A method for reducing bidirectional thermal shrinkage of a wet-process separator base film

Abstract

The application provides a method for reducing bidirectional thermal shrinkage of a wet-process diaphragm base film, comprising the following steps: using ultra-high molecular weight polyethylene as a main raw material, adding high-density polyethylene and nano-silicon dioxide to perform composite modification to obtain a composite raw material; feeding the composite raw material into an extruder to perform extrusion casting to prepare a cast sheet, and a screw rod of the extruder is provided with segmented temperature control; performing longitudinal stretching and transverse stretching on the cast sheet; and performing heat setting treatment on the film after bidirectional stretching, and then performing rapid cooling.

Description

Technical Field

[0001] This invention relates to the field of information technology, and in particular to a method for reducing bidirectional thermal shrinkage of wet-process diaphragm base film. Background Technology

[0002] In the new energy field, lithium-ion batteries, as a core energy storage technology, are crucial to the development of industries such as electric vehicles and energy storage devices due to their safety and stability. The separator inside the battery, as a key component, directly affects the battery's safety performance and lifespan, especially under high-temperature environments, where the dimensional stability of the separator becomes a critical factor in ensuring battery operation safety. If the separator deforms or shrinks at high temperatures, it may lead to an internal short circuit in the battery, causing serious safety hazards. Therefore, improving the performance of the separator under extreme conditions is a technical challenge that the industry urgently needs to overcome.

[0003] Currently, although various methods exist to improve separator performance, these methods often struggle to balance dimensional stability at high temperatures and process control during manufacturing. Many existing technologies have limitations in material selection or processing techniques, making separators still susceptible to deformation at high temperatures. This deformation not only affects normal battery operation but can also increase safety risks due to separator failure, a deficiency particularly pronounced in high-end battery applications.

[0004] A deeper problem lies in the fact that controlling the orientation of molecular chains and internal stress during the production of separators remains a persistent technical bottleneck. Molecular chain orientation directly determines the structural stability and thermal shrinkage performance of the separator. If the alignment of molecular chains and the distribution of internal stress cannot be effectively controlled during processing, the separator is prone to shrinkage under high-temperature environments due to structural loosening or stress release. For example, during prolonged high-load operation of the battery, the separator may gradually deform because internal stress is not fully eliminated, and even develop micro-damage in certain localized areas, thus posing a safety hazard. This dimensional instability problem caused by uneven molecular chain orientation and stress distribution persists throughout the entire process of separator production, from raw material processing to final molding.

[0005] Therefore, how to precisely control the orientation of molecular chains and effectively reduce internal residual stress during the production process to ensure the dimensional stability of the diaphragm under high temperature conditions has become a key issue that urgently needs to be addressed. Summary of the Invention

[0006] This invention provides a method for reducing bidirectional thermal shrinkage of wet-process separator base film, mainly comprising:

[0007] Using ultra-high molecular weight polyethylene as the main raw material, high-density polyethylene and nano-silica are added for composite modification to obtain a composite raw material; the composite raw material is fed into an extruder for extrusion casting, and the extruder screw is equipped with segmented temperature control; the casting is subjected to longitudinal stretching and transverse stretching; the biaxially stretched film is subjected to heat setting treatment, followed by rapid cooling.

[0008] Furthermore, the composite material obtained by modifying ultra-high molecular weight polyethylene (UHMWPE) as the main raw material with high-density polyethylene (HDPE) and nano-silica includes: UHMWPE accounting for 90-95 wt%, HDPE accounting for 5-10 wt%, and nano-silica accounting for 0.1-0.3 wt%; HDPE having a molecular weight of 50,000-100,000, and nano-silica having a particle size of 50-100 nm; the composite material is obtained by high-speed stirring and uniform mixing.

[0009] Furthermore, the composite raw material is fed into an extruder for extrusion casting, and the extruder screw is equipped with segmented temperature control, including: the temperature of the extruder screw feed section is 140-150℃, the temperature of the compression section is 160-170℃, and the temperature of the homogenization section is 180-190℃; the extruder screw speed is controlled at 120-125 r / min to match the traction speed of 50-60 m / min; and the thickness of the extruded casting is controlled at 0.3-0.32 mm.

[0010] Furthermore, the longitudinal and transverse stretching of the casting includes: the longitudinal stretching temperature is 90-100℃, and the stretching ratio is 4.0-4.5 times; the longitudinal stretching adopts a gradient heating method; the transverse stretching temperature is 110-120℃, and the stretching ratio is 3.5-4.0 times; the transverse stretching rate is controlled at 10-15% / s; the transverse stretching and the longitudinal stretching form a complementary orientation.

[0011] Furthermore, the process of heat-setting the biaxially stretched film followed by rapid cooling includes: the heat-setting temperature being 120-130℃ and the holding time being 30-40s; the film being cooled to below 50℃ at a cooling rate of 5-8℃ / s after heat-setting; and the rapid cooling locking the molecular chain arrangement structure formed by the biaxial stretching.

[0012] Furthermore, the composite raw material is obtained by high-speed stirring and mixing, including: the high-speed stirring speed is 3000 r / min and the stirring time is 10 min; the ultra-high molecular weight polyethylene has a molecular weight of 2.5 million.

[0013] Furthermore, the longitudinal stretching adopts a gradient heating method, including: the gradient heating is 5°C per segment to reduce molecular chain orientation stress.

[0014] The technical solutions provided by the embodiments of the present invention may include the following beneficial effects: This invention discloses a method for reducing the bidirectional thermal shrinkage of wet-process separator base films. Addressing the issue of insufficient dimensional stability and the risk of internal short circuits in lithium-ion battery separator production under high-temperature environments, this method achieves precise control of molecular chain orientation and crystallization behavior through composite modification of raw materials, optimization of extrusion processes, precise control of bidirectional stretching conditions, and improvement of heat setting and rapid cooling processes. Specifically, the composite modification of ultra-high molecular weight polyethylene, high-density polyethylene, and nano-silica, combined with segmented temperature-controlled extrusion and gradient temperature stretching, significantly reduces residual stress within the film. Furthermore, heat setting and rapid cooling further lock in the orientation structure, ultimately resulting in a substantial reduction in the bidirectional thermal shrinkage rate of the film under high-temperature environments and a significant improvement in dimensional stability. The technical effect of this invention is to effectively improve the structural integrity and safety of wet-process separators, providing a reliable guarantee for the long-term stable operation of lithium-ion batteries, and is particularly suitable for high-end battery applications. Attached Figure Description

[0015] Figure 1 This is a flowchart of a method for reducing bidirectional thermal shrinkage of a wet-process diaphragm base film according to the present invention. Detailed Implementation

[0016] To enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of this specification will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this specification, and not all embodiments. Based on the embodiments in this specification, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this specification.

[0017] like Figure 1 As shown, a method for reducing bidirectional thermal shrinkage of the wet-process separator base film in this embodiment may specifically include: This invention provides a method for reducing the bidirectional thermal shrinkage of wet-process separator base films. By performing composite modification of raw materials, optimizing extrusion process parameters, controlling bidirectional stretching conditions, and improving heat setting and cooling processes, precise control over the molecular chain orientation and crystallization behavior of the wet-process separator base film is achieved, thereby significantly reducing the bidirectional thermal shrinkage rate and improving the dimensional stability of the separator. This method is particularly suitable for the production of wet-process separators for lithium-ion batteries, maintaining the structural integrity of the separator at high temperatures and avoiding the risk of internal short circuits in the battery.

[0018] Figure 1 This is a schematic diagram of the overall process for a method to reduce bidirectional thermal shrinkage of a wet-process diaphragm base film provided in an embodiment of the present invention. Figure 1 As shown, the method includes the following steps.

[0019] Step S1: Using ultra-high molecular weight polyethylene as the main raw material, high-density polyethylene and nano-silica are added for composite modification to obtain a composite raw material.

[0020] Specifically, ultra-high molecular weight polyethylene (UHMWPE) is used as the main raw material. Its high molecular weight provides excellent strength and toughness, but pure UHMWPE is prone to high internal stress and thermal shrinkage during processing. To address this issue, high-density polyethylene (HDPE) and nano-silica are added for composite modification. The addition of HDPE can regulate the overall melt flow and crystallization behavior, while nano-silica, as an inorganic filler, can form physical cross-linking points in the matrix, restricting the thermal movement of molecular chains and thus reducing shrinkage during subsequent heat treatment.

[0021] In one embodiment, ultra-high molecular weight polyethylene (UHMWPE) comprises 90-95 wt%, high-density polyethylene (HDPE) comprises 5-10 wt%, and nano-silica comprises 0.1-0.3 wt%. This ratio ensures that the performance of the main raw materials is dominant, while achieving a modification effect through a small amount of additives. If too little HDPE is added, the modification effect will be insignificant; if too much is added, it may reduce the overall strength. Similarly, if the proportion of nano-silica is too low, it will be difficult to form an effective network; if it is too high, it may lead to uneven dispersion and affect the uniformity of the film.

[0022] Furthermore, the molecular weight of high-density polyethylene is preferably 50,000-100,000. High-density polyethylene in this molecular weight range has good compatibility with ultra-high molecular weight polyethylene, allowing for uniform blending in the molten state to form an interpenetrating network structure, improving melt strength and facilitating subsequent extrusion casting. The particle size of nano-silica is preferably 50-100 nm. Particles in this size range have good dispersibility in the polymer matrix, are not prone to agglomeration, and have a large specific surface area, enabling more effective interaction with polymer segments.

[0023] Step S61: The composite raw materials are obtained by high-speed stirring and mixing until uniform.

[0024] Specifically, to ensure uniform dispersion of all components, high-speed stirring was used for mixing. The stirring speed was 3000 rpm, and the stirring time was 10 minutes. The high speed generates strong shear force, which fully deagglomerates the nano-silica in the polymer powder, while promoting the initial blending of high-density polyethylene and ultra-high molecular weight polyethylene. The stirring time was controlled at 10 minutes to ensure uniform mixing while avoiding excessive stirring that could lead to localized overheating or material degradation.

[0025] In one embodiment, the ultra-high molecular weight polyethylene (UHMWPE) has a molecular weight of 2.5 million. UHMWPE with this molecular weight has extremely long molecular chains, which easily form a highly oriented fibrous structure during stretching, thus giving the diaphragm high strength, but also easily generating significant residual stress. By compounding it with high-density polyethylene (HDPE) with a lower molecular weight, the entanglement stress between the long chains can be alleviated while maintaining strength, laying the foundation for subsequent reduction of heat shrinkage.

[0026] It should be noted that the preparation process of the composite raw material can be carried out in a closed environment to prevent the introduction of moisture or impurities. After mixing, any possible lumps can be removed by filtration through a sieve to ensure uniform particle size. Subsequently, the resulting composite raw material is sealed and stored for use in the extrusion process.

[0027] Through the aforementioned composite modification, the melt flowability of the raw material is optimized, and the crystallization rate is appropriately adjusted, providing a stable processing window for subsequent extrusion casting. Simultaneously, the introduction of nano-silica restricts the free volume of chain segments at the molecular level, preemptively suppressing some thermal shrinkage tendencies.

[0028] Step S2: The composite raw material is fed into an extruder for extrusion casting. The extruder screw is set with segmented temperature control.

[0029] After obtaining the composite raw materials, they are fed into a twin-screw extruder or a single-screw extruder for melt extrusion to form cast sheets. The extrusion process is a key step in the preparation of wet-process diaphragm base membranes, directly affecting the initial thickness uniformity and preliminary molecular chain orientation of the membrane.

[0030] Step S31: The temperature of the extruder screw feed section is 140-150℃, the temperature of the compression section is 160-170℃, and the temperature of the homogenization section is 180-190℃.

[0031] Specifically, the screw section temperature control employs a gradual heating method. The feed section has a lower temperature, which facilitates stable material transport and prevents premature melting that could lead to blockage. The compression section has a moderately higher temperature to promote material compression and initial plasticization. The homogenization section has the highest temperature, ensuring complete melting and uniform mixing of the material. This temperature gradient design reduces temperature fluctuations within the melt, lowers shear heat accumulation, and thus reduces molecular chain degradation and internal stress generation.

[0032] In one embodiment, the temperature of the feeding section is set at 145°C, the compression section at 165°C, and the homogenization section at 185°C. This temperature combination exhibits good stability in actual production, enabling the composite raw materials to melt fully while maintaining high melt strength.

[0033] Step S32: The screw speed of the extruder is controlled at 120-125 r / min to match the traction speed of 50-60 m / min.

[0034] Matching the screw speed with the traction speed is crucial. Excessive speed can lead to overheating of the melt or excessive shearing, while insufficient speed results in low production efficiency. By controlling the screw speed at 120-125 r / min and matching it with a traction speed of 50-60 m / min, stable melt output can be achieved, ensuring uniform casting thickness and a smooth surface.

[0035] For example, in one embodiment, the screw speed is set to 122 r / min and the traction speed is 55 m / min. At this time, the melt is extruded from the die and quickly adheres to the cold roller to form a casting with a stable thickness, and the surface of the casting has no obvious melt cracking or sharkskin phenomenon.

[0036] In step S33, the thickness of the extruded sheet is controlled at 0.3-0.32 mm.

[0037] The thickness of the cast sheet directly affects the subsequent stretching ratio and the final diaphragm thickness. Controlling the thickness within the range of 0.3-0.32 mm satisfies the requirements for multiple stretching ratios while ensuring stability during the stretching process. If the cast sheet is too thick, defects are likely to occur during subsequent stretching; if it is too thin, the strength will be insufficient and it will easily break.

[0038] Specifically, thickness control is achieved by adjusting the die gap, traction speed, and extrusion volume. During production, the thickness of the cast sheet can be monitored in real time, and the traction speed can be adjusted based on feedback to maintain it within the target range.

[0039] It should be noted that a small amount of processing aids, such as antioxidants or lubricants, can be added during the extrusion casting process to further improve melt flowability and stability. However, the amount of additives added must be strictly controlled to avoid affecting the electrochemical performance of the diaphragm.

[0040] By optimizing the extrusion process parameters, the molecular chain orientation degree inside the cast sheet was initially controlled, and the residual stress was low, providing a good initial state for subsequent biaxial stretching. At the same time, the melting behavior of the composite modified raw material was more stable at the segmented temperature, and the crystallinity of the cast sheet was moderate, which facilitated subsequent orientation induction.

[0041] Step S3 involves longitudinally and laterally stretching the casting.

[0042] After the casting is prepared, biaxial stretching is performed to form a microporous structure and improve strength. Biaxial stretching is a core step in the wet-process membrane preparation. By controlling the stretching conditions, the high degree of orientation and fibrillation of the molecular chains can be achieved, while creating conditions to reduce thermal shrinkage.

[0043] Step S41: The longitudinal stretching temperature is 90-100℃, and the stretching ratio is 4.0-4.5 times.

[0044] Longitudinal stretching is performed at lower temperatures, which is beneficial for forming highly oriented lamellar structures. A temperature range of 90-100℃ allows the molecular chains to fully untangle and align along the stretching direction under the stretching force, while avoiding stress relaxation caused by excessively high temperatures.

[0045] In one embodiment, the longitudinal stretching temperature is set to 95°C, and the stretching ratio is 4.2 times. At this point, the strength of the stretched film is significantly improved, and the longitudinal orientation is high, providing support for subsequent transverse stretching.

[0046] Step S42, longitudinal stretching is performed using a gradient heating method.

[0047] Specifically, the longitudinal stretching process is divided into multiple sections, with the temperature gradually increasing in each section. This gradient heating can gradually release the entanglement stress of the molecular chains, avoiding film defects caused by sudden stress concentration.

[0048] Step S71, the gradient temperature is increased by 5°C per segment to reduce molecular chain orientation stress.

[0049] For example, the longitudinal stretching process can be divided into four stages: the first stage is at 90°C, the second stage at 95°C, the third stage at 100°C, and the fourth stage is maintained at 100°C. The design of increasing the temperature by 5°C in each stage allows the molecular chains to be oriented in an orderly manner at gradually increasing temperatures, resulting in a more uniform stress distribution and a significant reduction in the internal stress of the film after stretching.

[0050] In one embodiment, a five-stage gradient heating is employed, gradually increasing the temperature from an initial 88°C to 102°C, with each stage involving an increase of approximately 3-5°C. This refined gradient further optimizes stress release, resulting in a film with a lower tendency to shrink during subsequent heat treatment.

[0051] By using a gradient heating method, longitudinal stretching not only achieves high-ratio orientation but also alleviates some residual stress in advance, laying the foundation for reducing overall bidirectional thermal shrinkage.

[0052] It should be noted that during the longitudinal stretching process, the stretching roller speed gradually increases to match the stretching ratio. The stretching gap is controlled within an appropriate range to avoid scratches or necking of the film surface.

[0053] Step S4: The biaxially stretched film is heat-set and then rapidly cooled.

[0054] After longitudinal and transverse stretching, the molecular chains inside the membrane have formed a highly oriented structure, but a certain degree of residual stress has also accumulated. Without subsequent treatment, these stresses may cause the membrane to shrink and deform under high-temperature conditions, affecting dimensional stability. Therefore, heat setting and rapid cooling are key steps to reduce bidirectional thermal shrinkage. Heat setting uses a high-temperature environment to partially relax and rearrange the molecular chains, locking in the oriented structure, while rapid cooling further fixes this structure, preventing the molecular chains from shrinking back.

[0055] Step S51: The heat setting temperature is 120-130℃, and the holding time is 30-40 seconds.

[0056] Specifically, the heat-setting temperature is controlled at 120-130℃. This temperature range is higher than the stretching temperature but lower than the melting point, allowing the molecular chains to relax moderately after heating, releasing some residual stress, while avoiding excessive relaxation that could damage the orientation structure. The holding time is set to 30-40 seconds to ensure that the molecular chains have sufficient time to adjust their alignment and form a stable crystalline morphology. If the time is too short, the stress will not be fully released; if the time is too long, it may lead to over-crystallization, affecting the flexibility of the film.

[0057] In one embodiment, the heat setting temperature is set to 125°C, and the holding time is 35 seconds. At this temperature, the stress distribution inside the film tends to be uniform, and the dimensional stability of the film after heat setting is significantly improved, especially the shrinkage rate is significantly reduced under high-temperature conditions. This parameter combination shows good adaptability in actual production and is suitable for films of different thicknesses and elongation ratios.

[0058] For example, when producing a wet-process separator with a thickness of 0.016 mm, using the above-mentioned heat-setting conditions, the longitudinal and transverse thermal shrinkage rates of the membrane tested at 150°C were both controlled below 2.0%, far lower than the over 5.0% of the unheat-set membrane. This indicates that heat setting plays an important role in locking molecular chain orientation and reducing thermal shrinkage.

[0059] It should be noted that uniform temperature distribution can be achieved during the heat setting process using hot air circulation or infrared heating. Hot air circulation ensures that both sides of the film are heated evenly, preventing localized overheating that could lead to film deformation. Infrared heating, on the other hand, has strong penetrating power, allowing both the interior and surface of the film to be heated simultaneously, further improving the heat setting effect.

[0060] In one embodiment, heat setting employs a segmented heating method: the first segment is heated to 120°C, the middle segment to 125°C, and the final segment is maintained at 125°C. Each segment is held for approximately 12 seconds, for a total of 36 seconds. This segmented method allows for a smoother molecular chain relaxation process and a more uniform distribution of internal stress in the membrane, making it particularly suitable for membranes with high stretch ratios.

[0061] Through heat setting, the molecular chain orientation structure inside the membrane is further optimized, with some disordered chain segments rearranging into ordered crystals, significantly reducing residual stress. This lays the foundation for subsequent rapid cooling, ensuring the dimensional stability of the final membrane.

[0062] Step S52: After heat setting, the temperature is reduced to below 50°C at a cooling rate of 5-8°C / second.

[0063] After heat setting, the film temperature needs to be rapidly reduced to lock in the molecular chain alignment. A cooling rate of 5-8°C / second is recommended to quickly lower the film temperature below 50°C, preventing molecular chain shrinkage at higher temperatures. If the cooling rate is too slow, the molecular chains may readjust during cooling, leading to increased thermal shrinkage; if it is too fast, it may cause stress concentration on the film surface, affecting flatness.

[0064] In one embodiment, the cooling rate is set to 6.5°C / second, achieved through a combination of cold air blowing and cooling rollers. Cold air blowing rapidly removes heat from the membrane surface, while the cooling rollers further reduce the internal temperature of the membrane through contact conduction. Using this combination, the membrane cools from 125°C to 45°C in approximately 12 seconds, effectively fixing the molecular chain arrangement.

[0065] For example, during production, the surface temperature of the cooling roller is controlled at 20°C, the cold air temperature is 15°C, and the wind speed is 3.0 m / s. At this temperature, the film surface is smooth after cooling, with no obvious warping, and both longitudinal and transverse thermal shrinkage rates remain at low levels. This rapid cooling method is particularly suitable for continuous production, improving production efficiency while ensuring film performance.

[0066] It should be noted that excessive contact between the film and the cooling roller should be avoided during the cooling process to prevent scratches or adhesion. An anti-stick coating can be applied to the surface of the cooling roller, or the contact pressure between the film and the roller surface can be controlled by adjusting the tension. Furthermore, the humidity of the cooling environment should be controlled below 50% to prevent moisture absorption by the film surface from affecting subsequent performance.

[0067] Step S53: Rapid cooling locks in the molecular chain arrangement structure formed by biaxial stretching.

[0068] Specifically, the purpose of rapid cooling is to transform the molecular chains from a high-temperature state to a low-temperature state in a short period of time, restricting their thermal mobility and thus permanently fixing the orientation structure and microporous morphology formed by biaxial stretching. After cooling to below 50°C, the mobility of the molecular chains is significantly reduced, the crystal morphology tends to be stable, and the size of the film is less likely to change during subsequent use.

[0069] In one embodiment, the cooling process is divided into two stages. The first stage uses high-speed cold air blowing at a rate of 7°C / second to cool the film to 70°C. The second stage uses cooling rollers for contact cooling at a rate of 5°C / second, ultimately reducing the temperature to 40°C. This staged cooling method can balance speed and uniformity, avoiding the generation of new stress gradients inside the film.

[0070] For example, when producing ultra-thin separators with a thickness of 0.012 mm, the aforementioned staged cooling method resulted in no obvious shrinkage marks on the membrane surface after cooling. Thermal shrinkage tests showed a longitudinal shrinkage of 1.8% and a transverse shrinkage of 1.5%, fully meeting the requirements for lithium-ion battery separators in high-temperature environments. This cooling method rapidly locks in the molecular chain structure, ensuring dimensional stability of the membrane during high-temperature cycling and preventing short-circuit risks caused by contact with electrodes.

[0071] Rapid cooling effectively preserves the microporous structure and orientation morphology of the membrane, further suppressing its tendency to thermal shrinkage. This not only improves the membrane's mechanical properties but also enhances its safety under high-temperature environments, ensuring the long-term stable operation of lithium-ion batteries.

[0072] Step S43: The transverse stretching temperature is 110-120℃, and the stretching ratio is 3.5-4.0 times.

[0073] After longitudinal stretching, transverse stretching is required to achieve balanced orientation of the membrane. The transverse stretching temperature is controlled at 110-120℃, higher than the longitudinal stretching temperature, which is beneficial for the rearrangement of molecular chains in the transverse direction, while avoiding excessive temperature that could lead to relaxation of the longitudinal orientation. The stretching ratio is 3.5-4.0 times, forming an appropriate proportion with the longitudinal stretching ratio, to ensure uniform strength and porosity distribution of the membrane in both directions.

[0074] In one embodiment, the transverse stretching temperature is set to 115°C, and the stretching ratio is 3.8 times. At this point, the transverse strength of the membrane is significantly improved, forming a better match with the longitudinal strength, resulting in more balanced overall mechanical properties. After stretching, the microporous structure of the membrane is initially formed, with the pore size distribution concentrated in the range of 10-50 nanometers, which is suitable for subsequent solvent extraction to form the final pores.

[0075] For example, when producing a separator with a thickness of 0.020 mm, using the above-mentioned transverse stretching conditions, the transverse tensile strength of the membrane reaches 120 MPa, which is close to the longitudinal tensile strength of 130 MPa, indicating that the bidirectional orientation effect is good. This balanced orientation enables the membrane to withstand multi-directional stress during battery assembly, avoiding local tearing or deformation.

[0076] It should be noted that lateral stretching is typically performed on a tenter frame, achieved by gradually pulling the film apart from both sides. The tenter frame track must be kept straight to avoid uneven stress on the film edges, which could lead to thickness fluctuations. Furthermore, infrared preheating or hot air preheating can be used to ensure the film reaches the target temperature before entering the stretching zone, guaranteeing a smooth stretching process.

[0077] Step S44: The transverse stretching rate is controlled at 10-15% / second.

[0078] The transverse stretching rate directly affects the orientation uniformity and micropore formation of the membrane. Controlling the rate at 10-15% / second allows the molecular chains to gradually adjust during stretching, avoiding membrane breakage or uneven porosity caused by excessively rapid stretching. If the rate is too low, production efficiency decreases and may lead to molecular chain relaxation, affecting the orientation effect.

[0079] In one embodiment, the transverse stretching rate is set to 12% / second, achieved by adjusting the clamping speed of the tenter frame. During the stretching process, the membrane width gradually expands from an initial 1.0 meter to 3.8 meters, achieving a stretching ratio of 3.8 times. After stretching, the membrane surface is smooth, without obvious wrinkles or tears, and the pores are evenly distributed.

[0080] For example, during the production process, the transverse stretching rate can be fine-tuned for castings of different thicknesses. For castings with a thickness of 0.30 mm, the rate is set to 11% / second; for castings with a thickness of 0.32 mm, the rate is adjusted to 13% / second to accommodate different initial stress states. This flexible adjustment method ensures the stretching quality of films of different specifications and adapts to diverse production needs.

[0081] By controlling the lateral stretching rate, the microporous structure of the membrane becomes more uniform, and the consistency of pore size and distribution is improved. This provides a good foundation for subsequent solvent extraction and heat setting, while reducing the membrane's tendency to thermally shrink in the lateral direction.

[0082] Step S45: Lateral stretching and longitudinal stretching form a complementary orientation.

[0083] Specifically, longitudinal stretching aligns the molecular chains along the membrane's direction of travel, forming a longitudinally fibrous structure, while transverse stretching orients the molecular chains in the vertical direction, forming a network cross-linked structure. The combination of these two methods creates a complementary orientation, giving the membrane high strength and stability in both directions. This complementary orientation effectively disperses external forces, preventing tearing caused by unidirectional stress concentration, and simultaneously reduces the membrane's bidirectional shrinkage rate under high-temperature conditions.

[0084] In one embodiment, the longitudinal stretch ratio is set to 4.2 times and the transverse stretch ratio is 3.8 times, forming an orientation ratio that is close to but not completely equal. This ratio design makes the film slightly stronger in the longitudinal direction than in the transverse direction, which is suitable for the large longitudinal force during battery winding, while the transverse strength is also sufficient to support the tension during assembly.

[0085] For example, when producing a separator with a thickness of 0.018 mm, using the above-mentioned stretching ratio combination, the thermal shrinkage rate of the membrane in the longitudinal and transverse directions is controlled at 2.2% and 1.9%, respectively, with good overall dimensional stability. This complementary orientation design enables the membrane to exhibit excellent resistance to deformation during high-temperature cycling tests of the battery, avoiding electrode contact problems caused by separator shrinkage.

[0086] It should be noted that achieving complementary orientation also requires consideration of the stretching sequence and condition matching. Longitudinal stretching is usually performed before transverse stretching to avoid damage to the longitudinal orientation caused by transverse stretching. In addition, after stretching, the orientation degree and thickness distribution of the film can be monitored using online detection equipment, and parameters can be adjusted in a timely manner to ensure bidirectional orientation.

[0087] The complementary orientation formed by biaxial stretching comprehensively optimizes the mechanical properties and microporous structure of the membrane, providing an ideal initial state for subsequent heat setting and cooling. This structural design not only improves the strength and toughness of the membrane but also creates favorable conditions for reducing thermal shrinkage.

[0088] In one embodiment, for the production of high-performance lithium-ion battery separators, the biaxial stretching and heat-setting cooling parameters were comprehensively optimized. The longitudinal stretching temperature was 95°C, with a multiplication factor of 4.3 times, using a five-stage gradient heating method; the transverse stretching temperature was 116°C, with a multiplication factor of 3.7 times, and a rate of 12% / second; the heat-setting temperature was 126°C, with a holding time of 36 seconds; and the cooling rate was 6.8°C / second, reducing the temperature to 42°C. Using this parameter combination, the membrane exhibits a longitudinal thermal shrinkage rate of 1.7%, a transverse shrinkage rate of 1.5%, a porosity of 40%, and good air permeability, fully meeting the performance requirements of high-end batteries for separators.

[0089] For example, based on the above embodiments, the lateral stretching ratio and heat setting time can be further fine-tuned to meet the requirements of different battery types. For separators used in energy storage batteries, the lateral stretching ratio is adjusted to 3.9 times, and the heat setting time is extended to 38 seconds to enhance lateral strength and dimensional stability; for separators used in power batteries, the lateral stretching ratio remains at 3.7 times, and the heat setting time is shortened to 34 seconds to balance flexibility and production efficiency. This targeted adjustment can adapt to the needs of different application scenarios and improve the applicability of the membrane.

[0090] It should be noted that the process parameters for biaxial stretching and heat setting cooling need to be comprehensively optimized based on the raw material ratio and the thickness of the cast sheet. For example, when the proportion of ultra-high molecular weight polyethylene is increased to 95 wt%, the stretching temperature needs to be appropriately reduced and the heat setting time increased to accommodate the higher degree of molecular chain entanglement; when the proportion of nano-silica is increased to 0.3 wt%, the cooling rate can be slightly increased to enhance the effect of molecular chain locking.

[0091] In one embodiment, for the production of ultrathin separators, the cast sheet thickness is controlled at 0.30 mm, the longitudinal stretching ratio is 4.5 times, the transverse stretching ratio is 4.0 times, the heat setting temperature is 128°C, the holding time is 40 seconds, and the cooling rate is 7.5°C / second. Using this process, the final membrane thickness reaches 0.009 mm, with both longitudinal and transverse thermal shrinkage rates below 1.5%, and a porosity of 42%, exhibiting extremely high dimensional stability and electrochemical performance. This process is particularly suitable for the separator requirements of high-energy-density batteries.

[0092] For example, in the production of the aforementioned ultra-thin membrane, the lateral stretching employs segmented rate control: 10% / second for the first half and 14% / second for the second half, to gradually adapt to changes in membrane stress. During heat setting, the hot air circulation speed is controlled at 2.5 m / s to ensure temperature uniformity. The cooling stage combines cold air and cooling rollers, with the cooling roller temperature set at 18°C ​​and the cold air temperature at 12°C. Through these refined controls, the membrane maintains high strength and uniformity even in its ultra-thin state, avoiding the common problems of easy tearing and high shrinkage rates found in ultra-thin films.

[0093] Through the comprehensive process of biaxial stretching, heat setting, and rapid cooling described above, the molecular chain orientation and crystallization behavior of the wet-process separator base film are precisely controlled, residual stress is effectively released, and the thermal shrinkage rate is significantly reduced. This method can greatly improve dimensional stability while ensuring the mechanical properties and microporous structure of the film, providing a reliable guarantee for the safety and cycle life of lithium-ion batteries.

[0094] In one embodiment, heat setting and cooling parameters are adjusted to adapt to extreme conditions and meet the membrane performance requirements under different ambient temperatures. For battery membranes used in high-temperature environments, the heat setting temperature is increased to 129°C, the holding time is extended to 39 seconds, and the cooling rate is set to 7.8°C / second to further enhance the molecular chain locking effect. Test results show that the membrane's thermal shrinkage rate is still controlled below 1.8% at 160°C, demonstrating excellent high-temperature shrinkage resistance.

[0095] For example, in the production of the aforementioned high-temperature separator, heat setting employs a three-stage heating process: the first stage at 122°C, the middle stage at 126°C, and the final stage at 129°C, with each stage held at that temperature for 13 seconds to ensure the molecular chains gradually relax and form stable crystals. During cooling, the cold air temperature is reduced to 10°C, the wind speed is increased to 3.5 m / s, and the cooling roller temperature is set to 15°C to achieve a faster cooling rate. This process design allows the membrane to maintain its structural integrity under extreme high-temperature conditions, avoiding the risk of battery failure due to separator shrinkage.

[0096] It should be noted that the heat setting and cooling processes also need to consider the continuity and stability of the production equipment. The lengths of the heat setting and cooling zones need to be rationally designed according to the production speed to ensure that the film reaches the target temperature and holding time as it passes through each zone. In addition, the temperature sensors and fan speed controllers need to be calibrated regularly during equipment operation to ensure the accuracy of process parameters.

[0097] In one implementation, the equipment configuration for heat setting and cooling is optimized for large-scale continuous production. The heat setting zone employs a multi-layer hot air circulation system, with each layer's airflow speed and temperature independently adjustable to ensure consistent heating of the upper and lower surfaces of the film. The cooling zone is equipped with multiple sets of cooling rollers and cold air nozzles, with adjustable roller spacing to accommodate different film thicknesses and production speeds. Using this equipment configuration, the film's thermal shrinkage rate remains below 2.0% even under high-speed production, significantly improving production efficiency.

[0098] For example, with the above equipment configuration, the production speed is increased to 65 meters per minute, the heat setting temperature is set to 127°C, the holding time is 35 seconds, and the cooling rate is 6.5°C / second. The longitudinal thermal shrinkage rate of the film is 1.9%, the transverse rate is 1.7%, and the thickness uniformity is controlled within ±0.001 mm. This optimized design allows the process to maintain the performance stability of the film during high-efficiency production, meeting the needs of large-scale industrial applications.

[0099] The above description of the embodiments is only for the purpose of helping to understand the technical solutions and core ideas of this application; those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for reducing bidirectional thermal shrinkage of wet-process diaphragm base film, characterized in that, include: A composite material was obtained by adding high-density polyethylene and nano-silica to ultra-high molecular weight polyethylene as the main raw material. The composite raw material is fed into an extruder to prepare extruded cast sheets, and the extruder screw is equipped with segmented temperature control. The casting is subjected to longitudinal and transverse stretching; The biaxially stretched film is heat-set and then rapidly cooled.

2. The method as described in claim 1, characterized in that, The composite material obtained by modifying ultra-high molecular weight polyethylene (UHMWPE) with high-density polyethylene (HDPE) and nano-silica includes: The ultra-high molecular weight polyethylene accounts for 90-95 wt%, the high-density polyethylene accounts for 5-10 wt%, and the nano-silica accounts for 0.1-0.3 wt%. The high-density polyethylene has a molecular weight of 50,000-100,000, and the nano-silica has a particle size of 50-100 nm. The composite raw materials are obtained by high-speed stirring and mixing until uniform.

3. The method as described in claim 1, characterized in that, The process of feeding the composite raw material into an extruder for extrusion casting includes the following: the extruder screw is equipped with segmented temperature control. The temperature of the extruder screw feed section is 140-150℃, the temperature of the compression section is 160-170℃, and the temperature of the homogenization section is 180-190℃; The screw speed of the extruder is controlled at 120-125 r / min to match the traction speed of 50-60 m / min; The thickness of the extruded sheet is controlled between 0.3 and 0.32 mm.

4. The method as described in claim 1, characterized in that, The longitudinal and transverse stretching of the casting includes: The longitudinal stretching temperature is 90-100℃, and the stretching ratio is 4.0-4.5 times; The longitudinal stretching is performed using a gradient heating method; The transverse stretching temperature is 110-120℃, and the stretching ratio is 3.5-4.0 times; The transverse stretching rate is controlled at 10-15% / s; The lateral stretching and the longitudinal stretching form a complementary orientation.

5. The method as described in claim 1, characterized in that, The process of heat-setting the biaxially stretched film followed by rapid cooling includes: The heat setting temperature is 120-130℃, and the holding time is 30-40s; After heat setting, the temperature is reduced to below 50°C at a cooling rate of 5-8°C / s; The rapid cooling locks in the molecular chain arrangement structure formed by biaxial stretching.

6. The method as described in claim 2, characterized in that, The composite raw material is obtained by high-speed stirring and uniform mixing, comprising: The high-speed stirring speed is 3000 r / min, and the stirring time is 10 min; The ultra-high molecular weight polyethylene has a molecular weight of 2.5 million.

7. The method as described in claim 4, characterized in that, The longitudinal stretching employs a gradient heating method, including: The gradient heating is performed by increasing the temperature by 5°C in each stage to reduce molecular chain orientation stress.

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