Asynchronous stretching polytetrafluoroethylene film with high aperture uniformity and preparation method thereof

Abstract

The invention relates to the field of materials, and provides an asynchronous stretching polytetrafluoroethylene film with high aperture uniformity and a preparation method thereof. The method comprises the following steps: mixing polytetrafluoroethylene dispersion resin with mixed lubricating oil, curing, and then obtaining the polytetrafluoroethylene film through pre-pressing extrusion, multi-module longitudinal stretching and expanding and sizing, wherein the longitudinal stretching module is provided with three sections of temperature zones which are sequentially and adjacently connected, namely a core stretching deoiling zone, a high-temperature activation zone and an annealing rolling zone. Oil-containing stretching is realized by matching mixed lubricating oil with gradient temperature configuration and precise control of stretching speed in a longitudinal stretching stage, and meanwhile, controllability of pore size and high uniformity of pore size are realized by matching proper process parameters such as two-way stretching temperature, speed and multiple; the membrane can be applied to various different scenes as a filtering material.

Description

Technical Field

[0001] The present application relates to the field of materials, and more specifically, it relates to a polytetrafluoroethylene film with high pore size uniformity prepared by asynchronous stretching and a preparation method thereof. Background Art

[0002] Polytetrafluoroethylene material (PTFE) has excellent high and low temperature performance, chemical stability, electrical insulation, non-adhesion, weather resistance, incombustibility, and good lubricity. Due to its unique properties, it has currently been widely used in industrial sectors such as aerospace, petrochemical, machinery, electronics, construction, and textile. The PTFE microporous membrane formed by stretching has been used in multiple other fields such as membrane distillation, oil-water separation, clothing, and air filtration. Solving various problems existing in the application fields and further promoting the application of PTFE membranes are research hotspots in various countries, and the pore structure therein is the key to determining the application of the film.

[0003] Polytetrafluoroethylene is difficult to process. Since it is insoluble in any solvent and has poor fluidity at high temperatures, above its melting point (327 °C), it cannot transform from the high elastic state to the viscous flow state, and it does not flow even when heated to the decomposition temperature. This makes it impossible to use the forming methods of general thermoplastic plastics, but instead, methods similar to cold pressing and sintering or paste extrusion in powder metallurgy need to be used for forming. It can be prepared according to the method disclosed in US Patent US3953566A, but there are problems such as poor thickness uniformity. In response to this problem, Patent CN115230131A proposed to improve the thickness uniformity problem by adding fillers such as iron powder or graphene and improving the biaxial stretching machine method. However, this increases the processing cost and the addition of fillers reduces the wide range of application scenarios. In addition, there are still some problems in the control of the pore size of the microporous membrane at the present stage, such as free control of the micropore size and uniformity of the micropore distribution. Summary of the Invention

[0004] Based on the above technical problems, in order to enable the use of polytetrafluoroethylene membranes as filter membranes in different application scenarios, the present application provides a preparation method for stretched polytetrafluoroethylene membranes, which can improve the pore size uniformity of polytetrafluoroethylene membranes and can control the pore size.

[0005] The present application adopts the following technical solutions: In the first aspect, the present application provides a preparation method for a polytetrafluoroethylene film with high pore size uniformity prepared by asynchronous stretching, which includes: (1) Stir and mix polytetrafluoroethylene dispersion resin and a mixed lubricating oil according to a mass ratio of 1:(0.10 - 0.35), and then carry out aging to obtain an aged material; (2) Pre-press and form the aged material to make a blank, and the resin particles are tightly combined to obtain a columnar blank; after keeping the columnar blank warm, push and extrude it into a strip-shaped blank; roll the blank into a base belt; (3) Feed the baseband into the longitudinal stretching module. When drying and degreasing are completed, longitudinal stretching is carried out to obtain a polytetrafluoroethylene base film. The longitudinal stretching module has three adjacent temperature zones in sequence, namely the core stretching and degreasing zone, the high-temperature activation zone, and the annealing and winding zone. Among them, the core stretching and degreasing zone adopts stepwise stretching, with a total stretching multiple of 2 to 12 times, a stretching speed of 1 to 5 m / min, and the stretching temperature is set in a gradient within the range of 120 to 260 °C; (4) Carry out transverse stretching, heat setting, and cooling and winding on the polytetrafluoroethylene base film to finally obtain a polytetrafluoroethylene microporous film.

[0006] Further, when longitudinally stretching the baseband, the temperature of the high-temperature activation zone is 270 to 320 °C.

[0007] Further, when longitudinally stretching the baseband as described above, the temperature of the annealing and winding zone is 100 to 200 °C.

[0008] Due to the difficult processability of polytetrafluoroethylene and its unique characteristics of lamellar slip fibrillation, a lubricant that is easy to diffuse and infiltrate into polytetrafluoroethylene dispersed resin particles needs to be selected. The kinematic viscosity and distillation range of this lubricant have a great influence on the pore structure of the film. If the kinematic viscosity is too small, although it is easy to infiltrate and diffuse into the resin powder, it will reduce the fibrillation ductility; on the contrary, if it is too large, the infiltration difficulty is great and the curing and swelling are not sufficient.

[0009] The present application uses a mixed lubricating oil, and its distillation range is modulated corresponding to the temperature of the core stretching zone of the longitudinal stretching module. Further, the above-mentioned mixed lubricating oil is composed of isoparaffin and kerosene and / or liquid paraffin, and the mass ratio of isoparaffin is 40 - 50%. Preferably, the mixed lubricating oil is mixed by isoparaffin and kerosene and / or liquid paraffin according to a mass ratio of 1:1 - 1.5.

[0010] Feed the undegreased baseband into the longitudinal stretching module. The longitudinal stretching module consists of three parts: The first part is the core stretching and degreasing zone. This module is composed of six rollers. Stretching is achieved through the rotational speed difference among them, and the temperature gradient is configured. While realizing oil-containing stretching according to the distillation range of the modulated lubricating oil, part of the oil volatilizes. Different from other processes of drying and degreasing first and then stretching. On the one hand, oil-containing stretching can increase the fluidity of the polytetrafluoroethylene chain and prevent the formation of cracks in the initial fibrillation stage. On the other hand, due to longitudinal stretching, the lamellae in the polytetrafluoroethylene particles slide, and the lubricant diffuses between the crystal domains, reducing the activation energy of lamellar sliding and fibril formation. Based on these two points, it lays a foundation for the uniformity of the micropore distribution after subsequent transverse stretching; The second part is the high-temperature activation zone, which increases the molecular chain activity of the strip-shaped fibrils formed by the entanglement of the amorphous zone and the lamellar slip after longitudinal stretching at high temperature, and enhances the connection strength between the lamellae; The third part is the annealing and winding area. The winding roller is set at a certain temperature to make the molecular chains of polytetrafluoroethylene rearrange regularly, improving the entanglement firmness at the lamellar interfaces.

[0011] Further, when the polytetrafluoroethylene base film is transversely stretched, the stretching temperature is 210 - 320 °C, the stretching ratio is 3 - 20 times, the unwinding speed is 1 - 16 m / min, and the heat setting temperature is 330 - 390 °C.

[0012] Further, the crystallinity of the polytetrafluoroethylene dispersion resin is ≥97%, and the number average molecular weight is 2 million - 20 million.

[0013] Further, during the preparation of the curing material, the curing temperature is 30 - 70 °C, and the curing time is 24 - 48 h.

[0014] Further, during the process of obtaining the base belt, the blank-making treatment temperature is 30 - 50 °C, and the pressure holding time is 3 - 10 min; the temperature of the push-press extrusion treatment is 30 - 60 °C, and the compression ratio is 50 - 300.

[0015] In a second aspect, the present application provides a polytetrafluoroethylene film with high pore size uniformity and asynchronous stretching, which is obtained by the above preparation method. The pore size range of the polytetrafluoroethylene film is controllable within 0.05 - 2.5 μm.

[0016] Further, the above polytetrafluoroethylene film includes a microporous film, a micro-medium pore film, and a micro-large pore film; the microporous film is a liquid filtration film with a pore size of 0.05 - 0.5 μm and can be applied to the field of liquid filtration; the micro-medium pore film is a breathable and waterproof film with a pore size of 0.5 - 1.2 μm and can be applied to the field of electronic products; the micro-large pore film is an air filtration film with a pore size of 1.2 - 2.5 μm and can be applied to the field of air filtration.

[0017] In summary, the present application has the following beneficial effects: The preparation method provided by the present application realizes oil-containing stretching of the non-degreased base belt through a mixed lubricating oil and special multi-module longitudinal stretching, laying a foundation for the regulation of pore size uniformity and pore size. After longitudinal stretching, the fibers have no microcracks and high strength, which is more conducive to pore formation during subsequent transverse stretching. The lamellae fully slip, providing feasible conditions for the uniformity of pore formation during subsequent transverse stretching; coordinating the reasonable allocation of temperature, speed, and ratio of biaxial stretching can solve the problems of uneven pore size distribution and inaccurate controllability of pore size after current biaxial stretching.

[0018] The mixed lubricating oil of the method of the present invention in combination with the longitudinal stretching module greatly improves the strength of longitudinal fibers, reduces the possibility of transverse fiber fracture, and at the same time, the setting of the longitudinal stretching module can reduce the activation energy of the nodes and reduce the defect of poor pore size uniformity during transverse stretching.

[0019] The longitudinal stretching ratio and temperature have a great influence on the pore size and uniformity of the polytetrafluoroethylene membrane. The higher the longitudinal stretching ratio, the pore size first increases and then decreases, and the uniformity gradually increases; increasing the longitudinal stretching temperature within a certain range improves the pore size uniformity. At the same time, during the transverse stretching process, as the stretching ratio increases, the pore size also first increases and then decreases; as the transverse width expansion speed increases, the average pore size decreases, and the uniformity is optimal at a certain speed. Through full exploration of the biaxial stretching law, on the basis of using a mixed lubricating oil, this application selects process parameters of different biaxial stretching temperatures, ratios, and speeds to achieve controllability of the pore size of the polytetrafluoroethylene fiber membrane within the range of 0.05 - 2.5 μm.

[0020] The biaxial stretching process in the present invention has wide applicability. The precise control of the pore size enables its application in the preparation of filter membranes for chip etching solutions from dozens of nanometers to air filter membranes of several micrometers. Moreover, the pores with high uniformity result in better and more stable filtering effects, facilitating its popularization and application in industrial production. Description of the Drawings

[0021] Figure 1 is the flow chart of the preparation method of the polytetrafluoroethylene microporous membrane of the present invention; Figure 2 is the sampling schematic diagram of various tests in the examples and comparative examples; Figure 3 is the pore size distribution diagram of the polytetrafluoroethylene film prepared in Example 1; Figure 4 is the electron microscope picture of the polytetrafluoroethylene film prepared in Example 2. Detailed Embodiments

[0022] The following will describe the implementation schemes of the present invention in detail in combination with examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention. For the specific conditions not specified in the examples, they are carried out according to the conventional conditions or the conditions recommended by the manufacturer. For the reagents or instruments not specified by the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0023] The following will detail the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not used to limit the present invention.

[0024] Example 1 This example provides a polytetrafluoroethylene film with high pore size uniformity prepared by asynchronous stretching. Its preparation process is as Figure 1 shown and specifically includes: Step (1): Mix polytetrafluoroethylene dispersion resin (number-average molecular weight of 10 million and crystallinity of ≥97%) with a mixed lubricating oil in a mass ratio of 10:2.5 for curing to obtain a paste-like cured material; the mixed lubricating oil is prepared by mixing isoparaffin and kerosene, and the mass ratio of the two is 1:1; the curing temperature is 50°C and the curing time is 24 h; Step (2): Pre-press the cured material to form a blank, and the resin particles are tightly combined to obtain a columnar blank. The blanking pressure is 3 MPa and the pressure holding time is 5 minutes; Step (3): Push the columnar blank obtained in step (2) into an extruder, keep it warm in a 30°C material gun for 10 minutes, and extrude it into a strip-shaped blank with a compression ratio of 150:1; Step (4): Feed the strip-shaped blank obtained in step (3) into a two-roll mill and extrude it to obtain a base tape with a width of 250 mm and a thickness of 200 μm; Step (5): Place the base tape obtained in step (4) into a longitudinal stretching module. First, deoiling is achieved during the stretching process in the core stretching zone. The temperature in this zone is set according to a gradient of 170°C - 240°C, the stretching speed is 3 m / min, and the stretching ratio is 2 times; then it passes through a high-temperature activation zone where the temperature is 300°C and there is no stretching; finally, a longitudinally stretched film is collected in the annealing and winding zone with the temperature maintained at 200°C.

[0025] Step (6): Horizontally expand the longitudinally stretched film obtained in step (5). The horizontal stretching ratio is 8.4 times, the stretching temperature is 300°C, and the stretching speed is 10 m / min; then heat setting is completed while keeping the film clamped. The heat setting temperature is 360°C; after cooling, it is wound to obtain a polytetrafluoroethylene microporous membrane.

[0026] Example 2 This example provides a polytetrafluoroethylene film with high pore size uniformity and asynchronous stretching, and its preparation method includes: Step (1): Mix polytetrafluoroethylene dispersion resin (number-average molecular weight of 10 million and crystallinity of ≥97%) with a mixed lubricating oil in a mass ratio of 10:3 for curing to obtain a paste-like cured material; the mixed lubricating oil is prepared by mixing isoparaffin, kerosene and paraffin oil, and the mass ratio of the three is 1:1:0.5; the curing temperature is 40°C and the curing time is 48 h; Step (2): Pre-press the cured material to form a blank, and the resin particles are tightly combined to obtain a columnar blank. The blanking pressure is 3 MPa and the pressure holding time is 5 minutes; Step (3): Push the columnar blank obtained in step (2) into an extruder, keep it warm in a 30°C material gun for 10 minutes, and extrude it into a strip-shaped blank with a compression ratio of 150:1; Step (4): Feed the strip-shaped tape blank obtained in step (3) into a two-roll mill, and extrude to obtain a base tape with a width of 250 mm and a thickness of 200 μm. Step (5): Place the base tape obtained in step (4) into the longitudinal stretching module. First, realize oil removal during the stretching process in the core stretching zone. The temperature in this zone is set according to a gradient of 150°C - 250°C, the stretching speed is 3 m / min, and the stretching ratio is 8 times. Then pass through the high-temperature activation zone, where the temperature is 300°C and there is no stretching. Finally, collect the longitudinally stretched film in the annealing and winding zone, and keep the temperature at 200°C. Step (6): Perform transverse width expansion on the longitudinally stretched film obtained in step (5). The transverse stretching ratio is 8.4 times, the stretching temperature is 320°C, and the stretching speed is 10 m / min. Then complete heat setting while keeping the film in a clamped state. The heat setting temperature is 360°C. After cooling, wind up to obtain a polytetrafluoroethylene microporous film.

[0027] Example 3 This example provides a polytetrafluoroethylene film with high pore size uniformity and asynchronous stretching, and its preparation method includes: Step (1): Mix polytetrafluoroethylene dispersion resin (number average molecular weight is 10 million, crystallinity is ≥97%) and a mixed lubricating oil according to a mass ratio of 10:2.5 for ripening to obtain a paste-like ripened material. The mixed lubricating oil is prepared by mixing isoparaffin and kerosene, and the mass ratio of the two is 1:1. The ripening temperature is 50°C, and the ripening time is 24 h. Step (2): Perform pre-pressing and molding on the ripened material to form a blank. The resin particles are closely combined to obtain a columnar blank. The molding pressure is 5 MPa, and the pressure holding time is 10 minutes. Step (3): Push the columnar blank obtained in step (2) into an extruder, keep it warm in the material gun at 30°C for 10 minutes, and extrude it into a strip-shaped tape blank with a compression ratio of 128:1. Step (4): Feed the strip-shaped tape blank obtained in step (3) into a two-roll mill, and extrude to obtain a base tape with a width of 300 mm and a thickness of 230 μm. Step (5): Place the base tape obtained in step (4) into the longitudinal stretching module. First, realize oil removal during the stretching process in the core stretching zone. The temperature in this zone is set according to a gradient of 160°C - 240°C, the stretching speed is 1 m / min, and the stretching ratio is 8 times. Then pass through the high-temperature activation zone, where the temperature is 300°C. Finally, collect the longitudinally stretched film in the annealing and winding zone, and keep the temperature at 200°C.

[0028] Step (6): The longitudinally stretched film obtained in step (5) is subjected to transverse width expansion, with a transverse stretching ratio of 3 times, a stretching temperature of 300 °C, and a stretching speed of 10 m / min; then heat setting is completed while the film is in a clamped state, with a heat setting temperature of 360 °C; after cooling, winding is carried out to obtain a polytetrafluoroethylene microporous film.

[0029] Example 4 This example provides a polytetrafluoroethylene film with high pore size uniformity by asynchronous stretching, and its preparation method includes: Step (1): The polytetrafluoroethylene dispersion resin (number average molecular weight is 20 million, crystallinity is ≥97%) and the mixed lubricating oil are mixed in a mass ratio of 10:3 for ripening to obtain a paste-like ripened material; the mixed lubricating oil is prepared by mixing isoparaffin and kerosene, and their mass ratio is 1:1; the ripening temperature is 50 °C, and the ripening time is 24 h; Step (2): The ripened material is pre-pressed and formed into a blank, and the resin particles are closely combined to obtain a columnar blank body. The blanking pressure is 5 MPa, and the pressure holding time is 10 minutes; Step (3): The columnar blank body obtained in step (2) is pushed into an extruder and kept warm in a 30 °C material gun for 10 minutes, and then extruded into a strip-shaped blank with a compression ratio of 180:1; Step (4): The strip-shaped blank obtained in step (3) is fed into a two-roll wheel and extruded to obtain a base belt with a width of 200 mm and a thickness of 180 μm; Step (5): The base belt obtained in step (4) is placed in a longitudinal stretching module. First, defatting is achieved during the stretching process in the core stretching zone, and the temperature in this zone is set according to a gradient of 170 °C - 240 °C, the stretching speed is 3 m / min, and the stretching ratio is 10 times; then it passes through the high-temperature activation zone, where the temperature is 320 °C; finally, a longitudinally stretched film is collected in the annealing and winding zone, and the temperature is kept at 200 °C.

[0030] Step (6): The longitudinally stretched film obtained in step (5) is subjected to transverse width expansion, with a transverse stretching ratio of 18 times, a stretching temperature of 300 °C, and a stretching speed of 10 m / min; then heat setting is completed while the film is in a clamped state, with a heat setting temperature of 360 °C; after cooling, winding is carried out to obtain a polytetrafluoroethylene microporous film.

[0031] Example 5 This example provides a polytetrafluoroethylene film with high pore size uniformity by asynchronous stretching, and its preparation process is as Figure 1 shown, specifically including: Step (1): Mix polytetrafluoroethylene dispersion resin (number average molecular weight of 20 million, crystallinity ≥ 97%) with a mixed lubricating oil at a mass ratio of 10:2.5 for curing to obtain a paste-like cured material; the mixed lubricating oil is prepared by mixing isoparaffin and kerosene, and the mass ratio of the two is 1:1; the curing temperature is 30 °C and the curing time is 48 h; Step (2): Pre-press and form the cured material into a blank. The resin particles are tightly combined to obtain a columnar blank. The blanking pressure is 4 MPa and the pressure holding time is 10 minutes; Step (3): Push the columnar blank obtained in step (2) into an extruder, keep it warm in a material gun at 40 °C for 10 minutes, and extrude it into a strip-shaped blank with a compression ratio of 50:1; Step (4): Feed the strip-shaped blank obtained in step (3) into a two-roll mill and extrude to obtain a base belt with a width of 250 mm and a thickness of 200 μm; Step (5): Place the base belt obtained in step (4) into a longitudinal stretching module. First, realize deoiling during the stretching process in the core stretching area. The temperature in this area is set according to a gradient of 120 °C - 200 °C, the stretching speed is 5 m / min, and the stretching ratio is 10 times; then pass through the high-temperature activation area, where the temperature is 320 °C and the stretching ratio is 1.2 times; finally, collect the longitudinally stretched film in the annealing and winding area, and keep the temperature at 100 °C.

[0032] Step (6): Horizontally expand the longitudinally stretched film obtained in step (5). The horizontal stretching ratio is 3 times, the stretching temperature is 210 °C, and the stretching speed is 3 m / min; then complete heat setting while keeping the film in a clamped state. The heat setting temperature is 360 °C; after cooling, wind it up to obtain a polytetrafluoroethylene microporous membrane.

[0033] Example 6 This example provides a polytetrafluoroethylene film with high pore size uniformity and asynchronous stretching, and its preparation process is as Figure 1 shown, specifically including: Step (1): Mix polytetrafluoroethylene dispersion resin (number average molecular weight of 2 million, crystallinity ≥ 97%) with a mixed lubricating oil at a mass ratio of 10:1 for curing to obtain a paste-like cured material; the mixed lubricating oil is prepared by mixing isoparaffin and kerosene, and the mass ratio of the two is 1:1; the curing temperature is 70 °C and the curing time is 35 h; Step (2): Pre-press and form the cured material into a blank. The resin particles are tightly combined to obtain a columnar blank. The blanking pressure is 10 MPa and the pressure holding time is 3 minutes; Step (3): Push the columnar blank obtained in step (2) into an extruder, keep it warm in a material gun at 60 °C for 10 minutes, and extrude it into a strip-shaped blank with a compression ratio of 300:1; Step (4): Feed the strip-shaped green tape obtained in step (3) into a two-roll mill and extrude to obtain a base tape with a width of 200 mm and a thickness of 200 μm. Step (5): Place the base tape obtained in step (4) into the longitudinal stretching module. First, achieve oil removal during the stretching process in the core stretching zone, where the temperature of this zone is set according to a gradient of 180°C - 260°C, the stretching speed is 1 m / min, and the stretching ratio is 2 times. Then, pass through the high-temperature activation zone, where the temperature of this zone is 270°C and the stretching ratio is 1 time. Finally, collect the longitudinally stretched film in the annealing and winding zone, and keep the temperature at 150°C.

[0034] Step (6): Horizontally expand the longitudinally stretched film obtained in step (5), with a horizontal stretching ratio of 20 times, a stretching temperature of 320°C, and a stretching speed of 20 m / min. Then, complete heat setting while keeping the film in a clamped state, with a heat setting temperature of 360°C. After cooling, wind up to obtain the polytetrafluoroethylene microporous membrane.

[0035] Comparative Example 1 A method for preparing a polytetrafluoroethylene fiber membrane, which is different from the steps of Example 1, uses a single lubricating oil without oil-containing stretching, and its main steps are as follows; Step (1): Mix polytetrafluoroethylene dispersion resin (number average molecular weight of 10 million, crystallinity ≥ 97%) and lubricating oil (isoparaffin) according to a mass ratio of 10:2.5 for curing to obtain a paste-like cured material; the curing temperature is 50°C and the curing time is 24 h; Step (2): Pre-press and form the cured material to make a blank, and the resin particles are tightly combined to obtain a columnar blank. The blank-making pressure is 3 MPa and the pressure-holding time is 5 minutes; Step (3): Push the columnar blank obtained in step (2) into an extruder, keep it warm in a 30°C material gun for 10 minutes, and extrude it into a strip-shaped green tape, with a compression ratio of 150:1; Step (4): Feed the strip-shaped green tape obtained in step (3) into a two-roll mill and extrude to obtain a base tape with a width of 250 mm and a thickness of 200 μm; Step (5): Place the base tape obtained in step (4) into a dryer for oil removal treatment, and the oil removal temperature is 180°C; Step (6): Longitudinally stretch the base tape obtained in step (5), with a stretching ratio of 2 times, a stretching temperature of 200°C, and a stretching speed of 3 m / min; Step (7): Horizontally expand the horizontally stretched film obtained in step (6), with a horizontal stretching ratio of 8.4 times, a stretching temperature of 300°C, and a stretching speed of 10 m / min. Then, complete heat setting while keeping the film in a clamped state, with a heat setting temperature of 360°C. After cooling, wind up to obtain the polytetrafluoroethylene microporous membrane.

[0036] Comparative Example 2 A method for preparing a polytetrafluoroethylene fiber membrane, which is different from the steps of Example 1. Without using the oil-containing stretching and other longitudinal stretching modules with a mixed lubricant, the main steps are as follows; Step (1): Mix the polytetrafluoroethylene dispersion resin (number average molecular weight of 10 million and crystallinity of ≥97%) with the mixed lubricant at a mass ratio of 10:2.5 for curing to obtain a paste-like cured material; the mixed lubricant is prepared by mixing isoparaffin and kerosene, and the mass ratio of the two is 1:1; the curing temperature is 50°C and the curing time is 24 h; Step (2): Pre-press and mold the cured material to form a blank, and the resin particles are closely combined to obtain a columnar blank. The molding pressure is 3 MPa and the pressure holding time is 5 minutes; Step (3): Push the columnar blank obtained in step (2) into an extruder, keep it warm in a 30°C material gun for 10 minutes, and extrude it into a strip-shaped blank with a compression ratio of 150:1; Step (4): Feed the strip-shaped blank obtained in step (3) into a two-roll mill and extrude it to obtain a base belt with a width of 250 mm and a thickness of 200 μm; Step (5): Put the base belt obtained in step (4) into a dryer for deoiling treatment, and the deoiling temperature is 160°C; Step (6): Longitudinally stretch the base belt obtained in step (5), with a stretching ratio of 2 times, a stretching temperature of 200°C, and a stretching speed of 3 m / min; Step (7): Horizontally expand the longitudinally stretched film obtained in step (6), with a horizontal stretching ratio of 8.4 times, a stretching temperature of 300°C, and a stretching speed of 10 m / min; then complete heat setting while keeping the film in a clamped state, and the heat setting temperature is 360°C; after cooling, wind it up to obtain a polytetrafluoroethylene microporous membrane.

[0037] Comparative Example 3 A method for preparing a polytetrafluoroethylene fiber membrane, which is different from the steps of Example 3. Without using the oil-containing stretching with a single lubricant, the main steps are as follows; Step (1): Mix the polytetrafluoroethylene dispersion resin (number average molecular weight of 10 million and crystallinity of ≥97%) with the single lubricant (kerosene) at a mass ratio of 10:2.5 for curing to obtain a paste-like cured material; the curing temperature is 50°C and the curing time is 24 h; Step (2): Pre-press and mold the cured material to form a blank, and the resin particles are closely combined to obtain a columnar blank. The molding pressure is 5 MPa and the pressure holding time is 10 minutes; Step (3): Push the columnar blank obtained in step (2) into an extruder, keep it warm in a 30°C material gun for 10 minutes, and extrude it into a strip-shaped blank with a compression ratio of 128:1; Step (4): Feed the strip-shaped tape blank obtained in step (3) into a two-roll mill and extrude to obtain a base tape with a width of 300 mm and a thickness of 230 μm. Step (5): Put the base tape obtained in step (4) into a dryer for degreasing treatment, and the degreasing temperature is 180°C. Step (6): Longitudinally stretch the base tape obtained in step (5), the stretching ratio is 8 times, the stretching temperature is 200°C, and the stretching speed is 3 m / min. Step (7): Horizontally expand the longitudinally stretched film obtained in step (6), the horizontal stretching ratio is 3 times, the stretching temperature is 300°C, and the stretching speed is 10 m / min; then complete heat setting while keeping the film in a clamped state, and the heat setting temperature is 360°C; after cooling, wind up to obtain a polytetrafluoroethylene microporous membrane.

[0038] Perform pore size testing and air filtration efficiency testing on the polytetrafluoroethylene film: (I) Pore size testing (average pore size, most probable pore size) 1. Detection method Use PMI (pore size analyzer by gas permeation method) equipment for testing. Cut the sample to be tested into a circular or other shape with the diameter specified by the equipment. Note that the inside of the test sample should not be cut. After soaking the sample in the surfactant required for PMI equipment testing for 30 minutes, put the sample into the test tank of the instrument, and then tighten the lid of the tank to start the test. The test results are directly read by the equipment and converted into the distribution of the proportion (%) of each pore size (μm) of the filter material in the whole material (including average pore size, maximum pore size μm, minimum pore size μm, pore size distribution). Uniformly take three positions (A, B, C) along the transverse direction of the polytetrafluoroethylene film for sampling and testing, and then select three points (1, 2, 3) at each position, as Figure 2 shown.

[0039] 2. Detection results (1) Perform pore size testing on the polytetrafluoroethylene films prepared in Examples 1 to 4 and Comparative Examples 1 to 3. The average pore size, standard deviation and overall uniformity results at each position are shown in Table 1.

[0040] Table 1.

[0041] (2) Perform pore size testing on the polytetrafluoroethylene films prepared in Examples 1 to 4 and Comparative Examples 1 to 3. The most probable pore size (Mode) at each position and its corresponding proportion results are shown in Table 2.

[0042] Table 2.

[0043] As can be seen from Table 1 and Table 2, the polytetrafluoroethylene films provided in Examples 1-4 of the present application have good uniformity in pore size distribution, and accurate adjustment of pore size can be achieved.

[0044] (II) Air filtration efficiency 1. Detection method The filtration efficiency of the polytetrafluoroethylene films prepared in Example 3 and Comparative Example 3 was tested. The particle size distribution of sodium chloride (NaCl) aerosol used under the specified test conditions should be that the count median diameter (CMD) is 0.075 μm ± 0.020 μm, and the geometric standard deviation does not exceed 1.86 (equivalent to the aerodynamic mass median diameter (MMAD) of 0.24 μm ± 0.06 μm), and the gas flow rate is 85 L / min. The air filtration efficiency was detected at the same sampling points as the pore size test above.

[0045] 2. Detection results The average filtration efficiency and standard deviation results at each position of Example 3 and Comparative Example 3 are shown in Table 3.

[0046] Table 3.

[0047] As can be seen from Table 3, the pore size uniformity has a great influence on the filtration efficiency. High pore size uniformity can improve the filtration efficiency and stability. From Figure 3 the pore size test results, the uniform distribution of the pore size at a single measurement point can be intuitively seen, and from Figure 4 the scanning electron microscope images, it can also be seen that the fiber length distributions in the horizontal and vertical directions are uniform.

[0048] This specific embodiment is only an interpretation of the present application and does not limit the present application. Those skilled in the art can make modifications to this embodiment without creative contributions according to needs after reading this specification, but as long as it is within the scope of the claims of the present application, it is protected by the patent law.

Claims

1. A method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity, characterized in that: It includes: The polytetrafluoroethylene dispersion resin and the mixed lubricating oil are stirred and mixed in a mass ratio of 1: (0.10-0.35) and then aged to obtain an aged material; The matured material is pre-pressed and formed into a blank, and the resin particles are tightly combined to obtain a columnar embryo; after the columnar embryo is kept warm, it is pushed and extruded into a strip embryo, and the strip embryo is rolled into a base strip; The base tape is fed into a longitudinal stretching module, and longitudinal stretching is completed during drying and deoiling to obtain a polytetrafluoroethylene base film; the longitudinal stretching module has three temperature zones adjacent to each other in sequence, namely a core stretching and deoiling zone, a high-temperature activation zone, and an annealing and winding zone, wherein the core stretching and deoiling zone adopts step-by-step stretching, the total stretching multiple is 2 to 12 times, the stretching speed is 1 to 5 m / min, and the stretching temperature is set in a gradient range of 120 to 260°C; The polytetrafluoroethylene base film is transversely stretched, heat-set, cooled and rolled up to finally obtain a polytetrafluoroethylene microporous film.

2. The method for preparing the asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to claim 1, characterized in that: The mixed lubricating oil is composed of isoparaffin and kerosene and / or liquid paraffin, wherein the mass proportion of the isoparaffin is 40-50%.

3. The method for preparing the asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to claim 1, characterized in that: When the base tape is longitudinally stretched, the temperature of the high-temperature activation zone is 270-320°C.

4. The method for preparing the asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to claim 1, characterized in that: When the base tape is longitudinally stretched, the temperature of the annealing and winding zone is 100-200°C.

5. The method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to any one of claims 1 to 4, characterized in that: When the polytetrafluoroethylene base film is transversely stretched, the stretching temperature is 210 to 320° C., the stretching ratio is 3 to 20 times, and the unwinding speed is 1 to 16 m / min.

6. The method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to any one of claims 1 to 4, characterized in that: The polytetrafluoroethylene dispersed resin has a crystallinity of ≥97% and a number average molecular weight of 2 million to 20 million.

7. The method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to any one of claims 1 to 4, characterized in that: In the process of preparing the slaking material, the slaking temperature is 30 to 70° C. and the slaking time is 24 to 48 hours.

8. The method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to any one of claims 1 to 4, characterized in that: In the process of obtaining the base tape, the blanking treatment temperature is 30-50° C., and the holding time is 3-10 min; the pushing extrusion treatment temperature is 30-60° C., and the compression ratio is 50-300:

1.

9. An asynchronously stretched polytetrafluoroethylene film with high pore size uniformity, characterized in that: It is obtained according to the preparation method according to any one of claims 1 to 8, and the pore size of the polytetrafluoroethylene film ranges from 0.05 to 2.5 μm.

10. The asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to claim 9, characterized in that: The polytetrafluoroethylene film includes a microporous film, a micro-mesoporous film and a micro-macroporous film; The microporous membrane is a liquid filtration membrane with a pore size of 0.05 to 0.5 μm; The micro-mesoporous membrane is a breathable and waterproof membrane with a pore size of 0.5 to 1.2 μm; The micro-macroporous membrane is an air filter membrane with a pore size of 1.2 to 2.5 μm.

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