A method and apparatus for producing a biaxially oriented diaphragm

Abstract

The application discloses a diaphragm production method and device of bidirectional synchronous stretching, wherein the diaphragm production method is after the feeding, extruding and casting piece processes, the diaphragm is subjected to the processes of cutting switching, assembling double stretching, extraction, assembling horizontal stretching and multi-station winding, each partition in the assembling double stretching and the assembling horizontal stretching comprises a temperature control box, the diaphragm is sequentially subjected to preheating, stretching, heat setting and cooling in each temperature control box, each temperature control box can simultaneously realize temperature gradient control from the upper and lower directions and the direction of the film surface, and the air inlet speed can also be controlled individually, so that the problem of the speed and temperature difference existing on both sides and the middle of the film surface during stretching is solved, and the consistency of stretching and setting is realized. After the diaphragm is cut and switched, the diaphragm vertically advances, the gravity of paraffin oil between diaphragm molecules is the same as the direction of vertical stretching stress, the motion timeliness of diaphragm molecular chains is better, stress transmission is more uniform during the stretching process, a compact micro-fiber network structure is formed, and the stretching strength and puncture strength of the diaphragm are improved.

Description

Technical Field

[0001] This invention relates to a method and apparatus for producing a bidirectional synchronously stretched separator, belonging to the field of wet-process lithium battery separator production technology. Background Technology

[0002] The wet process for lithium-ion battery separators involves adding PE and paraffin oil (white oil) separately to a twin-screw extruder. After plasticizing in the extruder, the extruder delivers the plasticized material to the die head to extrude a film. The oil-containing film is then cooled and shaped before undergoing horizontal biaxial stretching in a synchronous biaxial stretching machine. After extraction and subsequent transverse stretching heat treatment, the film is sent to a winding machine for winding.

[0003] In the synchronous or asynchronous stretching process of wet-process lithium-ion battery separators, the oil film stretching stage has a significant impact on the separator's microstructure and mechanical properties. It is a critical stage for the formation of basic physical properties and the core stretching process for process control. Asynchronous sequential stretching of the film requires the oil-containing film, after being plasticized and cooled in an extruder, to be stretched longitudinally first, then transversely. It then undergoes the same extraction process as the synchronous stretching process, as well as a heat treatment process after extraction followed by transverse stretching, before being sent to a winding machine for winding, completing the entire winding process. However, both synchronous and asynchronous stretching processes use horizontal stretching with air intake from the top and bottom and return air from the sides. In wide-width separator stretching, the transverse airflow temperature differs between the edges and the middle (because the edges are closer to the airflow chamber wall, resulting in heat loss). Furthermore, as the width increases, the uniformity of the stretched thickness increases, leading to significant differences in thickness and permeability between the edges and the middle of the separator. Meanwhile, during the oil film stretching stage, oil mist accumulation is prone to occur in the stretching space. The accumulation of oil droplets on the oil film surface leads to changes in tensile stress and local melting point changes in the oil film, thus affecting the stretching consistency. Ultimately, after stretching, the air permeability, thickness, basis weight, strength, and puncture resistance of the diaphragm show significant differences between the two sides and the middle, affecting the stretching consistency.

[0004] In terms of process compatibility, the wet synchronous stretching process requires precise temperature matching for preheating, stretching, and heat treatment steps, as well as adaptation of the stretching strain rate to the temperature. If the temperature and stretching strain rate are not properly matched, or if there are significant differences in stretching temperature consistency, it can lead to insufficient optimization of the overall separator structure, reduced strength, and poor consistency in thickness and porosity, making it difficult to achieve good stretching consistency and stable yield. Equipment for the wet-process lithium-ion battery separator synchronous or asynchronous stretching process is designed and manufactured primarily based on parameters such as product width, machine speed, and process conditions. Once the separator width, machine speed, and process parameters are given, the separator production line specifications are determined. Therefore, after the stretching equipment production line is designed and selected, the structural dimensions of the stretching equipment are also fixed; that is, the dimensions of the lithium-ion battery separator stretching equipment are fixed for each width and machine speed. With fixed width and machine speed, it is difficult to optimize or adjust the stretching equipment or the process. Summary of the Invention

[0005] The purpose of this invention is to provide a method and apparatus for producing diaphragms by bidirectional synchronous stretching, so as to improve the uniformity and stability of diaphragm stretching and solve the technical problem that the performance of diaphragms produced by the prior art is significantly different on both sides and in the middle.

[0006] The bidirectional synchronous stretching diaphragm production method of the present invention adopts the following technical solution: A bidirectional synchronous stretching diaphragm production method, characterized in that, after the feeding, extrusion, and casting processes, the following steps are performed: Separation and reversal: The cast oil film is divided into two or more parallel independent cast oil films; each independent cast oil film is rotated 90° to one side to change its orientation, and each independent cast oil film changes from a state parallel to the horizontal plane to a vertical state perpendicular to the horizontal plane; Assembly and bidirectional stretching: Each independent cast oil film is bidirectionally stretched in the vertical state, and the bidirectional stretching includes preheating, longitudinal and transverse stretching, heat setting, and cooling steps in sequence. After bidirectional stretching, each cast oil film forms a porous structure. Oil film extraction: The porous oil film enters the extraction device vertically for vertical extraction. The paraffin oil in the porous oil film is extracted using an extractant. Each porous oil film is then dried to obtain a thin film. Assembly and transverse stretching: Each thin film is stretched laterally in a vertical state. The transverse stretching includes preheating, stretching, heat setting, and cooling steps. After transverse stretching, a diaphragm is obtained. Multi-station winding: Each diaphragm is wound up at different stations. In the assembly double stretching and assembly transverse stretching, each step uses air intake from both sides and return air from the top and bottom. The air intake enters horizontally in a direction perpendicular to the diaphragm surface, and the return air is discharged from the top and bottom edges of the diaphragm. The temperature of each step in the assembly double stretching and assembly transverse stretching is independently controlled by the air intake.

[0007] In the assembly double stretching process, the preheating temperature is 100-130℃, the longitudinal and transverse stretching ratio is 5-9, the longitudinal and transverse stretching temperature is 110-135℃, the heat setting temperature is 110-135℃, and the cooling temperature is 15-25℃.

[0008] In the biaxial stretching process, the preheating step is divided into 3 zones, the biaxial stretching step into 6 zones, the heat setting step into 3 zones, and the cooling step into 3 zones; the tensile strain rate in the biaxial stretching step is 0.1–0.2 s⁻¹. -1 The air intake for the preheating, biaxial stretching, heat setting and cooling steps is divided into three sections vertically from top to bottom. The air intake temperature of the upper and lower sections is 1 to 3°C higher than that of the middle section.

[0009] In the extraction process, the extraction temperature is 5–25℃ and the extraction time is 1–3 min.

[0010] In the assembly and stretching process, the preheating step is divided into 2 zones, the stretching step into 4 zones, the heat setting step into 10 zones, and the cooling step into 3 zones; the tensile strain rate in the stretching step is between 0.02 and 0.05 s⁻¹. -1 The air intake for the preheating, stretching, heat setting and cooling steps is divided into three sections vertically from top to bottom. The air intake temperature of the upper and lower sections is 1 to 3°C higher than that of the middle section.

[0011] In each step of the assembly of the double-pull process and the assembly of the horizontal pull process, oil mist is adsorbed and purified to improve the cleanliness of the air field.

[0012] The bidirectional synchronous stretching diaphragm production apparatus of the present invention adopts the following technical solution: A bidirectional synchronous stretching diaphragm production apparatus includes a separating and reversing device, an assembly double stretching device, an extraction device, and an assembly horizontal stretching device. The separating and reversing device includes a slitting mechanism and a reversing mechanism. The slitting mechanism is used to slit the cast oil film into two or more independent cast oil films. The reversing mechanism includes a set of steering rollers, which is used to rotate each independent cast oil film 90° to one side, so that the cast oil film is in a vertical state perpendicular to the horizontal plane. The assembly double stretching device includes a number of independent cast oil films... The extraction device includes an extraction tank for vertical extraction of the diaphragm, and a vertically arranged roller assembly is provided in the extraction tank. The horizontal stretching zone includes horizontal stretching units equal in number to the independent cast film oil film. Each double stretching unit and each horizontal stretching unit includes a preheating zone, a stretching zone, a heat setting zone, and a cooling zone in sequence. Each zone of the double stretching unit and the horizontal stretching unit includes a temperature control box. The film surface passes through each temperature control box in a horizontal direction in sequence. Each temperature control box has independent air inlet and outlet. Each temperature control box in each zone includes an air inlet box located on the left and right sides of the film surface and an air outlet box located above and below the film surface.

[0013] The walls of the temperature control box on both sides of the diaphragm are provided with oil-absorbing fiber layers.

[0014] The temperature control box is equipped with chain clamping mechanisms on both the upper and lower sides for clamping the edge of the film surface.

[0015] In the double-stretch unit, there are 3 temperature control boxes in the preheating zone, 6 temperature control boxes in the stretching zone, 3 temperature control boxes in the heat setting zone, and 3 temperature control boxes in the cooling zone; in the horizontal stretching unit, there are 2 temperature control boxes in the preheating zone, 4 temperature control boxes in the stretching zone, 10 temperature control boxes in the heat setting zone, and 3 temperature control boxes in the cooling zone.

[0016] The beneficial effects of this invention are as follows: This invention adds the processes of separation and reversal, assembly double stretching, vertical extraction, and assembly horizontal stretching. The oil film thick sheet formed by feeding, extrusion, and casting is first separated into independent cast oil films by the separation and reversal process, and then realizes a vertical state from parallel to the horizontal plane to perpendicular to the horizontal plane. The assembly double stretching achieves parallel longitudinal and transverse stretching. After vertical extraction, it enters the assembly horizontal stretching process for stretching and shaping. Finally, the diaphragm production is completed by multi-station winding.

[0017] Both the assembly of double stretching and assembly of horizontal stretching are set up with parallel double stretching units and horizontal stretching units according to the number of independent cast oil films. Each stretching device is equipped with preheating, stretching, heat setting and cooling functional areas. Each parallel stretching unit operates independently and can be matched with the cutting specifications according to the die head size. Each independent parallel stretching unit can operate at different stretching ratios, making efficient use of three-dimensional elevation space to arrange the production line, improving production efficiency and adapting to the stretching process.

[0018] Each section of the assembled double-stretch and assembled horizontal-stretch systems includes a temperature control chamber. The diaphragm undergoes preheating, stretching, heat setting, and cooling sequentially within each chamber. Each chamber can achieve gradient temperature control, simultaneously controlling temperature gradients from top to bottom and from the direction of membrane travel. The vertical air inlet side is divided into upper, middle, lower, and more functional air zones to meet gradient temperature control requirements based on the stretching process, ensuring process controllability of the diaphragm stretching. Each temperature control chamber is equipped with chain clamping mechanisms at the top and bottom. Each chain clamping mechanism includes chains with clamps that tightly grip the two edges of the membrane surface and support it on guide rails with variable longitudinal and transverse dimensions, enabling dynamic stretching in the same direction and synchronously through the two chain clamps. Each temperature control box in both the double-stretch and horizontal-stretch assembly systems features a structure with horizontal air intake on both sides and vertical air return. Air intake is controlled at 5–35 m / s, adjustable via electrically controlled air valves with a speed control accuracy of ±0.5 m / s. This precise speed regulation resolves the issue of wind speed and temperature differences between the sides and the middle of the membrane during stretching, ensuring consistent stretching and shaping. After stretching, the differences in air permeability, thickness, basis weight, strength, and puncture resistance between the sides and the middle of the diaphragm are reduced, improving the consistency of diaphragm stretching.

[0019] Both the assembled double-stretch and assembled horizontal-stretch devices adopt a vertical structure. The diaphragm is stretched vertically. The gravitational force of the paraffin oil between diaphragm molecules is in the same direction as the vertical stretching stress, resulting in better diaphragm molecular chain movement and more uniform stress transmission during the stretching process. This forms a dense microfiber network structure, thereby improving the tensile strength and puncture strength of the diaphragm.

[0020] An oil-absorbing fiber layer is installed on the air inlet side of the temperature control box to effectively remove the oil mist evaporated by paraffin oil during the assembly of the double stretching process. This effectively solves the problems of thin spots and uneven stretching caused by oil droplets falling on the horizontal oil film during horizontal stretching in the existing technology. Moreover, the paraffin oil that is separated by vertical stretching of the oil film is easier to fall and less likely to accumulate on the membrane surface, thus improving the appearance quality of the diaphragm.

[0021] In summary, this invention achieves uniform preheating, stretching, heat setting, and cooling of the non-roller contact area of ​​the oil film, reducing temperature differences across different regions of the diaphragm while ensuring temperature consistency during lateral heating and cooling. This results in more thorough and uniform stretching, leading to better consistency and stability of the diaphragm. The simultaneous stretching process employs a flexible assembly structure with slit parallel stretching units and series-connected stretching zones, improving the stretching strain rate and temperature adaptability of the stretching device, ensuring stretching consistency, and increasing diaphragm yield. This efficient use of elevation space reduces floor space. The oil-absorbing fiber layer adsorbs and purifies the oil in the stretching unit, improving airflow cleanliness, reducing diaphragm appearance defects, and enhancing overall appearance quality. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the bidirectional synchronous stretching diaphragm production apparatus of an embodiment of the present invention, showing the division of the bidirectional stretching unit and the transverse stretching unit. Figure 2 This is a schematic diagram of the independent parallel arrangement of stretching units in a bidirectional synchronous stretching diaphragm production apparatus according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the temperature control box in the biaxial stretching unit and the transverse stretching unit of the biaxial synchronous stretching diaphragm production device according to an embodiment of the present invention; Figure 4 This is a flowchart of a bidirectional synchronous stretching diaphragm production method according to an embodiment of the present invention; Figure 5 These are the physical property data of a 7-micron membrane prepared according to one embodiment of the present invention; Figure 6 These are the physical property data of a 7-micron membrane prepared using existing technology.

[0023] In the figure: 1-Stretching unit, 1.1-Preheating zone, 1.2-Stretching zone, 1.3-Heat setting zone, 1.4-Cooling zone, 2-Temperature control box, 2.1-Air inlet box, 2.2-Air outlet box, 2.3-Oil-absorbing fiber layer, 2.4-Chain clamping mechanism, 3-Film surface. Detailed Implementation

[0024] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0025] An embodiment of the present invention discloses a bidirectional synchronous stretching diaphragm production apparatus, comprising a separating and reversing device, an assembly double-stretching device, an extraction device, and an assembly horizontal stretching device. The separating and reversing device is disposed after the casting device and includes a slitting mechanism and a reversing mechanism. The slitting mechanism is used to slit the cast oil film into two or more independent cast oil films. The reversing mechanism includes a set of steering rollers, which is used to rotate each independent cast oil film 90° to one side, so that the cast oil film is in a vertical state perpendicular to the horizontal plane. The assembly double-stretching device includes a double-stretching unit equal to the number of independent cast oil films. The extraction device includes an extraction tank for vertical extraction of the diaphragm, the number of which is equal to the number of independent cast oil films, and the extraction tank is provided with a vertically arranged roller set. The assembly horizontal stretching zone includes a horizontal stretching unit equal to the number of independent cast oil films. Each double-stretching unit and each horizontal stretching unit sequentially includes a preheating zone 1.1, a stretching zone 1.2, a heat setting zone 1.3, and a cooling zone 1.4. Figure 1 The diagram shows a top view of each zone, with the arrows indicating the forward direction of the membrane surface. Each zone of the double-pull unit and the horizontal pull unit includes a temperature control box 2. The membrane surface 3 passes horizontally through each temperature control box 2 sequentially. Each temperature control box 2 has independent air inlet and outlet. Each zone's temperature control box 2 includes an air inlet box 2.1 located on the left and right sides of the membrane surface and an air outlet box 2.2 located above and below the membrane surface. The temperature control box 2 has chain clamping mechanisms 2.4 on its upper and lower sides for clamping the edges of the membrane surface. The walls of the temperature control box 2 on both sides of the membrane surface are provided with oil-absorbing fiber layers 2.3. The structure of the temperature control box is as follows... Figure 3 As shown, Figure 3 The middle film surface 3 advances in a direction perpendicular to the paper surface.

[0026] In this embodiment, in the double-stretch unit, there are 3 temperature control boxes in the preheating zone, 6 temperature control boxes in the stretching zone, 3 temperature control boxes in the heat setting zone, and 3 temperature control boxes in the cooling zone; in the horizontal stretching unit, there are 2 temperature control boxes in the preheating zone, 4 temperature control boxes in the stretching zone, 10 temperature control boxes in the heat setting zone, and 3 temperature control boxes in the cooling zone.

[0027] For ease of description, both the double-tension unit and the transverse tension unit are referred to as tension units. In the assembly of the double-tension device and the assembly of the transverse tension device, the number of tension units is equal to the number of independent cast sheet oil films after slitting. Each independent cast sheet oil film enters its respective tension unit, and the tension units are independently arranged in parallel, as shown in the following diagram. Figure 2 As shown.

[0028] An embodiment of the present invention provides a method for producing a bidirectional synchronously stretched diaphragm, which includes, after feeding, extrusion, and casting processes, separation and reversal, bidirectional stretching, extraction, cross stretching, and multi-station winding processes, as follows: Figure 4 As shown, each process is specifically as follows: (1) Feeding: Paraffin oil and polyethylene raw materials are preheated and mixed by paraffin oil conveying and powder conveying devices, respectively, and then metered and conveyed to the extruder; the mass ratio of polyethylene raw material to paraffin oil raw material is 1.6~2.3:7.7~8.4. Paraffin oil is conveyed by injecting oil into the extruder through 3 channels, with the oil injection rate of each channel being 100~300Kg / h, and the polyethylene conveying rate being 100~300Kg / h.

[0029] (2) Extrusion: The paraffin oil and polyethylene raw materials entering the extrusion system are mixed and heated to melt in the extruder, and then extruded through the die to obtain a polyethylene-paraffin oil mixed melt with a uniform phase. The extrusion temperature used in the extruder is 200℃~235℃.

[0030] (3) Casting: The mixed melt obtained in step (2) is cooled and shaped under the cooling of the casting roller with a roller surface temperature of 0 to 15°C to obtain a casting oil film with a thickness of 0.5 mm to 5 mm and a width of 350 mm to 900 mm.

[0031] (4) Divide and change direction: Divide the cast oil film into two or more parallel independent cast oil films. According to the process requirements, the cast oil film can be divided into parallel independent cast oil films with specifications of 175mm to 450mm. Flip each independent cast oil film to one side by 90° to change the direction of the cast oil film. Each independent cast oil film changes from a state parallel to the horizontal plane to a vertical state perpendicular to the horizontal plane.

[0032] (5) Assembly and bidirectional stretching: Each independent cast oil film is bidirectionally stretched in a vertical state. The bidirectional stretching includes preheating, longitudinal and transverse stretching, heat setting and cooling steps. After bidirectional stretching, each cast oil film forms a porous oil film. Each step of this process adopts air intake from both sides and air return from the top and bottom. The air intake is horizontally introduced from a direction perpendicular to the diaphragm surface, and the air return is discharged from the top and bottom edges of the diaphragm. The temperature of each step is independently controlled by the air intake.

[0033] In the assembly double stretching process, the preheating temperature is 100-130℃, the longitudinal and transverse stretching ratio is 5-9, the longitudinal and transverse stretching temperature is 110-135℃, the heat setting temperature is 110-135℃, and the cooling temperature is 15-25℃.

[0034] In the biaxial stretching process, the preheating step is divided into 3 zones, the biaxial stretching step into 6 zones, the heat setting step into 3 zones, and the cooling step into 3 zones; the tensile strain rate in the biaxial stretching step is 0.1–0.2 s⁻¹. -1 The air intake for the preheating, biaxial stretching, heat setting and cooling steps is divided into three sections vertically from top to bottom. The air intake temperature of the upper and lower sections is 1-3°C higher than that of the middle section, so as to avoid the air permeability of the two sides of the diaphragm being higher than that of the middle.

[0035] (6) Extraction: The porous oil film after being assembled and stretched vertically enters the extraction device for vertical extraction. The number of extraction devices is equal to the number of independent cast oil films. The paraffin oil in the porous oil film is extracted using an extractant. Each porous oil film is extracted and dried to obtain a thin film. In the extraction process, the extraction temperature is 5-25℃, preferably 15-20℃, and the extraction time (total time of the film in dichloromethane liquid) is 1-3 min, preferably 1.5 min.

[0036] The extracted membrane is dried within a specific temperature range to ensure complete evaporation of the extractant. The drying temperature is 20–60°C, preferably 35–50°C.

[0037] (7) Assembly and horizontal stretching: Each film is stretched horizontally in a vertical state. The horizontal stretching includes preheating, stretching, heat setting and cooling steps. After horizontal stretching, a diaphragm is obtained. Each step in the assembly and horizontal stretching adopts air intake from both sides and air return from the top and bottom. The air intake is horizontally introduced from a direction perpendicular to the diaphragm surface, and the air return is discharged from the top and bottom edges of the diaphragm. The temperature of each step in the assembly and horizontal stretching is independently controlled by the air intake.

[0038] In the assembly and stretching process, the preheating step is divided into 2 zones, the stretching step into 4 zones, the heat setting step into 10 zones, and the cooling step into 3 zones; the tensile strain rate in the stretching step is between 0.02 and 0.05 s⁻¹. -1 The air intake for the preheating, stretching, heat setting and cooling steps is divided into three sections vertically from top to bottom. The air intake temperature of the upper and lower sections is 1 to 3°C higher than that of the middle section.

[0039] In each step of the assembly of the double-pull process and the assembly of the horizontal pull process, oil mist is adsorbed and purified to improve the cleanliness of the air field.

[0040] (8) Multi-station winding: Each separator is wound up at different stations; the oil-free separator, after being assembled, stretched, heat-set and cooled as described above, is wound up at multiple stations to complete the production of wet lithium battery separator. Multi-station independent parallel winding is adopted, with winding tension controlled within the range of 5 to 10 N / m and winding diameter of 600 to 1000 mm.

[0041] In this embodiment, the slitting step divides the cast oil film into two cast oil films. Therefore, the assembly double stretching process and the assembly transverse stretching process both use two parallel independent stretching units. The extraction step uses two independent extraction devices. The multi-station winding step uses dual-station independent parallel winding.

[0042] The physical properties (winding width, left, middle, and right) of the 7-micron membrane prepared according to the above embodiments are as follows: Figure 5The table shows the physical properties of existing 7-micron membranes (winding width, left, center, right). Figure 6 As shown in the table below. The diaphragm prepared in the embodiments of the present invention has the following significant characteristics: (1) transverse air permeability ≤ 5s / 100cc; (2) transverse porosity ≤ 1%; (3) tensile thickness uniformity ≤ 1%.

[0043] Average areal density: 4.45 g / m³ 2 The surface density range is 0.02 g / m³. 2 The average thickness is 7.12 μm, the thickness range is 0.06, the average porosity is 33.73%, the porosity range is 0.9, the average air permeability is 141 s / 100cc, the air permeability range is 4, and the average tensile strength in the MD direction is 4175 kgf / cm. 2 The tensile strength range in the MD direction is 23, and the mean tensile strength in the TD direction is 2720 kgf / cm². 2 TD tensile strength range 5, MD elongation average 73%, MD elongation range 5%, TD elongation average 144%, TD elongation range 12%, puncture strength average 516 gf, puncture strength range 7%, MD heat shrinkage average 2.13%, MD heat shrinkage range 0.15%, TD heat shrinkage average 1.87%, TD heat shrinkage range 0.1%.

[0044] The method for detecting the areal density of the diaphragm is as follows: According to GB / T 451.2-2002 (Paper Surface Density Test), five square separator samples were cut along the transverse (TD) direction using a 10cm×10cm mold for testing. If the TD direction was less than 10cm, a 10cm sample was cut along the longitudinal (MD) direction; in this case, the sample was not square. The mass of the sample was weighed using an analytical balance (Mettler-Toledo Instruments (Shanghai) Co., Ltd., ME204E / 02) with a measurement accuracy of 0.0001g. The separator surface density = mass of separator sample / (length of separator sample × width of separator sample). The average value of the five samples was taken as the surface density of the multi-separator. The longitudinal direction refers to the length of the battery separator, and the transverse direction refers to the width of the battery separator.

[0045] The test method for diaphragm thickness is as follows: The test was conducted in accordance with the requirements of GB / T 36363-2018 "Polyolefin Separators for Lithium-ion Batteries".

[0046] Five square diaphragm samples were cut along the TD direction using a 10cm×10cm mold and tested. If the TD direction sample was less than 10cm, a 10cm sample was cut along the MD direction; in this case, the sample was not square. The four corners and the center point of each sample were measured using a Mahr thickness gauge (C1202). The average value of these five points was taken as the thickness of a single sample. The average value of the five samples was taken as the thickness of the diaphragm.

[0047] Porosity testing methods: Cut a 10cm × 10cm sample and measure its thickness (Mahr thickness gauge, C1202) and mass (electronic balance, Mettler Toledo Instruments (Shanghai) Co., Ltd., ME204E / 02). Calculate the surface density (in g / cm³). 2 ), where m represents the mass of the sample (in g), L represents the length of the sample (in cm), and b represents the width of the sample (in cm). According to Calculate the porosity, where p is the porosity of the sample (in %) and d is the thickness of the sample (in cm).

[0048] Methods for testing air permeability: The test was conducted in accordance with the requirements of GB / T 36363—2018 "Determination of air permeability of polyolefin separators for lithium-ion batteries". A 600mm×100mm separator sample was cut, and an air permeability meter (ASAHI Corporation, EG01-55-1MR) was used. The test time was 3s. The air permeability of the separator was measured at any position at 100mm intervals along the 600mm TD direction. The average value of the above 5 test points was recorded as the air permeability of the separator.

[0049] Test methods for tensile strength and elongation: The test was conducted according to GB / T 1040.3-2006. A 2.5cm × 20cm specimen was cut and marked with the MD / TD direction of the diaphragm. The specimen was tested using a tensile testing machine (High-speed Rail Testing Instruments (Dongguan) Co., Ltd., AL-300-U). The specimen was fixed between the upper and lower clamps of the tensile testing machine (the distance between the clamps was 100±5 mm). The specimen was ensured to be flat and wrinkle-free, and vertical and not skewed. The tensile speed was 250mm / min. The tensile testing machine output the tensile strength value based on the width and thickness of the specimen. The test was performed 3 times and the average value was taken as the tensile strength (MPa) and elongation (%) in the MD / TD direction.

[0050] Puncture strength test method: According to the method specified in GB / T 36363-2018, a diaphragm with a size of 500*100 mm was cut along the TD direction. The diaphragm was fixed on the sample fixture of the puncture testing machine (model: AI-3000-S, High-speed Rail Testing Instruments (Dongguan) Co., Ltd.). A steel needle with a diameter of 1.0 mm was used to puncture the diaphragm at a speed of 300 mm / min. The maximum load of the steel needle penetrating the diaphragm was read. The test was performed more than 5 times and the arithmetic mean was taken.

[0051] Method for testing heat shrinkage rate: The test was conducted according to the requirements of GB / T36363-2018. A 10cm × 10cm sample was cut, and the transverse (TD) and longitudinal (MD) widths were marked on the sample. The transverse and longitudinal widths were measured using a fully automatic image measuring projector (Kunshan Gaopin Precision Instrument Co., Ltd., GP-300C). The sample was held between two sealed A4 sheets of paper and placed in a 105℃ oven for 1 hour. After the sample returned to room temperature, the transverse and longitudinal widths were measured again using the fully automatic image measuring projector. The measurement was repeated three times, and the average value was taken.

[0052] The assembly of the double-stretch and horizontal-stretch steps in this invention enables gradient temperature control for each functional zone, as well as vertical temperature gradient control. Each functional zone employs an independent temperature control device (using steam or electric heating). Combined with the preheating functional zone and the upper-middle-lower air intake structure, independent temperature control heating devices are also installed on the upper, middle, and lower sides of the membrane surface to address the temperature difference between the upper, middle, and lower sides due to heat loss caused by the environment within the airflow field. Simultaneously, upper, middle, lower, and more functional air zones are set on the vertical air intake side to achieve gradient temperature control based on the stretching process requirements. Electrically controlled air valves enable precise control of flow rate and temperature. The stretching diaphragm adopts vertical stretching. The gravitational force of the paraffin oil between diaphragm molecules is in the same direction as the vertical stretching stress, resulting in better molecular chain movement and more uniform stress transmission during stretching, forming a dense microfiber network structure, thereby improving the consistency of stretched properties.

[0053] All temperature control boxes are equipped with a structure of horizontal air intake on both sides and vertical air return, forming a circulation of air supply on both sides and exhaust on both sides. The air intake speed is controlled at 5-35 m / s, and is adjusted and controlled by an electrically controlled air valve with a speed control accuracy of ±0.5 m / s, achieving precise air speed regulation and effectively ensuring temperature consistency. The air intake side of the temperature control box is equipped with an oil-absorbing fiber layer device for oil mist filtration, which is made of nano-grade carbon fiber material with a thickness of 2mm-20mm. It can be set to offline replacement and electric winding online replacement modes.

Claims

1. A method for producing a diaphragm through bidirectional synchronous stretching, characterized in that, After the feeding, extrusion, and casting processes, it undergoes the following steps: Separation and reversal: The cast oil film is divided into two or more parallel independent cast oil films; each independent cast oil film is flipped 90° to one side to change the direction of the cast oil film, and each independent cast oil film changes from a state parallel to the horizontal plane to a state perpendicular to the horizontal plane. Biaxial stretching: Each independent cast oil film is subjected to biaxial stretching in a vertical state. The biaxial stretching process includes preheating, longitudinal and transverse stretching, heat setting, and cooling. After biaxial stretching, each cast oil film forms a porous oil film. In the biaxial stretching process, the preheating temperature is 100–130℃, the longitudinal and transverse stretching ratio is 5–9, the longitudinal and transverse stretching temperature is 110–135℃, the heat setting temperature is 110–135℃, and the cooling temperature is 15–25℃. The preheating step is divided into 3 zones, the longitudinal and transverse stretching steps into 6 zones, the heat setting step into 3 zones, and the cooling step into 3 zones. The tensile strain rate in the biaxial stretching step is 0.1–0.2 s⁻¹. -1 The air intake for the preheating, longitudinal and transverse stretching, heat setting and cooling steps is divided into 3 sections from top to bottom in the vertical direction. The air intake temperature of the upper and lower sections is 1 to 3°C higher than that of the middle section. Extraction: The porous oil film enters the extraction device vertically for vertical extraction. The paraffin oil in the porous oil film is extracted using an extractant. Each porous oil film is then dried after extraction to obtain a thin film. Assembly and transverse stretching: Each film is stretched laterally in a vertical position. The transverse stretching process includes preheating, stretching, heat setting, and cooling steps, resulting in a diaphragm. In the assembly and transverse stretching process, the preheating step is divided into 2 zones, the stretching step into 4 zones, the heat setting step into 10 zones, and the cooling step into 3 zones. The tensile strain rate in the stretching step is between 0.02 and 0.05 s⁻¹. -1 The air intake for the preheating, stretching, heat setting and cooling steps is divided into three sections vertically from top to bottom. The air intake temperature of the upper and lower sections is 1 to 3 degrees Celsius higher than that of the middle section. Multi-station winding: Each diaphragm is wound up at a different station; In the assembly of double-stretch and horizontal-stretch, each step adopts air intake from both sides and air return from the top and bottom. The air intake is horizontally introduced from a direction perpendicular to the diaphragm surface, and the air return is discharged from the top and bottom edges of the diaphragm respectively. The temperature of each step in the assembly of double-stretch and horizontal-stretch is independently controlled by the air intake.

2. The method for producing a diaphragm by bidirectional synchronous stretching according to claim 1, characterized in that: In the extraction process, the extraction temperature is 5–25℃ and the extraction time is 1–3 min.

3. The method for producing a diaphragm by bidirectional synchronous stretching according to claim 1, characterized in that: In each step of the assembly of the double-pull process and the assembly of the horizontal pull process, oil mist is adsorbed and purified to improve the cleanliness of the air field.

4. A diaphragm production apparatus for bidirectional synchronous stretching, characterized in that: A diaphragm production apparatus is used for performing the bidirectional synchronous stretching diaphragm production method according to any one of claims 1 to 3. The diaphragm production apparatus includes a separating and reversing device, an assembly double-stretching device, an extraction device, and an assembly horizontal stretching device. The separating and reversing device includes a slitting mechanism and a reversing mechanism. The slitting mechanism is used to slit the cast oil film into two or more independent cast oil films. The reversing mechanism includes a set of steering rollers, which is used to rotate each independent cast oil film 90° to one side, so that the cast oil film is in a vertical state perpendicular to the horizontal plane. The assembly double-stretching device includes a number of independent cast oil films... The extraction device includes an extraction tank for vertical extraction of porous oil films, with vertically arranged rollers inside the extraction tank; the assembly of horizontal stretching devices includes horizontal stretching units equal in number to the independent cast oil films; each double stretching unit and each horizontal stretching unit sequentially includes a preheating zone, a stretching zone, a heat setting zone, and a cooling zone, and each zone of the double stretching unit and the horizontal stretching unit includes a temperature control box, with the film surface passing through each temperature control box horizontally in sequence, each temperature control box having independent air inlet and outlet, and each zone's temperature control box including an air inlet box located on the left and right sides of the film surface and an air outlet box located above and below the film surface.

5. The bidirectional synchronous stretching diaphragm production apparatus according to claim 4, characterized in that: The walls of the temperature control box on both sides of the diaphragm are provided with oil-absorbing fiber layers.

6. The diaphragm production apparatus for bidirectional synchronous stretching according to claim 4, characterized in that: The temperature control box is equipped with chain clamping mechanisms on both the upper and lower sides for clamping the edge of the film surface.

7. The diaphragm production apparatus for bidirectional synchronous stretching according to claim 4, characterized in that: In the double-stretch unit, there are 3 temperature control boxes in the preheating zone, 6 temperature control boxes in the stretching zone, 3 temperature control boxes in the heat setting zone, and 3 temperature control boxes in the cooling zone; in the horizontal stretching unit, there are 2 temperature control boxes in the preheating zone, 4 temperature control boxes in the stretching zone, 10 temperature control boxes in the heat setting zone, and 3 temperature control boxes in the cooling zone.

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