Microporous membrane, microporous membrane preparation process and cast sheet cooling device of microporous membrane

Abstract

The invention discloses a microporous membrane, a microporous membrane preparation process and a cast sheet cooling device of the microporous membrane, the microporous membrane is a polyolefin microporous membrane for a secondary battery, the matte surface stripping force of the microporous membrane is 60-90N / m, and the thickness range of a cast sheet is 0.5-0.53 mm; the cast piece cooling device comprises an air cooling cover, the outer side of the air cooling cover is connected with an air supply mechanism, two or more back cooling rollers are arranged on the inner side of the air cooling cover, air knives are arranged in gaps of the back cooling rollers in a penetrating mode, the air knives are communicated with the air supply mechanism, and side face air exhaust areas are arranged in the air cooling cover on the two sides of the air knives and the back cooling rollers. A scraper used for scraping white oil on the back cold roller is installed on the lower edge of the air knife, the angle between the scraper and the back cold roller is adjusted through an angle adjusting mechanism, and the white oil scraped by the scraper flows to the side face exhaust area along the scraper and is discharged after being collected. Through the mode of combining the small-diameter back cooling roller set and the air-cooling air knife, the contact area of the back cooling rollers and the membrane surface is increased, meanwhile, uniform cold air is assisted, and the optimal effect of cooling the membrane is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to a microporous membrane, a microporous membrane preparation process and a microporous membrane casting sheet cooling device, and belongs to the technical field of lithium battery separator production and manufacturing. BACKGROUND

[0002] The lithium battery separator is an important battery component for preventing the positive and negative electrodes from contacting and short-circuiting, and providing a channel for lithium ion migration between the electrodes through the nanoscale microporous structure. As the base material of the separator, the microporous membrane is usually made of polyester as the main raw material, and is formed into a polyester film through extrusion casting and longitudinal and lateral stretching. In the production and application of the microporous membrane, there are still problems, such as the non-ideal flatness and adhesion of the two sides of the microporous membrane product, and the low A yield of the product. In the cooling of the lithium battery microporous membrane casting sheet, the current wind cooling device is usually directly used to cool the casting sheet through cold air, but the wind cooling device is unstable, and has a great impact on the consistency of the two sides of the product and the microstructure. The peeling force of the membrane surface on one side of the wind cooling device is much lower than that of the membrane surface on the side in contact with the chill roller. In addition, the cooling air is not uniform, and the cooling effect on the casting sheet is different. In addition, there is a way of using a back cooling roller to directly contact the casting sheet for cooling, but the back cooling roller has a small contact area, and the cooling effect is insufficient, which has a great impact on the subsequent process. SUMMARY

[0003] The purpose of the present application is to provide a polyolefin microporous membrane for secondary batteries with good consistency on both sides.

[0004] Another purpose of the present application is to provide a polyolefin microporous membrane preparation process capable of producing a microporous membrane with good consistency and high A yield.

[0005] Still another purpose of the present application is to provide a microporous membrane casting sheet cooling device, which combines a small-diameter back cooling roller group with a wind cooling air knife to increase the contact area between the back cooling roller and the membrane surface and assist in uniform cooling air to achieve the best effect of membrane sheet cooling.

[0006] To achieve the above purposes, the technical solutions adopted by the present application are as follows:

[0007] On the one hand, the present application provides a polyolefin microporous membrane for secondary batteries, one side of the microporous membrane is a smooth surface, and the other side is a matte surface. The peeling force of the matte surface of the microporous membrane is 60-90 N / m, and the peeling force of the smooth surface of the microporous membrane is 60-90 N / m. The smooth surface is the membrane surface formed by stretching the casting sheet on the side close to the chill roller in the casting sheet stage of the microporous membrane, and the matte surface is the membrane surface formed by stretching the casting sheet on the side away from the chill roller in the casting sheet stage of the microporous membrane. The thickness difference of the casting sheet in the casting sheet stage of the microporous membrane is 0.5-0.53 mm.

[0008] Further, the peeling force of the smooth surface of the microporous membrane is 60-90 N / m, and the peeling force of the smooth surface of the microporous membrane is 60-90 N / m.

[0009] Furthermore, the matte surface peel strength of the microporous membrane is 80-90 N / m, and the thickness difference of the microporous membrane casting is 0.528 mm.

[0010] On the other hand, the present invention also provides a process for preparing a polyolefin microporous membrane, the steps of which are as follows:

[0011] Step 1: Feeding and extrusion;

[0012] A certain amount of polyolefin resin powder is added to the twin-screw extruder through a metering system. A pore-forming agent is simultaneously added to the twin-screw extruder through multiple oil filling ports set on the twin-screw extruder at a set temperature via a metering pump. The mixture is then melted and extruded to the die under set temperature and pressure.

[0013] Step 2, casting;

[0014] The molten slurry extruded from the die head is rapidly cooled by the quench roller to form a casting film. The casting film enters the air-cooled hood of the casting cooling device as the quench roller rotates. The sheet substrate is cooled by multiple sets of air-cooled and back-cooled cooling in the casting cooling device. Finally, after exiting the casting cooling device, before entering the double-drawing inlet, the sheet temperature is reduced to 25-35℃.

[0015] Step 3, double pull;

[0016] After cooling, the sheet enters the biaxial stretching unit and undergoes preheating, biaxial stretching, shaping, and cooling in sequence to form a thick film of a certain thickness and width.

[0017] Step 4: Extraction;

[0018] The thick film is trimmed by a traction machine and then enters an extraction tank containing dichloromethane to extract the paraffin oil from the film. The film that comes out of the extraction outlet tank will be dried by a temperature-controlled roller and then enter a drying room for further drying.

[0019] Step 5: Pull horizontally;

[0020] After drying, the membrane enters the transverse stretching heat treatment unit for slight transverse expansion, followed by slight transverse and longitudinal shrinkage to obtain a thin film, namely a microporous membrane.

[0021] Step 6: Rewind and slit;

[0022] After being trimmed by a traction machine, the microporous membrane is wound up under low tension and then slit into the required width by a slitting machine before being wound up again.

[0023] Furthermore, in step 1, the polyolefin resin powder can be fed simultaneously with the antioxidant.

[0024] Furthermore, in step 1, the polyolefin resin can be ultra-high molecular weight polyethylene, high-density polyethylene, and low-density polyethylene, or any combination of two of the above.

[0025] Furthermore, in step 1, the pore-forming agent is paraffin oil, nonane, decane, or naphthane, and paraffin oil may be selected.

[0026] Furthermore, in step 1, the mass ratio of polyolefin resin to pore-forming agent is (10-50):(50-90).

[0027] Furthermore, in step 1, the extrusion pressure is 4-8 MPa.

[0028] Furthermore, in step 2, the die extrusion temperature can be 140-270℃, the extrusion speed can be 0.2-15m / min, and the film thickness can be adjusted by adjusting the extrusion amount of the polyolefin resin composition.

[0029] Furthermore, in step 2, the gap between the back cooling roller and the quenching roller is 1-2mm, for example, it can be 1mm, 1.2mm, 1.4mm, 1.6mm, 1.8mm, or 2mm, and can be selected as 1-1.6mm.

[0030] Furthermore, the air-cooled unit has a cooling air temperature of 2-5℃ and an air volume of 4000-6000 m³ / h. 3 / h, wind pressure 3000Pa.

[0031] Furthermore, the temperature of the back-cooling roller is 8-12℃.

[0032] Furthermore, the diameter of the back cooling roller is between 100-150mm, and there are no fewer than two back cooling rollers. Each back cooling roller is equipped with a separate drive motor for individual drive to ensure that the speed is consistent with the speed of the quenching roller, and the speed difference between the two is controlled within ±0.01m / min.

[0033] Furthermore, in step 3, the bi-stretch can be either bi-directional synchronous stretching or bi-directional asynchronous stretching.

[0034] Furthermore, in step 3, the unidirectional stretching magnification factor can be 2 times or more, for example, 3-30 times. In the case of bidirectional stretching, the stretching magnification factor in each direction can be 3 times or more, for example, 4-30 times. Moreover, the amount of stretching in each direction does not need to be the same; the magnification factor affects the film size in a multiplicative manner.

[0035] Furthermore, in step 3, the stretching temperature is set within the range of the polyolefin resin's crystallization dispersion temperature (Tcd) of Tcd+30°C, preferably within the range of Tcd+5°C to Tcd+28°C. When the stretching temperature is within the above range, film breakage caused by stretching of the polyolefin resin can be suppressed, and high-ratio stretching can be performed.

[0036] Furthermore, in step 5, the stretching temperature can be 80-130℃.

[0037] Furthermore, in step 5, the stretching ratio is 1-1.8 times.

[0038] In another aspect, the present invention also provides a casting cooling device for microporous membranes, which includes an air-cooled shroud. An air supply mechanism is connected to the outer side of the air-cooled shroud, and two or more back-cooling rollers are arranged on the inner side of the air-cooled shroud. Air knives are interspersed in the gaps between the back-cooling rollers. The air knives are connected to the air supply mechanism. Side exhaust areas are provided in the air-cooled shroud on both sides of the air knives and the back-cooling rollers. A scraper for scraping off white oil on the back-cooling rollers is installed on the lower edge of the air knives. The angle between the scraper and the back-cooling rollers is adjusted by an angle adjustment mechanism. The white oil scraped off by the scraper flows along the scraper to the side exhaust areas on both sides and is discharged after being collected.

[0039] Furthermore, the air supply mechanism includes a cold air distribution chamber. One end of the cold air distribution chamber is connected to an external cooling unit via an air inlet pipe, and the other end is connected to an air knife via an air knife inlet pipe. Each air knife is equipped with a corresponding air knife inlet pipe. The cold air provided by the external cooling unit enters the cold air distribution chamber through the air inlet pipe, and the cold air distribution chamber evenly delivers the cold air to the air knife through the air knife inlet pipe.

[0040] Furthermore, the angle adjustment mechanism includes an adjustment knob, a movable nut, and a connecting rod. The two ends of the adjustment knob are rotatably mounted on a fixed bracket. The movable nut is fitted onto the adjustment knob. The two ends of the connecting rod are hinged to the movable nut and the scraper, respectively. By rotating the adjustment knob, the movable nut is moved back and forth, thereby causing the connecting rod to change the angle of the scraper.

[0041] Furthermore, the back cooling roller is fixed to the outer shell of the air-cooling shroud via a bearing seat, and the back cooling rollers are parallel to each other with their tangents on the same circle. The diameter of the back cooling roller is between 100-150mm.

[0042] Furthermore, each back-cooling roller is equipped with a separate drive motor to ensure that its speed is consistent with that of the quenching roller, and the speed difference between the back-cooling roller and the quenching roller is controlled within ±0.01m / min.

[0043] Furthermore, the air-cooled shroud serves as the main frame of the device, and its overall structure is arc-shaped.

[0044] Furthermore, the side of the air-cooling shroud is provided with a distance adjustment mechanism, which is used to adjust the distance between the back cooling roller and the quenching roller.

[0045] Furthermore, the distance adjustment mechanism includes a distance adjustment block connected to the air-cooling shroud. The distance adjustment block is slidably mounted on the quench roller mounting wall plate. The end of the distance adjustment block that slides away from the quench roller is connected to a hydraulic cylinder, and the end that slides closer to the quench roller is provided with a limiting slider. The limiting slider and the distance adjustment block are contacted and connected through an inclined structure. At the same time, the limiting slider is mounted on a vertically set lead screw, and the end of the lead screw is provided with an adjustment handle. By rotating the adjustment handle, the limiting slider is driven to move up and down on the lead screw, thereby limiting the distance that the distance adjustment block can advance.

[0046] Furthermore, the side exhaust area is equipped with an exhaust pipe and an oil drain pipe at the bottom. The side exhaust area is equipped with an inner baffle and an outer baffle on both sides. The front end of the outer baffle protrudes from the inner baffle to ensure that the cold air effectively diffuses into the side exhaust area after hitting the membrane surface and is discharged without overflowing.

[0047] Furthermore, the area between the two air knives is connected by an arc-shaped plate to ensure that the cold air blown out by the air knives does not escape into the dead zone behind the back cooling roller.

[0048] Furthermore, the casting curve of the microporous membrane prepared after cooling by the casting cooling device is generally flattened, and the casting thickness range is 0.528 mm. Compared with before the casting cooling device was installed, the thickness range is reduced by 10%, the overall thickness of the casting is slightly reduced (0.6%-7.1%), and the casting width is more stable.

[0049] On the other hand, the present invention also provides a microporous membrane prepared using the above-described casting cooling device.

[0050] The beneficial effects of this invention are as follows: This invention can significantly improve the microstructure of the matte surface of the diaphragm, enhancing the consistency of the structure on both sides; it improves thickness flatness and reduces film thickness deviation through the back-cooling roller; it can significantly improve the peel force of the matte surface, making it more consistent with the glossy surface. By improving the uniformity of airflow from the air knife, it avoids defects on the film surface caused by uneven airflow, while effectively improving the cooling effect. The adjustable-angle scraper can effectively remove white oil from the back-cooling roller surface, improving the adhesion between the back-cooling roller and the film surface, achieving flatness of the film surface adhesion, avoiding thin spots and other defects, thereby significantly improving the overall diaphragm performance and achieving a significant increase in the A-grade yield. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the overall structure of the casting cooling device of the present invention;

[0052] Figure 2This is a schematic diagram of the air supply mechanism of the casting cooling device of the present invention;

[0053] Figure 3 This is a schematic diagram of the interior of the air-cooled shroud and the angle adjustment mechanism of the casting cooling device of the present invention;

[0054] Figure 4 This is a cross-sectional view of the air-cooled shroud of the casting cooling device of the present invention;

[0055] Figure 5 This is a schematic diagram of the distance adjustment mechanism of the casting cooling device of the present invention;

[0056] Figure 6 This is a curve showing the change in casting thickness before and after using this invention;

[0057] Figure 7 is a comparison of electron microscope images of the cast plates before and after using the present invention. (7-1) is the glossy surface, (7-2) is the matte surface without the present invention, and (7-3) is the matte surface with the present invention.

[0058] Figure 8 This is a diagram showing the improvement in peel force before and after using the present invention.

[0059] The diagram is labeled as follows: 1-Cold air distribution chamber; 2-Air cooling hood; 3-Drive motor; 4-Angle adjustment mechanism; 4a-Adjusting knob; 4b-Moving nut; 4c-Connecting rod; 5-Inner baffle; 6-Air knife; 7-Back cooling roller; 8-Scraper; 9-Air knife inlet pipe; 10-Main air inlet pipe; 11-Exhaust pipe; 12-Arc-shaped plate; 13-Long strip-shaped air outlet; 14-Side exhaust area; 15-Outer baffle; 16-Oil drain pipe; 17-Top exhaust; 18-Adjusting handle; 19-Distance adjusting block; 20-Hydraulic cylinder; 21-Quick cooling roller mounting wall plate; 22-Quick cooling roller; 23-Screw; 24-Limit slider; 25-Die head. Detailed Implementation

[0060] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0061] Example 1

[0062] A microporous membrane, wherein one side of the microporous membrane is glossy and the other side is matte, wherein the matte surface peel strength is 60-90 N / m, preferably 80-90 N / m; and the glossy surface peel strength is 60-90 N / m, preferably 80-90 N / m; the glossy surface is the membrane surface formed by stretching the cast sheet near the chill roller during the microporous membrane casting stage, and the matte surface is the membrane surface formed by stretching the cast sheet away from the chill roller during the microporous membrane casting stage; and the thickness difference of the cast sheet during the casting stage is 0.5-0.53 mm, preferably 0.528 mm.

[0063] In this application, the thickness range of the casting is obtained by testing with an online thickness gauge.

[0064] Example 2

[0065] 1) Feeding and extrusion

[0066] Polyethylene powder (25 parts by weight) and antioxidant (0.3 parts by weight) are added to the twin-screw extruder through a metering system. Paraffin oil (75 parts by weight) is added to the twin-screw extruder simultaneously through a metering pump at a set temperature (110°C) from multiple oil filling ports set on the twin-screw extruder (70% for the first port, 20% for the second port, and 10% for the third port). The mixture is then extruded to the die at a pressure of 5.5 MPa and a temperature of 215°C.

[0067] 2) Casting

[0068] The slurry extruded from the die head is rapidly cooled on a chiller to form a casting film. The temperature of the chiller roller 22 is set at 12°C. The casting film rotates with the chiller roller and then enters the cooling device of Example 3 (the gap between the back cooling roller 7 and the chiller roller 22 is adjusted to 1.6mm by means of a distance adjustment structure). First, the top exhaust fan 17 will quickly exhaust the oil fumes generated during cooling. Then, the air knife 6 will blow cold air onto the sheet to cool it down (cold air temperature set at 3°C, air volume at 5000m3 / h, air pressure at 3000Pa). Then, the sheet will come into contact with the back cooling roller 7 (back cooling roller temperature set at 10°C) for contact cooling. This contact surface is matte. Since the sheet surface contains a lot of white oil, it will stick to the roller surface when it comes into contact with the back cooling roller. When contacting the sheet, it will create a gap between the sheet and the roller surface, affecting the cooling effect. The white oil on the surface of the back cooling roller can be scraped off by the scraper 8 to ensure the cooling effect and avoid defects such as thin spots. Excess white oil flows along the scraper 8 to the side exhaust areas 14 on both sides and is discharged from the oil drain pipe for recycling. The sheet substrate undergoes multiple sets of combined air-cooled and back-cooled cooling in this device. Finally, after exiting the device, the sheet temperature drops to about 30°C before the double-draw inlet.

[0069] 3) Double pull

[0070] The cooled sheet (1300μm thick and 650mm wide) enters the biaxial synchronous stretching unit (the biaxial stretching ratio is 7 times), and goes through preheating, biaxial stretching, shaping and cooling in sequence to produce a thick film with a thickness of 30μm and a width of 4550mm.

[0071] 4) Extraction

[0072] The thick film is trimmed by a traction machine (thickness 30 μm, width 4300 mm), and then enters an extraction tank containing dichloromethane (including 8 extraction tanks from No. 1 to No. 8) to extract the paraffin oil from the film. After that, the film is dried in a drying room at 45°C.

[0073] 5) Horizontal pull

[0074] The dried membrane (12μm thick, 3150mm wide) enters the transverse stretching heat treatment unit and undergoes preheating, stretching, shrinkage, and first shaping steps. Under set temperature and wind speed conditions, it undergoes a small transverse expansion (stretching ratio of 1.5 times), followed by a small transverse and longitudinal shrinkage (TD direction, 10% shrinkage; MD direction, 10% shrinkage) and a second shaping to obtain a thin film (microporous membrane).

[0075] 6) Rewinding and slitting

[0076] After being trimmed by a traction machine, the microporous membrane is wound up under low tension (winding tension 8N / m), and then slit into the required width by a slitting machine (width after slitting 840mm).

[0077] In this embodiment, the preheating, stretching, shrinking, shaping, and cooling temperatures are not particularly limited, as long as they meet the production requirements of the microporous membrane.

[0078] Example 3

[0079] like Figures 1 to 5 As shown, a casting sheet cooling device employs multiple sets of back-cooling rollers to directly cool the casting sheet, while air knives are interspersed between the back-cooling rollers to assist in cooling the casting sheet, thereby maximizing the cooling effect. Specifically, this device includes an air-cooling hood 2, with an air supply mechanism connected to the outer side of the air-cooling hood 2. Multiple back-cooling rollers 7 are arranged and installed on the inner side of the air-cooling hood 2, and air knives 6 are interspersed between the gaps of the back-cooling rollers 7. The air knives 6 are connected to the air supply mechanism. Side exhaust areas 14 are provided inside the air-cooling hood 2 on both sides of the air knives 6 and the back-cooling rollers 7. A scraper 8 for scraping off white oil on the back-cooling rollers 7 is installed on the lower edge of the air knives 6. The angle between the scraper 8 and the back-cooling roller is adjusted by an angle adjustment mechanism 4. The white oil scraped off by the scraper flows along the scraper to the side exhaust areas 14 on both sides and is discharged after being collected.

[0080] In this embodiment, the air supply mechanism includes a cold air distribution chamber 1. One end of the cold air distribution chamber 1 is connected to an external cooling unit via an air inlet pipe 10, and the other end is connected to an air knife 6 via an air knife inlet pipe 9. Each air knife 6 is configured with an air knife inlet pipe 9. The external cooling unit introduces clean air at 0-15℃ into the cold air distribution chamber 1 through the air inlet pipe. The cold air distribution chamber 1 evenly delivers the cold air to the air knife 6 through the air knife inlet pipe 9. The cold air is blown onto the film surface through the air knife 6 to achieve auxiliary cooling of the casting sheet. The clean air is filtered by a filtration device and then pressurized by a high-pressure variable frequency air supply fan to deliver clean high-pressure air. The total rated air volume of one or more high-pressure variable frequency air supply fans is ≥3000m3 / h, and the air pressure is 3000Pa~4000Pa.

[0081] In this embodiment, the angle adjustment mechanism 4 includes an adjustment rod 4a, a movable nut 4b, and a connecting rod 4c. The two ends of the adjustment rod 4a are rotatably mounted on a fixed bracket. The movable nut 4b is fitted onto the adjustment rod 4a. The two ends of the connecting rod 4c are hinged to the movable nut 4b and the scraper 8, respectively. By rotating the adjustment rod 4a, the movable nut 4b is moved back and forth, thereby causing the connecting rod 4c to change the angle of the scraper 8.

[0082] In this embodiment, cooling water treated by a mold temperature controller flows through the multiple back-cooling rollers 7, achieving a temperature control of 6-15℃. The back-cooling rollers 7 are fixed to the outer shell of the air-cooling cover 2 via bearing seats. The back-cooling rollers 7 are parallel to each other, and their tangents lie on the same circle. The diameter of the back-cooling rollers 7 is between 100-150mm. Each back-cooling roller 7 is equipped with an individual drive motor for individual driving, ensuring that its speed is consistent with the speed of the quenching roller 22. The speed difference between the two can be controlled within ±0.01m / min.

[0083] In this embodiment, a distance adjustment mechanism is provided on the side of the air-cooling shroud 2. This mechanism is used to adjust the distance between the back-cooling roller 7 and the quenching roller 22. The distance adjustment mechanism includes a distance adjustment block 19 connected to the air-cooling shroud 2. The distance adjustment block 19 is slidably mounted on the quenching roller mounting wall plate 21. One end of the distance adjustment block 19, which is slidably away from the quenching roller 22, is connected to a hydraulic cylinder 20, and the other end, which is close to the quenching roller 22, is provided with a limiting slider 24. The limiting slider 24 and the distance adjustment block 19 are contacted and connected through an inclined structure. At the same time, the limiting slider 24 is mounted on a vertically arranged lead screw 23. An adjustment handle 18 is provided at the end of the lead screw 23. By rotating the adjustment handle 18, the limiting slider 24 is driven to move up and down on the lead screw 23, thereby limiting the distance that the distance adjustment block 19 can move forward, and thus achieving fine adjustment of the distance between the back-cooling roller 7 and the quenching roller 22. The distance adjustment block 19 moves back and forth through the hydraulic cylinder 20.

[0084] In this embodiment, the side exhaust area 14 is equipped with an exhaust pipe 11. Inner baffles 5 and outer baffles 15 are provided on both sides of the side exhaust area. The front end of the outer baffle 15 protrudes beyond the inner baffle 5. The outer baffle 15 is close to the membrane surface, ensuring that the cold air effectively diffuses to the side exhaust area 14 after hitting the membrane surface and is discharged without overflowing. Simultaneously, a long strip-shaped air outlet is provided in the middle of the air knife 6. The area between two air knives 6 is connected by an arc-shaped plate 12 to ensure that the cold air blown out by the air knife does not escape into the dead zone behind the back cooling roller, thus improving air cooling efficiency.

[0085] In this embodiment, an oil drain pipe 16 is provided at the bottom of the side exhaust area 14. The white oil hanging down by the scraper 8 will flow along the scraper 8 to the two side exhaust areas 14. The collected white oil can be discharged from the bottom oil drain pipe 16 at regular intervals and returned to the liquid recovery system for re-collection and treatment. The top exhaust 17 and the side exhaust 14 are interconnected, forming a three-sided exhaust shape to maximize the exhaust effect.

[0086] The working principle of this casting cooling device is as follows: the diaphragm coming out of the die head first contacts the cooling roller 22, and at this time, it undergoes short-term single-sided cooling. The large amount of smoke generated by the cooling is discharged through the top exhaust 17. As the cooling roller 22 rotates, the diaphragm enters this cooling device, and the back cooling roller 7 will directly contact the diaphragm surface for back-side cooling. Driven by the drive motor 3, the back cooling roller 7 will maintain the same rotation speed as the cooling roller 22 to achieve relative stillness with the diaphragm surface, avoiding relative sliding with the diaphragm surface and causing defects. At this time, the membrane surface is sandwiched between the back cooling roller 7 and the quench roller 22 for two-sided cooling. Then the membrane rotates over the contact surface to reach the cooling range of the air knife 6. The cold air with the set temperature will enter the cold air distribution chamber 1 through the air inlet pipe 10. After being processed by the cold air distribution chamber 1, the cold air will be evenly distributed through the air inlet pipe 10 into each air knife 6, and finally blown onto the membrane surface through the long strip air outlet 13 for cooling. As the cold air blows onto the membrane surface, the smoke mixed with a large amount of white oil will be discharged through the gap between the two back cooling rollers 7 and the air knife 6 to the smoke exhaust areas 14 on both sides, and then discharged to the gas treatment device through the exhaust pipe 11 to remove the smoke.

[0087] After the cooling roller 7 contacts the film, some white oil adheres to the roller surface and rotates with the roller. On the next rotation towards the film surface, it will first contact the doctor blade 8. Under the action of the doctor blade 8, the white oil on the roller surface will be intercepted. As the accumulation increases, excess white oil will flow along the doctor blade 8 to the side exhaust areas 14 on both sides, collect at the bottom, and finally be discharged from the device through the oil drain pipe 16. The doctor blade 8 can adjust the contact angle with the cooling roller through the angle adjustment mechanism 4 to achieve the optimal oil scraping angle.

[0088] After the film surface is cooled by air, it will continue to enter the working range of the back cooling roller, and then circulate until it finally exits the cooling device to achieve cooling.

[0089] like Figure 6 Under the condition that parameters such as material quantity, rotation speed, and quench roller temperature remain unchanged, comparing the casting thickness with and without using this device, it can be clearly seen that using this device will produce the following effects:

[0090] 1. It will cause the overall casting curve to flatten into an M-shape, and the casting thickness range will be 0.528mm, which is 10% lower than the casting thickness range obtained without using this device;

[0091] 2. The overall thickness of the cast sheet decreased slightly (0.6%-7.1%);

[0092] 3. The width of the cast sheet is more stable.

[0093] in, Figure 6 The vertical axis represents thickness in mm, and the horizontal axis (1-17) represents 17 detection points. The selection method for these points is as follows: Using the left side as the starting zero point, construct an XY rule (X represents the short side direction of the casting, Y represents the long side direction). Select 17 points along the Y direction at intervals of 10cm from the zero point, and at 5cm intervals along the X direction (corresponding to...). Figure 6 The thickness of the material is measured on the horizontal axis (corresponding to the horizontal axis). Figure 6 The vertical axis represents the casting thickness curve corresponding to 10cm; 17 points are taken sequentially at positions 30cm from zero in the Y direction and 5cm apart in the X direction (corresponding to...). Figure 6 The thickness of the material is measured on the horizontal axis (corresponding to the horizontal axis). Figure 6 The vertical axis (Y-axis) is the casting thickness curve corresponding to 30cm; similarly, the casting thickness curves corresponding to 50cm, 70cm, 90cm, 110cm, 130cm, 150cm, 170cm and 190cm away from the zero point in the Y direction were detected respectively.

[0094] As shown in Figure 7, by Figure 7-1 and Figure 7-2 The comparison shows that before using this device, the microstructure of matte and glossy surfaces differed significantly, with the matte surface exhibiting low porosity and uneven pore size. Figure 7-1 and Figure 7-3 The comparison shows that after using this device, the porosity and pore size of the matte and glossy surfaces in the microstructure are more consistent with those of the glossy surface, effectively improving the porosity and air permeability of the base film.

[0095] in, Figure 7-1 7-1 shows the microstructure of the smooth surface, while 7-2 shows the microstructure of the matte surface before using this device. Figure 7-3 This refers to the microstructure diagram of the matte surface after using this device.

[0096] Among them, the smooth side refers to the casting sheet on the side in contact with the cooling roller, while the matte side refers to the casting sheet on the side away from the cooling roller.

[0097] like Figure 8 Using this device, the peel strength of the matte surface is significantly improved, increasing by 350%, and is nearly identical to that of the glossy surface. Under the condition of ensuring consistent film-forming process parameters and coating processes, without using the casting cooling device of this application, the peel strength of the glossy surface is 81.7 N / m, and the peel strength of the matte surface is 18 N / m; with the casting cooling device of this application, the peel strength of the glossy surface is 87.3 N / m, and the peel strength of the matte surface is 81.5 N / m.

[0098] The peel strength test involves coating both sides of the membrane prepared in this application with a coating of the same thickness and composition to form a coated diaphragm. A 2.5cm × 30cm mold is used to cut the sample. Double-sided adhesive tape for peel strength is pasted onto the test plate, and the surface sticker is peeled off, leaving approximately 2cm of the sticker. The sample is then flatly pasted onto the end of the double-sided adhesive tape with the peeled sticker removed. A pressure roller is used to roll the sample back and forth three times, and 1cm is manually peeled off to form the sample strip to be tested. The end with the remaining sticker is clamped in the lower jaw of the tensile testing machine, and the other end of the sample strip is clamped in the upper jaw, ensuring no tilting. The tensile speed is 50mm / min, and the average of three measurements is taken.

[0099] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the scope of protection of the present invention in any way, and all technical solutions obtained by equivalent substitution or other means fall within the scope of protection of the present invention. Parts not covered in this invention are the same as or can be implemented using existing technology.

Claims

1. A polyolefin microporous membrane for a secondary battery, characterized by, One side of the microporous membrane is a smooth surface, and the other side is a matte surface. The peeling force of the matte surface of the microporous membrane is 60-90 N / m, the smooth surface is a microporous membrane casting stage, and the casting film side close to the chill roll is formed into a membrane surface after stretching. The matte surface is a microporous membrane casting stage, and the casting film side away from the chill roll is formed into a membrane surface after stretching. The microporous membrane is in the casting stage, and the casting film thickness difference is 0.5-0.53 mm.

2. The polyolefin microporous membrane for a secondary battery according to claim 1, wherein The peeling force of the matte surface of the microporous membrane is 80-90 N / m, and the peeling force of the smooth surface is 60-90 N / m. The casting film thickness difference of the microporous membrane is 0.528 mm.

3. A process for producing a polyolefin microporous membrane for a secondary battery as claimed in claim 1 or 2, characterized by, The steps are as follows: Step 1, feeding and extruding; A certain amount of polyolefin resin powder is added to the double screw extruder through the metering system, and the pore-forming agent is added to the double screw extruder through the metering pump from the multiple oil ports set on the double screw extruder at the set temperature. It is melted and extruded to the die under the set temperature and pressure; Step 2, casting; The molten slurry extruded from the die is rapidly cooled by the chill roll to form a casting film. The casting film enters the casting cooling device with the chill roll rotating. The sheet-shaped substrate is cooled by multiple groups of air-cooled back cooling in the casting cooling device. Before entering the double-drawing inlet, the temperature of the sheet is reduced to 25-35℃; Step 3, double-drawing; The cooled sheet enters the two-way stretching machine group, and is sequentially preheated, two-way stretched, shaped, and cooled to form a thick film with a certain thickness and width; Step 4, extraction; The thick film is cut by a traction machine, and then enters an extraction tank containing dichloromethane to extract paraffin oil from the film. The film after the extraction outlet tank is dried by a temperature control roller, and then enters a drying room for further drying; Step 5, horizontal drawing; The dried film enters the horizontal drawing heat treatment machine group for micro horizontal expansion, and then is subjected to micro shrinkage in the horizontal and vertical directions to obtain a thin film, i.e. a microporous membrane; Step 6, winding and slitting; The microporous membrane is cut by a traction machine after winding at low tension, and then is cut into the required width by a slitting machine.

4. The process for preparing a polyolefin microporous membrane according to claim 3, characterized by, The gap between the back cooling roller and the chilling roller of the casting sheet cooling device in step 2 is 1-2 mm, the air cooling temperature is 2-5℃, the air volume is 4000-6000 m 3 / h, and the back cooling roller temperature is controlled at 8-12℃.

5. The process for the preparation of a polyolefin microporous membrane according to claim 4, characterized in that, The diameter of the back cooling roll is between 100-150 mm, and the number of back cooling rolls is not less than two. Each back cooling roll is separately driven by a separate drive motor to ensure that the speed is uniform with the chill roll speed, and the speed difference between the two is controlled within ±0.01 m / min.

6. A casting sheet cooling device for use in the production of the microporous membrane of claim 1 or 2 or for use in the production process of claim 3, characterized in that, The back cooling roll is provided with a wind knife, and the wind knife is in communication with the air supply mechanism. Side air exhaust areas are provided on both sides of the wind knife and the back cooling roll in the wind cooling cover. A scraper is installed at the lower edge of the wind knife for scraping white oil on the back cooling roll. The angle between the scraper and the back cooling roll is adjusted by an angle adjusting mechanism. The white oil scraped by the scraper flows to the side air exhaust areas on both sides of the scraper and is discharged after being collected.

7. The casting sheet cooling device according to claim 6, characterized by The air supply mechanism comprises a cold air distribution chamber, one end of which is connected with an external cooling unit through an air inlet main pipe, and the other end is communicated with an air knife through an air knife air inlet pipe, wherein one air knife corresponds to one air knife air inlet pipe; the cold air provided by the external cooling unit enters the cold air distribution chamber through the air inlet main pipe, and the cold air distribution chamber uniformly delivers the cold air to the air knife through the air knife air inlet pipe; the back cooling roller is fixed on the shell of the air cooling cover through a bearing seat, and the back cooling rollers are parallel to each other with their tangent lines on the same circle.

8. The casting sheet cooling device according to claim 6, characterized by The angle adjusting mechanism comprises an adjusting rotary rod, a moving nut and a connecting rod, both ends of the adjusting rotary rod are rotatably installed on a fixed support, the moving nut is sleeved on the adjusting rotary rod, and both ends of the connecting rod are respectively hinged with the moving nut and the scraper; the moving nut is moved back and forth by rotating the adjusting rotary rod, and the connecting rod changes the angle of the scraper.

9. The casting sheet cooling device according to claim 6, characterized by The air cooling cover serves as a device main frame, the side surface of the air cooling cover is provided with a distance adjusting mechanism, the distance adjusting mechanism is used for adjusting the distance between the back cooling roller and the chill roller; the side surface exhaust area is provided with an exhaust pipe, the bottom is provided with an oil discharge pipe, and both sides of the side surface exhaust area are provided with an inner baffle and an outer baffle, the front end of the outer baffle protrudes from the inner baffle, and is used for ensuring that the cold air blown to the film surface is effectively diffused to the side surface exhaust area and discharged without overflowing; the area between the two air knives is connected through an arc plate, which is used for ensuring that the cold air blown by the air knife does not escape to the dead zone behind the back cooling roller; the distance adjusting mechanism comprises a distance adjusting block connected with the air cooling cover, the distance adjusting block is slidably installed on the chill roller installation wall plate, one end of the distance adjusting block away from the chill roller is connected with a hydraulic cylinder, and the other end close to the chill roller is provided with a limiting sliding block, the limiting sliding block and the distance adjusting block are in contact and butt joint through a slope structure, and the limiting sliding block is installed on a vertically arranged lead screw, an adjusting handle is arranged at the end of the lead screw, and the limiting sliding block is driven to move up and down on the lead screw by rotating the adjusting handle, so as to limit the distance of the distance adjusting block.

10. A microporous membrane prepared by the cast sheet cooling device of any one of claims 6-9.

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