) The longitudinally stretched film is subjected to a transverse stretching with a stretching ratio of 18 times and a stretching temperature of 95-115℃.

[0082] 5) Extract the film after one horizontal stretching with dichloromethane to obtain a microporous membrane;

[0083] 6) The microporous membrane is subjected to a second transverse stretching, with a stretching ratio of 1.3 times, at a temperature of 125-135℃;

[0084] 7) The finished film obtained after high-temperature setting is wound up and slit, with a thickness of 5.5μm. Figure 5 As shown.

[0085] Comparative Example 2:

[0086] 1) Select a weight-average molecular weight of 60 × 10⁻⁶ 4 Ultra-high molecular weight polyethylene: 40 parts, weight average molecular weight 240×10 4 60 parts of ultra-high molecular weight polyethylene were mixed for 5 minutes to obtain the formulation.

[0087] 2) Mix 23 parts of the formulated material and 77 parts of white oil as a diluent. Feed the above raw materials separately into a twin-screw extruder for melt mixing. Set the extrusion temperature to 185-215℃ and the screw torque to 78%. The resulting film is cast to obtain a base film with a thickness of 2.5 mm.

[0088] 3) The base film is stretched longitudinally at a ratio of 12 times and a stretching temperature of 90-115℃;

[0089] 4) The longitudinally stretched film is then stretched laterally at a stretch ratio of 22 times and a stretching temperature of 95-120℃.

[0090] 5) The stretched film was extracted with dichloromethane to obtain a microporous membrane;

[0091] 6) The microporous membrane is subjected to a second transverse stretching, with a stretching ratio of 1.3 times, at a temperature of 125-135℃;

[0092] 7) The finished film obtained after high-temperature setting is wound up and slit, and the thickness of the finished film is 5.0μm.

[0093] Comparative Example 3:

[0094] 1) Select a weight-average molecular weight of 60 × 10⁻⁶ 4 Ultra-high molecular weight polyethylene: 50 parts, weight average molecular weight 400×10 4 50 parts of ultra-high molecular weight polyethylene were mixed for 5 minutes to obtain the formulation.

[0095] 2) Mix 20 parts of the formulated material and 80 parts of white oil as a diluent. Feed the above raw materials separately into a twin-screw extruder for melt mixing. Set the extrusion temperature to 185-215℃ and the screw torque to 85%. The resulting film is cast to obtain a base film with a thickness of 2.5mm. Due to the excessive torque of the twin-screw extruder, the surface of the cast sheet is rough, with too much unplasticized material, making stable extrusion impossible.

[0096] The properties of the ultrathin polyethylene diaphragms prepared in Examples 1-3 and Comparative Examples 1-3 are shown in Table 1.

[0097] Table 1. Performance of the ultrathin polyethylene films prepared in Examples 1-3 and Comparative Examples 1-3

[0098]

[0099]

[0100] As shown in Table 1, the ultra-thin polyethylene diaphragm of the present invention has a thickness of 4.5-5.5 μm. The cast base film exhibits a smooth, crystalline-free appearance, while Comparative Examples 1-2 show a rough, crystalline-free appearance, and Comparative Example 3 shows a rough, crystalline appearance. Figure 4 The modified ultrathin polyethylene diaphragm is uniformly plasticized and contains no unplasticized resin particles, which improves the consistency of the product. Figure 5 Comparative Example 1 shows the ultrathin polyethylene diaphragm prepared without modification, which clearly contains unplasticized resin particles and shows uneven plasticization.

[0101] Because the formulation in this application incorporates a specific high weight-average molecular weight ultra-high molecular weight polyethylene modifier, the torque of the twin-screw extruder in Examples 1-3 is reduced, which helps to reduce energy consumption, while simultaneously improving puncture strength and reducing thickness range / standard deviation.

[0102] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Based on the technical essence of the present invention, any simple modifications, equivalent substitutions, and improvements made to the above embodiments within the spirit and principles of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing an ultrathin polyethylene separator based on ultra-high molecular weight polyethylene modification, characterized in that, Includes the following steps: Step 1: Modify ultra-high molecular weight polyethylene with high weight average molecular weight using mechanical methods to obtain modified ultra-high molecular weight polyethylene material. The mechanical method is mechanical ball milling, with a milling time of 30s-30min. The weight average molecular weight of the ultra-high molecular weight polyethylene is 1.5-4 million. Step 2: Blend the modified ultra-high molecular weight polyethylene with ultra-high molecular weight polyethylene with low weight average molecular weight to obtain a formulation, wherein the ultra-high molecular weight polyethylene has a weight average molecular weight of 200,000 to 1,200,000 and includes 20 to 90 parts by weight of ultra-high molecular weight polyethylene, and the modified ultra-high molecular weight polyethylene has 10 to 80 parts by weight. Step 3: Melt and plasticize the formulated materials with the diluent to obtain the base film; Step 4: The base film is stretched uniformly to obtain an ultra-thin polyethylene diaphragm.

2. The method for preparing an ultrathin polyethylene diaphragm based on ultra-high molecular weight polyethylene modification according to claim 1, characterized in that, In step three, the diluent is at least one of paraffin oil, mineral oil, soybean oil, phthalate, and fatty acid ester.

3. The method for preparing an ultrathin polyethylene separator based on ultra-high molecular weight polyethylene modification according to claim 1, characterized in that, 10-50 parts by weight of the formulation material and 50-90 parts by weight of the diluent are injected into a twin-screw extruder for melt plasticization. The extrusion processing temperature is 170-230℃ to obtain a base film with a thickness of 0.8-4mm.

4. The method for preparing an ultrathin polyethylene separator based on ultra-high molecular weight polyethylene modification according to claim 1, characterized in that, The base film is subjected to longitudinal stretching, first transverse stretching, and second transverse stretching in sequence; In the longitudinal stretching, the stretching temperature is 90-120℃ and the stretching ratio is 10-15 times; In the first transverse stretching, the stretching temperature is 100-130℃ and the stretching ratio is 18-25 times. In the secondary transverse stretching, the stretching temperature is 110-140℃, and the stretching ratio is 1-3 times.

5. The method for preparing an ultrathin polyethylene separator based on ultra-high molecular weight polyethylene modification according to claim 4, characterized in that, The extraction is performed after the first transverse stretching, and the extractant is dichloromethane.

6. An ultra-thin polyethylene diaphragm, characterized in that, The ultra-thin polyethylene diaphragm is prepared according to any one of claims 1 to 5, wherein the thickness of the ultra-thin polyethylene diaphragm is 4.0 to 5.5 μm.

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