High-aperture-uniformity asynchronous-stretching polytetrafluoroethylene film and method of making same

Abstract

The application relates to the field of material design, and provides a high-aperture-uniformity asynchronous stretching polytetrafluoroethylene film and a preparation method thereof. The method comprises the following steps: mixing polytetrafluoroethylene dispersion resin with mixed lubricating oil, then aging, and then obtaining the polytetrafluoroethylene film through pre-pressing extrusion, multi-module longitudinal stretching and expansion shaping; wherein the longitudinal stretching module has three sections of temperature zones which are sequentially adjacent, and the three sections of temperature zones are respectively a core stretching oil removal zone, a high-temperature activation zone and an annealing winding zone. The oil-containing stretching is realized by using the mixed lubricating oil in cooperation with the gradient temperature configuration of the longitudinal stretching stage and the precise control of the stretching speed, meanwhile, the process parameters such as the appropriate bidirectional stretching temperature, speed and multiple are matched to realize the controllable aperture size and the high uniformity of the aperture, and the film can be applied to various different scenes as a filtering material.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of materials, more particularly, it relates to a high-pore-diameter-uniformity asynchronous-stretching polytetrafluoroethylene film and a preparation method thereof. BACKGROUND

[0002] Polytetrafluoroethylene material (PTFE) has excellent high and low temperature performance and chemical stability, electrical insulation, non-adhesion, weather resistance, non-combustibility and good lubricity. Due to its unique performance, it has been widely used in aerospace, petrochemical, machinery, electronics, construction, light textile and other industrial departments. PTFE microporous membrane formed by stretching has been used in membrane distillation, oil-water separation, clothing field, air filtration and other fields, and further promoting the application of PTFE membrane to solve various problems existing in the application field is the research focus of various countries, and the pore structure is the key to determine the application of the film.

[0003] Polytetrafluoroethylene is difficult to process, because it is insoluble in any solvent, and has poor flowability at high temperature, it cannot be transformed from high elastic state to viscous flow state above the melting point (327℃), even if heated to the decomposition temperature, it does not flow. This makes it impossible to use the molding method of general thermoplastic plastics, but to use cold pressing sintering or paste extrusion method similar to powder metallurgy. According to the method disclosed in US3953566A, it can be prepared, but there are problems such as poor thickness uniformity, and the patent CN115230131A proposes to improve the thickness uniformity problem by adding fillers such as iron powder or graphene and improving the method of two-way stretching machine, but the processing cost is increased and the addition of fillers reduces the universality of the application scene. In addition, there are still some problems in the pore size control of microporous membrane at the present stage, such as free control of micropore size, uniformity of micropore distribution, etc. SUMMARY

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

[0005] The present application adopts the following technical scheme:

[0006] In a first aspect, the present application provides a preparation method of a high-pore-diameter-uniformity asynchronous-stretching polytetrafluoroethylene film, which comprises:

[0007] (1) stirring and mixing polytetrafluoroethylene dispersion resin and mixed lubricating oil according to a mass ratio of 1:(0.10-0.35), and then aging to obtain an aging material;

[0008] (2) Pre-pressing the matured material to form a blank, and the resin particles are tightly combined to obtain a columnar embryo; after heat preservation, the columnar embryo is pushed and extruded into a strip-shaped embryo; and the embryo is extended to form a base strip;

[0009] (3) Feeding the base strip into a longitudinal stretching module to complete longitudinal stretching to obtain a polytetrafluoroethylene base film when drying and deoiling; the longitudinal stretching module has three sections of temperature zones 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 2-12, the stretching speed is 1-5 m / min, and the stretching temperature is set in a gradient range of 120-260℃;

[0010] (4) Transversely stretching, heat setting, and cooling and winding the polytetrafluoroethylene base film to finally obtain a polytetrafluoroethylene microporous film.

[0011] Further, when the base strip is longitudinally stretched, the temperature of the high-temperature activation zone is 270-320℃.

[0012] Further, when the base strip is longitudinally stretched, the temperature of the annealing and winding zone is 100-200℃.

[0013] Based on the difficult machinability and unique lamellar slip fiberization characteristics of polytetrafluoroethylene, a lubricant that is easy to diffuse and infiltrate into the dispersed resin particles of polytetrafluoroethylene is selected. The kinematic viscosity and distillation range of the lubricant have a great influence on the pore structure of the film. If the kinematic viscosity is too small, it is easy to infiltrate and diffuse into the resin powder, but it will reduce the fiberization ductility; on the contrary, it is difficult to infiltrate, and the swelling of the maturation is not sufficient.

[0014] The mixed lubricating oil used in the present application has a distillation range that is adjusted based on the temperature of the core stretching zone of the longitudinal stretching module. Further, the mixed lubricating oil is composed of isoparaffin and kerosene and / or liquid paraffin, wherein the mass fraction of isoparaffin is 40-50%. Preferably, the mixed lubricating oil is mixed by isoparaffin and kerosene and / or liquid paraffin at a mass ratio of 1:1-1.5.

[0015] Feeding the base strip without deoiling into a longitudinal stretching module, which is composed of three parts:

[0016] The first section is the core stretching and deoiling zone. This module consists of six rollers, which achieve stretching through a speed difference. A temperature gradient is configured, and oil-containing stretching is achieved based on the distillation range of the modulated lubricant, with some oil volatilization occurring simultaneously. This differs from other processes that first dry and deoil before stretching. On the one hand, oil-containing stretching increases the fluidity of the PTFE chains, preventing crack formation during the initial fiber formation stage. On the other hand, longitudinal stretching causes the lamellae in the PTFE particles to slide, allowing the lubricant to diffuse between crystal domains, reducing the activation energy for lamellae sliding and fibril formation. These two factors lay the foundation for uniform micropore distribution after subsequent transverse stretching.

[0017] The second part is the high-temperature activation zone, which increases the mobility of the molecular chains at high temperatures and strengthens the connection strength between the lamellae by the strip-shaped fibrils formed by the entanglement of the amorphous zone and the slip of the lamellae after longitudinal stretching.

[0018] The third part is the annealing and winding area, where the winding roller is set to a certain temperature so that the polytetrafluoroethylene molecular chains are rearranged regularly, thereby improving the entanglement strength at the interface of the crystal sheets.

[0019] Furthermore, 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.

[0020] Furthermore, the polytetrafluoroethylene dispersed resin has a crystallinity of ≥97% and a number average molecular weight of 2 million to 20 million.

[0021] Furthermore, 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.

[0022] Furthermore, 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.

[0023] In a second aspect, the present application provides an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity, which is obtained by the above-mentioned preparation method. The pore size of the polytetrafluoroethylene film is controllable within the range of 0.05 to 2.5 μm.

[0024] Furthermore, the above-mentioned polytetrafluoroethylene film includes microporous membrane, micro-mesoporous membrane and micro-macroporous membrane; the microporous membrane is a liquid filtration membrane with a pore size of 0.05 to 0.5 μm, which can be used in the field of liquid filtration; the micro-mesoporous membrane is a breathable and waterproof membrane with a pore size of 0.5 to 1.2 μm, which can be used in the field of electronic products; the micro-macroporous membrane is an air filtration membrane with a pore size of 1.2 to 2.5 μm, which can be used in the field of air filtration.

[0025] In summary, this application has the following beneficial effects:

[0026] The preparation method provided in this application utilizes a mixed lubricant combined with a special multi-module longitudinal stretching process to achieve oil-containing stretching of an unde-oiled base tape, laying the foundation for pore uniformity and pore size adjustment. After longitudinal stretching, the fibers are free of microcracks, and their high strength further facilitates subsequent pore formation during transverse stretching. The lamellar crystals experience sufficient slippage, ensuring uniform pore formation during subsequent transverse stretching. The rational adjustment of the temperature, speed, and ratio of biaxial stretching can address the current issues of uneven pore size distribution and the inability to precisely control pore size in biaxially stretched films.

[0027] The mixed lubricant of the method of the present invention is combined with the longitudinal stretching module to greatly improve the longitudinal fiber strength and reduce the possibility of transverse fiber breakage. At the same time, the setting of the longitudinal stretching module can reduce the node activation energy and reduce the defect of poor pore uniformity during the transverse stretching process.

[0028] 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; the longitudinal stretching temperature improves the pore size uniformity within a certain range. At the same time, during the transverse stretching process, the stretching ratio increases, and the pore size also first increases and then decreases; the transverse expansion speed increases, the average pore size decreases, and the uniformity is optimal at a certain speed. This application fully explores the laws of biaxial stretching, and on the basis of using a mixed lubricant, selects different process parameters of biaxial stretching temperature, ratio, and speed to achieve controllable polytetrafluoroethylene fiber membrane pore size between 0.05 and 2.5 μm.

[0029] The biaxial stretching process of the present invention has wide applicability. The precise control of pore size makes it applicable to the preparation of filter membranes for chip etching liquids ranging from tens of nanometers to air filter membranes of several microns. The highly uniform pore size makes the filtering effect better and more stable, which facilitates promotion and application in industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a flow chart of the method for preparing the polytetrafluoroethylene microporous membrane of the present invention;

[0031] Figure 2 Schematic diagram of sampling for various tests in Examples and Comparative Examples;

[0032] Figure 3 is a pore size distribution diagram of the polytetrafluoroethylene film prepared in Example 1;

[0033] Figure 4 This is an electron microscope image of the polytetrafluoroethylene film prepared in Example 2. DETAILED DESCRIPTION

[0034] The embodiments of the present invention will be described in detail below with reference to the 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 construed as limiting the scope of the present invention. Specific conditions not specified in the examples are carried out according to conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used without indicating the manufacturer are all conventional products that can be purchased commercially.

[0035] The following is a detailed description of 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 intended to limit the present invention.

[0036] Example 1

[0037] This embodiment provides an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity, and its preparation process is as follows: Figure 1 As shown, specifically including:

[0038] Step (1), mixing a polytetrafluoroethylene dispersion resin (number average molecular weight of 10 million, crystallinity of ≥97%) with a mixed lubricating oil in a mass ratio of 10:2.5 and aging to obtain a paste-like aging material; the mixed lubricating oil is prepared by mixing isoparaffin and kerosene in a mass ratio of 1:1; the aging temperature is 50°C, and the aging time is 24 hours;

[0039] Step (2): pre-press the matured material into a blank, and the resin particles are tightly combined to obtain a columnar blank. The blanking pressure is 3 MPa and the holding time is 5 minutes;

[0040] Step (3), pushing the columnar embryo obtained in step (2) into an extruder, keeping it warm in a feed gun at 30°C for 10 minutes, and extruding it into a strip embryo with a compression ratio of 150:1;

[0041] Step (4), feeding the strip-shaped ribbon obtained in step (3) into two rollers, and extruding to obtain a base tape with a width of 250 mm and a thickness of 200 μm;

[0042] Step (5), placing the base tape obtained in step (4) into the longitudinal stretching module, first passing through the core stretching zone to achieve deoiling during the stretching process, the temperature of this zone is set according to the gradient of 170℃-240℃, the stretching speed is 3m / min, and the stretching ratio is 2 times; then passing through the high-temperature activation zone, the temperature of this zone is 300℃, and there is no stretching in this zone; finally, collecting in the annealing and winding zone to obtain the longitudinal stretched film, the temperature is maintained at 200℃.

[0043] Step (6), the longitudinal stretched film obtained in step (5) is subjected to transverse expansion, the transverse stretching multiple is 8.4 times, the stretching temperature is 300℃, and the stretching speed is 10 m / min; the heat setting is completed in a state that the film is clamped, the heat setting temperature is 360℃; and the polytetrafluoroethylene microporous film is obtained after cooling and winding.

[0044] Example 2

[0045] The embodiment provides a high-aperture-uniformity asynchronous stretching polytetrafluoroethylene film, and a preparation method thereof.

[0046] Step (1), polytetrafluoroethylene dispersion resin (the number average molecular weight is 10 million, and the crystallinity is greater than or equal to 97%) is mixed with mixed lubricating oil according to a mass ratio of 10:3 to obtain a paste-like curing material; the mixed lubricating oil is prepared by mixing isomeric alkanes, kerosene and paraffin oil, and the mass ratio of the three is 1:1:0.5; the curing temperature is 40℃, and the curing time is 48 h;

[0047] Step (2), the cured material is subjected to pre-pressing to form a blank, and the resin particles are combined tightly to obtain a columnar embryo, the embryo forming pressure is 3 MPa, and the pressure maintaining time is 5 minutes;

[0048] Step (3), the columnar embryo obtained in step (2) is pushed into an extruder, and is kept warm in a 30℃ material gun for 10 minutes, and is extruded into a strip-shaped embryo with a compression ratio of 150:1;

[0049] Step (4), the strip-shaped embryo obtained in step (3) is fed into a two-roller wheel, and a base tape with a width of 250 mm and a thickness of 200 μm is obtained through extrusion;

[0050] Step (5), the base tape obtained in step (4) is placed into a longitudinal stretching module, is stretched in a core stretching zone to realize oil removal, the temperature of the zone is set according to a gradient of 150℃-250℃, the stretching speed is 3 m / min, and the stretching multiple is 8 times; is subjected to high-temperature activation in a high-temperature activation zone, the temperature of the zone is 300℃, and no stretching is performed; and finally, the longitudinal stretched film is collected in an annealing and winding zone, and the temperature is maintained at 200℃;

[0051] Step (6), the longitudinal stretched film obtained in step (5) is subjected to transverse expansion, the transverse stretching multiple is 8.4 times, the stretching temperature is 320℃, and the stretching speed is 10 m / min; the heat setting is completed in a state that the film is clamped, the heat setting temperature is 360℃; and the polytetrafluoroethylene microporous film is obtained after cooling and winding.

[0052] Example 3

[0053] The embodiment provides a high-aperture-uniformity asynchronous stretching polytetrafluoroethylene film, and a preparation method thereof.

[0054] Step (1), mix the polytetrafluoroethylene dispersion resin (number average molecular weight 10 million, crystallinity ≥97%) and mixed lubricating oil according to a mass ratio of 10:2.5 to perform aging, to obtain a paste-like aged material; the mixed lubricating oil is prepared by mixing isomeric alkanes and kerosene, and the mass ratio of the two is 1:1; the aging temperature is 50°C, and the aging time is 24h;

[0055] Step (2), pre-press the aged material to form a blank, and the resin particles are combined tightly to obtain a columnar embryo, the blanking pressure is 5MPa, and the pressure holding time is 10 minutes;

[0056] Step (3), push the columnar embryo obtained in step (2) into an extruder, and keep it in a 30°C material gun for 10 minutes, to extrude a strip-shaped embryo with a compression ratio of 128:1;

[0057] Step (4), feed the strip-shaped embryo obtained in step (3) into a two-roller wheel, and extrude a base tape with a width of 300mm and a thickness of 230μm;

[0058] Step (5), put the base tape obtained in step (4) into a longitudinal stretching module, first pass through the core stretching zone to realize oil removal, the temperature of this zone is set according to a gradient of 160°C-240°C, the stretching speed is 1m / min, and the stretching multiple is 8 times; then pass through the high-temperature activation zone, the temperature of this zone is 300°C; finally, collect the longitudinal stretched film in the annealing and winding zone, and the temperature is kept at 200°C.

[0059] Step (6), stretch the longitudinal stretched film obtained in step (5) in the transverse direction, the transverse stretching multiple is 3 times, the stretching temperature is 300°C, and the stretching speed is 10m / min; then complete heat setting of the film in a clamped state, the heat setting temperature is 360°C; and after cooling, wind up to obtain a polytetrafluoroethylene microporous film.

[0060] Example 4

[0061] The embodiment provides a high-pore-aperture-uniformity asynchronous stretching polytetrafluoroethylene film, and a preparation method thereof.

[0062] Step (1), mix the polytetrafluoroethylene dispersion resin (number average molecular weight 10 million, crystallinity ≥97%) and mixed lubricating oil according to a mass ratio of 10:2.5 to perform aging, to obtain a paste-like aged material; the mixed lubricating oil is prepared by mixing isomeric alkanes and kerosene, and the mass ratio of the two is 1:1; the aging temperature is 50°C, and the aging time is 24h;

[0063] Step (2), pre-press the aged material to form a blank, and the resin particles are combined tightly to obtain a columnar embryo, the blanking pressure is 5MPa, and the pressure holding time is 10 minutes;

[0064] Step (3), pushing the columnar embryo obtained in step (2) into an extruder, keeping it warm in a feed gun at 30°C for 10 minutes, and extruding it into a strip embryo with a compression ratio of 180:1;

[0065] Step (4), feeding the strip-shaped ribbon obtained in step (3) into two rollers, and extruding to obtain a base tape with a width of 200 mm and a thickness of 180 μm;

[0066] Step (5), placing the base tape obtained in step (4) into the longitudinal stretching module, first deoiling during the stretching process in the core stretching zone, the temperature of which is set according to a gradient of 170°C-240°C, the stretching speed is 3m / min, and the stretching ratio is 10 times; then passing through the high-temperature activation zone, the temperature of which is 320°C; finally collecting in the annealing and winding zone to obtain the longitudinally stretched film, the temperature of which is maintained at 200°C.

[0067] Step (6): The longitudinally stretched film obtained in step (5) is subjected to transverse expansion, with a transverse stretching ratio of 18 times, a stretching temperature of 300°C, and a stretching speed of 10 m / min; the film is then maintained in a clamped state to complete heat setting, with the heat setting temperature being 360°C; and after cooling, the film is rolled up to obtain a polytetrafluoroethylene microporous membrane.

[0068] Example 5

[0069] This embodiment provides an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity, and its preparation process is as follows: Figure 1 As shown, specifically including:

[0070] Step (1), mixing polytetrafluoroethylene dispersion resin (number average molecular weight of 20 million, crystallinity ≥97%) and mixed lubricating oil in a mass ratio of 10:2.5 and aging to obtain a paste-like aging material; the mixed lubricating oil is prepared by mixing isoparaffin and kerosene in a mass ratio of 1:1; the aging temperature is 30°C, and the aging time is 48 hours;

[0071] Step (2): pre-press the matured material into a blank, and the resin particles are tightly combined to obtain a columnar blank. The blanking pressure is 4 MPa and the holding time is 10 minutes;

[0072] Step (3), pushing the columnar embryo obtained in step (2) into an extruder, keeping it warm in a feed gun at 40°C for 10 minutes, and extruding it into a strip embryo with a compression ratio of 50:1;

[0073] Step (4), feeding the strip-shaped ribbon obtained in step (3) into two rollers, and extruding to obtain a base tape with a width of 250 mm and a thickness of 200 μm;

[0074] Step (5), placing the base tape obtained in step (4) into the longitudinal stretching module, first passing through the core stretching zone to achieve deoiling, the temperature of this zone is set according to the gradient of 120℃-200℃, the stretching speed is 5m / min, and the stretching ratio is 10 times; then passing through the high-temperature activation zone, the temperature of this zone is 320℃, and the stretching ratio is 1.2 times; finally, collecting in the annealing and winding zone to obtain the longitudinal stretched film, the temperature is maintained at 100℃.

[0075] Step (6): The longitudinally stretched film obtained in step (5) is subjected to transverse expansion, with a transverse stretching ratio of 3 times, a stretching temperature of 210°C, and a stretching speed of 3 m / min; the film is then maintained in a clamped state to complete heat setting, with a heat setting temperature of 360°C; and after cooling, the film is rolled up to obtain a polytetrafluoroethylene microporous membrane.

[0076] Example 6

[0077] This embodiment provides an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity, and its preparation process is as follows: Figure 1 As shown, specifically including:

[0078] Step (1), mixing polytetrafluoroethylene dispersion resin (number average molecular weight of 2 million, crystallinity ≥97%) and mixed lubricating oil in a mass ratio of 10:1 and aging to obtain a paste-like aging material; the mixed lubricating oil is prepared by mixing isoparaffin and kerosene in a mass ratio of 1:1; the aging temperature is 70°C and the aging time is 35 hours;

[0079] Step (2): pre-press the matured material into a blank, and the resin particles are tightly combined to obtain a columnar blank. The blanking pressure is 10 MPa and the holding time is 3 minutes;

[0080] Step (3), pushing the columnar embryo obtained in step (2) into an extruder, keeping it warm in a feed gun at 60°C for 10 minutes, and extruding it into a strip embryo with a compression ratio of 300:1;

[0081] Step (4), feeding the strip-shaped ribbon obtained in step (3) into two rollers, and extruding to obtain a base tape with a width of 200 mm and a thickness of 200 μm;

[0082] Step (5), placing the base tape obtained in step (4) into the longitudinal stretching module, first passing through the core stretching zone to achieve deoiling during the stretching process, the temperature of this zone is set according to the gradient of 180℃-260℃, the stretching speed is 1m / min, and the stretching ratio is 2 times; then passing through the high-temperature activation zone, the temperature of this zone is 270℃, and the stretching ratio is 1 times; finally, collecting in the annealing and winding zone to obtain the longitudinal stretched film, the temperature is maintained at 150℃.

[0083] Step (6), the longitudinally stretched film obtained in step (5) is transversely expanded, the transverse stretching multiple is 20 times, the stretching temperature is 320℃, the stretching speed is 20 m / min; the heat setting is completed under the condition that the film is clamped, the heat setting temperature is 360℃; after cooling, the polytetrafluoroethylene microporous film is obtained by winding.

[0084] Comparative Example 1

[0085] A method for preparing a polytetrafluoroethylene fiber film, which is different from the steps of Example 1, does not use a single lubricating oil for oil-containing stretching, and the main steps are as follows:

[0086] Step (1), the polytetrafluoroethylene dispersion resin (number average molecular weight is 10 million, crystallinity is ≥97%) is mixed with the lubricating oil (isomeric alkanes) according to a mass ratio of 10:2.5 to obtain a paste-like curing material; the curing temperature is 50℃, and the curing time is 24h;

[0087] Step (2), the cured material is pre-pressed to form a blank, and the resin particles are tightly combined to obtain a columnar embryo, the blanking pressure is 3 MPa, and the pressure holding time is 5 minutes;

[0088] Step (3), the columnar embryo obtained in step (2) is pushed into an extruder, and is kept at 30℃ in the material gun for 10 minutes to be extruded into a strip-shaped embryo with a compression ratio of 150:1;

[0089] Step (4), the strip-shaped embryo obtained in step (3) is fed into a two-roller wheel to be extruded into a base tape with a width of 250 mm and a thickness of 200 μm;

[0090] Step (5), the base tape obtained in step (4) is placed in a drying machine for oil removal treatment, and the oil removal temperature is 180℃;

[0091] Step (6), the base tape obtained in step (5) is longitudinally stretched, the stretching multiple is 2 times, the stretching temperature is 200℃, and the stretching speed is 3 m / min;

[0092] Step (7), the longitudinally stretched film obtained in step (6) is transversely expanded, the transverse stretching multiple is 8.4 times, the stretching temperature is 300℃, and the stretching speed is 10 m / min; the heat setting is completed under the condition that the film is clamped, the heat setting temperature is 360℃; after cooling, the polytetrafluoroethylene microporous film is obtained by winding.

[0093] Comparative Example 2

[0094] A method for preparing a polytetrafluoroethylene fiber film, which is different from the steps of Example 1, uses a mixed lubricating oil without other longitudinal stretching modules such as oil-containing stretching, and the main steps are as follows:

[0095] Step (1), mix the polytetrafluoroethylene dispersion resin (number average molecular weight 100 million, crystallinity ≥97%) and mixed lubricating oil according to a mass ratio of 10:2.5 to perform aging, to obtain paste-like aged material; the mixed lubricating oil is prepared by mixing isomeric alkanes and kerosene, and the mass ratio of the two is 1:1; the aging temperature is 50°C, and the aging time is 24h;

[0096] Step (2), pre-press the aged material to form a blank, and the resin particles are combined tightly to obtain a columnar embryo, the blanking pressure is 3MPa, and the pressure holding time is 5 minutes;

[0097] Step (3), push the columnar embryo obtained in step (2) into an extruder, and keep it in a 30°C material gun for 10 minutes, and extrude it into a strip-shaped embryo with a compression ratio of 150:1;

[0098] Step (4), feed the strip-shaped embryo obtained in step (3) into a two-roller wheel, and extrude it to obtain a base tape with a width of 250mm and a thickness of 200μm;

[0099] Step (5), put the base tape obtained in step (4) into a drying machine for oil removal treatment, and the oil removal temperature is 160°C;

[0100] Step (6), longitudinally stretch the base tape obtained in step (5), the stretching multiple is 2 times, the stretching temperature is 200°C, and the stretching speed is 3m / min;

[0101] Step (7), longitudinally stretch the base tape obtained in step (6), the stretching multiple is 2 times, the stretching temperature is 200°C, and the stretching speed is 3m / min; after the stretching, the film is kept in a clamped state to complete heat setting, the heat setting temperature is 360°C; after cooling, the polytetrafluoroethylene microporous film is obtained by winding.

[0102] Comparative Example 3

[0103] A method for preparing a polytetrafluoroethylene fiber film, which is different from the steps of Example 3, uses a single lubricating oil and does not perform oil-containing stretching, and the main steps are as follows:

[0104] Step (1), mix the polytetrafluoroethylene dispersion resin (number average molecular weight 100 million, crystallinity ≥97%) and single lubricating oil (kerosene) according to a mass ratio of 10:2.5 to perform aging, to obtain paste-like aged material; the aging temperature is 50°C, and the aging time is 24h;

[0105] Step (2), pre-press the aged material to form a blank, and the resin particles are combined tightly to obtain a columnar embryo, the blanking pressure is 5MPa, and the pressure holding time is 10 minutes;

[0106] Step (3), the columnar embryo obtained in step (2) is pushed into an extruder, and is extruded into a strip-shaped embryo with an extrusion ratio of 128:1 at 30°C for 10 minutes;

[0107] Step (4), the strip-shaped embryo obtained in step (3) is fed into a two-roller wheel, and a base tape with a width of 300 mm and a thickness of 230 μm is obtained by extrusion;

[0108] Step (5), the base tape obtained in step (4) is placed into a drying machine for oil removal treatment at an oil removal temperature of 180°C;

[0109] Step (6), the base tape obtained in step (5) is longitudinally stretched at a stretching ratio of 8 times at a stretching temperature of 200°C and a stretching speed of 3 m / min;

[0110] Step (7), the longitudinally stretched film obtained in step (6) is transversely expanded at a transverse stretching ratio of 3 times at a stretching temperature of 300°C and a stretching speed of 10 m / min; and heat setting is completed at a heat setting temperature of 360°C while the film is clamped; and the polytetrafluoroethylene microporous film is obtained after cooling and winding.

[0111] The pore size of the polytetrafluoroethylene film is tested, and the air filtration efficiency is tested.

[0112] (I) Pore size test (average pore size, most probable pore size)

[0113] 1. Detection method

[0114] The PMI (gas permeation method pore size analyzer) equipment is used for testing, the sample to be tested is cut into a circular or other shape with a diameter specified by the equipment, and the sample cannot be cut inside. After the sample is soaked in the surfactant required by the PMI equipment test for 30 minutes, the sample is placed in the test slot of the instrument, and the cover of the slot is tightened. The test can be performed, and the test results are directly read by the equipment and converted into the distribution of each pore size (μm) of the filter material (%). The proportion of the entire material (including average pore size, maximum pore size μm, minimum pore size μm, pore size distribution), three positions (A, B, C) are uniformly taken along the transverse direction of the polytetrafluoroethylene film, and three points (1, 2, 3) are selected at each position, as shown in Figure 2

[0115] 2. Detection results

[0116] (1) The pore size of the polytetrafluoroethylene film prepared in Examples 1-4 and Comparative Examples 1-3 is tested, and the average pore size and standard deviation of each position and the overall uniformity are shown in Table 1.

[0117] Table 1. ​

[0118]

[0119] (2) The pore size test was performed 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 are shown in Table 2.

[0120] Table 2.

[0121]

[0122] 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 can achieve accurate adjustment of pore size.

[0123] (2) Air filtration efficiency

[0124] 1. Detection method

[0125] The PTFE membranes prepared in Example 3 and Comparative Example 3 were tested for filtration efficiency. The test conditions specified a sodium chloride (NaCl) aerosol particle size distribution with a number median diameter (CMD) of 0.075 μm ± 0.020 μm and a geometric standard deviation of no more than 1.86 (equivalent to a mass median aerodynamic diameter (MMAD) of 0.24 μm ± 0.06 μm). Air filtration efficiency was tested at the same sampling points as in the pore size test above.

[0126] 2. Test results

[0127] The average filtration efficiency and standard deviation results at each position of Example 3 and Comparative Example 3 are shown in Table 3.

[0128] Table 3.

[0129]

[0130] As can be seen from Table 3, pore size uniformity has a great influence on filtration efficiency. High pore size uniformity can improve filtration efficiency and stability. Figure 3 The aperture test results can intuitively show the uniform distribution of the aperture at a single measurement point. Figure 4 The scanning electron microscope image also shows that the fiber length is evenly distributed in the horizontal and vertical directions.

[0131] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but as long as they are within the scope of the claims of the present application, they are 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 slaked material is pre-pressed and formed into a blank, and the resin particles are tightly combined to obtain a columnar blank; after the columnar blank is kept warm, it is pushed and extruded into a sheet-like strip blank, and the strip blank 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 adjacent temperature zones, namely a core stretching and deoiling zone, a high-temperature activation zone, and an annealing and winding zone; wherein the temperature of the high-temperature activation zone is 270-320°C, the core stretching and deoiling zone adopts stepwise stretching, the total stretching ratio is 2-12 times, the stretching speed is 1-5 m / min, and the stretching temperature is set in a gradient within the range of 120-260°C; The polytetrafluoroethylene base film is subjected to transverse stretching, heat setting, cooling and winding to finally obtain a polytetrafluoroethylene microporous film.

2. The method for preparing an 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 an 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.

4. The method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to any one of claims 1 to 3, 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.

5. The method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to any one of claims 1 to 3, characterized in that: The polytetrafluoroethylene dispersed resin has a crystallinity of ≥97% and a number average molecular weight of 2 million to 20 million.

6. The method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to any one of claims 1 to 3, characterized in that: In the process of preparing the slaking material, the slaking temperature is 30-70° C. and the slaking time is 24-48 hours.

7. The method for preparing an asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to any one of claims 1 to 3, 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 minutes; the pushing extrusion treatment temperature is 30-60° C., and the compression ratio is 50-300.

8. An asynchronously stretched polytetrafluoroethylene film with high pore size uniformity, characterized in that: The polytetrafluoroethylene film is obtained according to the preparation method according to any one of claims 1 to 7, and the pore size of the polytetrafluoroethylene film is controllable within a range of 0.05 to 2.5 μm.

9. The asynchronously stretched polytetrafluoroethylene film with high pore size uniformity according to claim 8, characterized in that: The polytetrafluoroethylene film is a microporous film, a micro-mesoporous film or 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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