Polytetrafluoroethylene membrane with gradient pore structure and method for producing same

Abstract

The present invention provides a polytetrafluoroethylene (PTFE) membrane with a gradient pore structure and a method for producing the same, which has a gradient pore structure in its cross section. The first and second porous outer surfaces of the polytetrafluoroethylene (PTFE) membrane have fibers and nodes with different microstructures, with the first porous outer surface having an island-like microstructure composed of a plurality of interconnected, relatively small nodes, and the second porous outer surface having an H-shaped ladder-like microstructure composed of a plurality of interconnected, relatively large nodes. The polytetrafluoroethylene membrane material with a gradient pore structure produced by this method has low resistance and high flow rate without reducing thickness or filtration accuracy, and enhances contaminant capture through improved filtration efficiency. For a given transmembrane pressure drop, the membrane's high permeability or high flow capacity reduces resistance losses and shortens filtration time, thereby reducing energy costs.

Description

[Technical Field]

[0001] The present invention relates to the technical field of polymer filtration membrane materials, and in particular to a polytetrafluoroethylene membrane having a gradient pore structure and a method for producing the same. [Background technology]

[0002] Expanded polytetrafluoroethylene (PTFE) membranes with porous structures are widely used in the fields of liquid and gas filtration. These membranes have high chemical inertness and thermal stability at extreme temperatures, allowing them to be used under harsh operating conditions.

[0003] As described in U.S. Patent No. 3,953,566A, a PTFE resin and a lubricant are mixed to form a raw material, the PTFE mixed raw material is blanked, the paste is extruded, the lubricant is removed from the extruded material, and the PTFE product is expanded to produce a porous PTFE product. The expanded PTFE material has a porous microstructure with nodes connected through the original fibers, and the pore sizes inside and on both surfaces of the membrane are symmetrical.

[0004] Although symmetrical pore structure filter media have high filtration accuracy, and the smaller the pore size, the more accurately they can be filtered or separated, the smaller the pore size usually reduces the permeability of liquids or gases through the membrane and cannot provide high dirt capacity. Conversely, increasing the pore size of the filter media increases the membrane's flow-through capacity, but this also reduces the number of particles captured by the membrane, thereby reducing filtration efficiency.

[0005] To increase the throughput per unit area and unit time and increase the filtration rate, the number of pores must be significantly increased or the membrane thickness must be minimized while maintaining a constant pore size. However, under certain manufacturing conditions, significantly increasing the number of pores per unit area is extremely difficult. Furthermore, while filtration rate can be increased by reducing the thickness of the filter material, this weakens the membrane's mechanical strength. The focus of filtration material development is to find the optimal combination of high permeability and high filtration accuracy, ensuring filtration efficiency while also providing low resistance, high flow rate, and high contaminant absorption capacity. As a technology that overcomes this deficiency, asymmetric porous membranes offer greater throughput under the same filtration or separation conditions.

[0006] Although the filtration properties of porous PTFE membranes are improving, a PTFE filtration membrane that can simultaneously provide small particle size and low flow resistance remains one of the technical challenges in this field.

[0007] Filtration materials with asymmetric pore structures can ensure high filtration accuracy while also offering advantages such as small pressure differentials and long service life, making them widely used in the field of filtration and separation. For example, Chinese Patent No. CN107810047A discloses an asymmetric polytetrafluoroethylene composite with a macroscopic textured surface, in which the two membrane surfaces of this polytetrafluoroethylene composite have different bubble point pressure values. In this method, the second PTFE film, when wetted (i.e., without removing the lubricant), must be expanded longitudinally or transversely from the rolled ribbon, a process that is difficult to carry out and likely to cause irreversible damage to the PTFE ribbon. Although membranes produced by this method have a unique macroscopic textured surface, the membrane surface has one or more strands, resulting in raised membrane surfaces, which can lead to uneven interlayer force-bearing when combined with other support layers.

[0008] As in U.S. Patent No. 4,248,924 A, a porous membrane material made of polytetrafluoroethylene with an asymmetric structure was provided and produced by stretching the membrane using a pair of rotating rolls with a temperature difference of 50°C or more. The temperature difference between the front and back surfaces of the membrane creates a temperature gradient in the thickness direction, which, combined with a compressive force in the thickness direction, creates an asymmetric structure in the resulting porous membrane material, characterized by a fiber structure in the front surface that is different from the fiber structure in the back surface. Because the thickness of the stretched film is thin, this method limits the temperature gradient in the thickness direction, making it difficult to effectively control the asymmetric structure in the film material.

[0009] Therefore, in order to solve the above problems, there is a need to improve the asymmetric membrane materials and the manufacturing methods thereof in the prior art. Summary of the Invention

[0010] The objective of the present invention is to disclose a polytetrafluoroethylene membrane material with a gradient pore structure, which combines the characteristics of low flow resistance and small pore size, and has excellent filtration rate and reliable cutoff accuracy in terms of liquid and gas filtration.

[0011] To achieve the above object, the present invention provides a polytetrafluoroethylene membrane having a gradient pore structure, wherein the cross section of the polytetrafluoroethylene membrane has a gradient pore structure, and the first and second porous outer surfaces of the polytetrafluoroethylene membrane have fibers and nodes with different microstructures, the first porous outer surface having an island-like microstructure constructed from a plurality of interconnected, relatively small nodes, and the second porous outer surface having an H-shaped ladder-type microstructure constructed from a plurality of interconnected, relatively large nodes.

[0012] In some embodiments, the relatively small nodes are circular in structure and have an average diameter of 0.30 to 1.50 μm.

[0013] In some embodiments, the relatively small nodes are formed by connecting a plurality of thick and short microfibers having an average length in the range of 0.2 to 2.5 μm, an average diameter in the range of 0.05 to 0.2 μm, and an aspect ratio of 2 to 20.

[0014] In some embodiments, the larger nodes are elongated oval structures with an average length of 0.5-10 μm and an average width of 0.5-2 μm.

[0015] In some embodiments, the relatively large nodes are formed by connecting a plurality of elongated microfibers having an average length in the range of 1-10 μm, an average diameter in the range of 5-200 nm, and an aspect ratio in the range of 10-200.

[0016] Another object of the present invention is to provide a method for producing a polytetrafluoroethylene membrane having a gradient pore structure, First, a step (1) of uniformly mixing an isoalkane and a PTFE-dispersed resin to obtain a PTFE resin mixture, and then pressing the PTFE resin mixture into a cylindrical preform; Step (2) using an extruder to extrude the cylindrical preform into three continuous strip-shaped sheets of equal width and rectangular cross section with different thicknesses through T-shaped extrusion dies of different specifications, which are designated as a first sheet, a second sheet, and a third sheet, respectively; Step (3) of stacking the first sheet, the second sheet, and the third sheet in the same direction along the width in order of gradually increasing or decreasing thickness, and rolling them into a pair of metal rolls along the longitudinal direction in the thickness direction to a range of 0.05 to 1 mm to form a laminate; (4) drying the laminate in an environment of 200 to 250°C, and then stretching the laminate along the speed direction at a temperature of 250 to 350°C, a stretching speed of 20% / sec to 3000% / sec, and an expansion ratio of 50% to 900%, to form a unidirectionally stretched product; and (5) stretching the unidirectionally stretched product in a direction perpendicular to the stretching direction at a temperature of 200-400°C, a stretching rate of 5% / sec-500% / sec, and a stretching ratio of 300%-3000%, thereby finally forming a PTFE membrane material with a gradient pore structure.

[0017] In some embodiments, in step (1), isoalkane and PTFE-dispersed resin are uniformly mixed in a weight ratio of 15 to 30% by weight to obtain a PTFE resin mixture, which is then dried at 20°C or higher for 12 hours or more, and the PTFE resin mixture is pressed into a cylindrical preform.

[0018] In some embodiments, in step (2), the cylindrical preform is extruded through a T-shaped extrusion die with different specifications at a compression ratio of 20 to 500 into three continuous strip-shaped sheets with rectangular cross-sections and equal widths and different thicknesses, respectively, designated as a first sheet, a second sheet, and a third sheet, wherein the thickness of the second sheet is at least 1.2 times that of the first sheet, the thickness of the third sheet is at least 1.2 times that of the second sheet, and the compression ratio of the first sheet is at least 2.5 times that of the second sheet and 3 times that of the third sheet.

[0019] In some embodiments, in step (4), the laminate is stretched along the speed direction at a stretching rate of 50% / sec to 2000% / sec and an expansion ratio of 80% to 800% to form a unidirectionally stretched product.

[0020] In some embodiments, in step (5), the stretching rate is 20% / s to 300% / s, and the stretching ratio is 500% to 2500% in the direction perpendicular to the stretching rate, thereby finally forming a PTFE membrane material with a gradient pore structure. [Effects of the Invention]

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: the polytetrafluoroethylene membrane material with a gradient pore structure produced by this method has the characteristics of low resistance and high flow rate without reducing thickness and filtration accuracy, and enhances the capture of contaminants by improving filtration efficiency. For a given transmembrane pressure drop, the high permeability or high flow capacity of the membrane reduces resistance losses and shortens filtration time, thereby reducing energy costs. Furthermore, these characteristics can provide a system that requires less installation space and reduces costs. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a cross-sectional view of a polytetrafluoroethylene film laminate according to the present invention. [Figure 2] 1 is a scanning electron microscope photograph at 2000 times magnification of the first porous outer surface of the polytetrafluoroethylene membrane shown in Example 1 of the present invention. [Figure 3] 1 is a scanning electron microscope photograph at 2000 times magnification of the second porous outer surface of the polytetrafluoroethylene membrane shown in Example 1 of the present invention. [Figure 4] 1 is a scanning electron microscope photograph at 2000 times magnification of the first porous outer surface of the polytetrafluoroethylene membrane shown in Example 2 of the present invention. [Figure 5] 1 is a scanning electron microscope photograph at 2000 times magnification of the second porous outer surface of the polytetrafluoroethylene membrane shown in Example 2 of the present invention. [Figure 6] 1 is a scanning electron microscope photograph at 2000 times magnification of the first porous outer surface of the polytetrafluoroethylene membrane shown in Example 3 of the present invention. [Figure 7] 1 is a scanning electron microscope photograph at 2000 times magnification of the second porous outer surface of the polytetrafluoroethylene membrane shown in Example 3 of the present invention. [Figure 8] 1 is a scanning electron microscope photograph at 2000 times magnification of the first porous outer surface of the polytetrafluoroethylene membrane shown in Example 4 of the present invention. [Figure 9]1 is a scanning electron microscope photograph at 2000 times magnification of the second porous outer surface of the polytetrafluoroethylene membrane shown in Example 4 of the present invention. [Figure 10] 1 is a scanning electron microscope photograph at 2000 times magnification of the first porous outer surface of the polytetrafluoroethylene membrane shown in Example 5 of the present invention. [Figure 11] 1 is a scanning electron microscope photograph at 2000 times magnification of the second porous outer surface of the polytetrafluoroethylene membrane shown in Example 5 of the present invention. [Figure 12] 1 is a scanning electron microscope photograph at 2000 times magnification of the first porous outer surface of the polytetrafluoroethylene membrane shown in Example 6 of the present invention. [Figure 13] 1 is a scanning electron microscope photograph at 2000 times magnification of the second porous outer surface of the polytetrafluoroethylene membrane shown in Example 6 of the present invention.

[0023] The present invention will be described in detail below based on the embodiments shown in the drawings. However, these embodiments are not limitations on the present invention, and any equivalent conversions or substitutions in function, method, or structure made by a person skilled in the art based on these embodiments all fall within the scope of protection of the present invention.

[0024] The method for producing a polytetrafluoroethylene membrane having a gradient pore structure of the present invention comprises the following steps: Step (1): First, an isoalkane-based lubricant and a polytetrafluoroethylene (PTFE) dispersion resin are uniformly mixed in a weight ratio of 15 to 30% by weight to obtain a PTFE resin mixture, which is then dried at 20°C or higher for 12 hours or more, and then the PTFE resin mixture is pressed into a cylindrical preform.

[0025] Step (2), using an extruder, extrude the cylindrical preform through a T-shaped extrusion die with different specifications at a compression ratio of 20 to 500 into three continuous strip-shaped sheets with rectangular cross sections and equal widths and different thicknesses, respectively, which are designated as a first sheet, a second sheet, and a third sheet; Here, the thickness of the second sheet is at least 1.2 times, preferably 1.2 to 5 times, that of the first sheet, the thickness of the third sheet is at least 1.2 times, preferably 1.2 to 5 times, that of the second sheet, and the compression ratio of the first sheet is at least 2.5 times that of the second sheet and 3 times that of the third sheet.

[0026] In step (3), as shown in FIG. 1, the first sheet, the second sheet, and the third sheet are stacked in the same width direction in an order of gradually increasing or decreasing thickness, and then rolled into a pair of metal rolls in the longitudinal direction to a thickness of 0.05 to 1 mm, preferably 0.15 to 0.6 mm, to form a laminate.

[0027] Step (4): After drying the laminate in an environment of 200 to 250°C, the laminate is stretched along the speed direction at a temperature of 250 to 350°C at a stretching speed of 20% / sec to 3000% / sec, preferably 50% / sec to 2000% / sec, and an expansion ratio of 50% to 900%, preferably 80% to 800%, to form a unidirectionally stretched product.

[0028] Step (5): The unidirectionally stretched product is stretched in the direction perpendicular to the stretching direction at a temperature of 200-400°C, a stretching rate of 5% / sec-500% / sec, preferably 20% / sec-300% / sec, and a stretching ratio of 300%-3000%, preferably 500%-2500%, to finally form a polytetrafluoroethylene (PTFE) membrane material with a gradient pore structure.

[0029] The cross section of this membrane material has a gradient pore structure, and the polytetrafluoroethylene (PTFE) membrane material has two outer surfaces, a first porous outer surface and a second porous outer surface, each with fibers and nodes of different microstructures.

[0030] The first porous outer surface has an island-like microstructure constructed from a plurality of interconnected, relatively small nodes, each of which has a circular structure and an average diameter of 0.30-1.50 μm, and is formed by a plurality of thick and short microfibers connected together, each of which has an average length in the range of 0.2-2.5 μm, an average diameter in the range of 0.05-0.2 μm, and an aspect ratio of 2-20.

[0031] The second porous outer surface has an H-shaped ladder microstructure constructed from a plurality of interconnected, relatively large nodes. The relatively large nodes are elongated elliptical structures with an average length of 0.5-10 μm and an average width of 0.5-2 μm. The relatively large nodes are formed by connecting a plurality of elongated microfibers with an average length in the range of 1-10 μm, an average diameter in the range of 5-200 nm, and an aspect ratio of 10-200.

[0032] Regarding the measurement method of the polytetrafluoroethylene membrane, Thickness measurement The thickness of the film was measured by placing the film or sheet between the measuring head and anvil of a Mitotoyo 7327 thickness dial gauge, and the average value of three measurements was used.

[0033] Bubble Point Measurement The bubble point and mean flow pore size were measured using a Porolux porometer (Model Porolux 500, Porometer NV, Promet GmbH, Belgium) according to the general guidelines of ASTM F316-03. The sample membrane was placed in the sample chamber and wetted with a test liquid with a surface tension of 16 dynes / cm. The values ​​shown for bubble point and mean flow pore size are the average of two measurements.

[0034] Scanning electron microscope photographs Generate SEM images using a cold cathode field emission scanning electron microscope (Hitachi Regulus 8100).

[0035] Node and fiber size measurements The SEM photographs were geometrically measured using SolidWorks 2014 software (Dassault Systems, France), and the measurement results were converted to the actual size of the nodes and fibers using the scale of the SEM photographs.

[0036] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0037] Example 1 Daikin's F-106 polytetrafluoroethylene dispersion resin was uniformly mixed with 18.3% by weight of isoalkane (ExxonMobil ISOPAR M), and then left in a thermostatic chamber at 25°C for 18 hours. The PTFE resin mixture was pressed into a cylindrical preform, and the cylindrical preform was extruded using a T-shaped die at a compression ratio of 195 to obtain a continuous first sheet with a rectangular cross section and a thickness of 0.4 mm.

[0038] AGC's CD-126E polytetrafluoroethylene dispersion resin was uniformly mixed with 25.6% by weight of isoalkane (ExxonMobil ISOPAR K), and then left in a 25°C thermostatic chamber for 18 hours. The PTFE resin mixture was pressed into a cylindrical preform, which was then extruded through a T-shaped die with different specifications at a compression ratio of 49 to obtain a continuous second sheet with a rectangular cross section and a thickness of 0.9 mm, and then extruded at a compression ratio of 37 to obtain a continuous third sheet with a rectangular cross section and a thickness of 1.2 mm.

[0039] Three sheets with different specifications were stacked in the same width direction in order of increasing or decreasing thickness, and then rolled longitudinally between a pair of metal rolls to a thickness of 0.3 mm to form a laminate. The dried laminate, from which the lubricant had been removed, was stretched longitudinally using a longitudinal stretching mechanism at a roll temperature of 300°C, a stretching speed of 117% / s, and an expansion ratio of 230%. The longitudinally stretched unidirectionally stretched product (base band) was then stretched transversely using a transverse expander at a stretching temperature of 250°C, a stretching speed of 22% / s, and an expansion ratio of 2000%, and finally heat-set to form a film.

[0040] The resulting polytetrafluoroethylene (PTFE) microporous membrane has a first porous outer surface and a second porous outer surface with different microstructures, as described above. As shown in Figures 2 and 3, Figure 2 is a scanning electron micrograph (SEM) of the first porous outer surface of the PTFE membrane taken at 2000x magnification, showing an island-like microstructure constructed from several interconnected, relatively small nodes. Figure 3 is a scanning electron micrograph (SEM) of the second porous outer surface of the PTFE membrane taken at 2000x magnification, showing an H-shaped ladder-like microstructure constructed from several interconnected, larger nodes.

[0041] The nodes in Figure 2 were geometrically measured using Solidworks 2014 software, and the node sizes are shown in Table 1 below.

[0042] [Table 1]

[0043] The fibers in Figure 2 were measured geometrically using Solidworks 2014 software, and the fiber sizes are shown in Table 2 below.

[0044] [Table 2]

[0045] The nodes in Figure 3 were geometrically measured using Solidworks 2014 software, and the node sizes are shown in Table 3 below.

[0046] [Table 3]

[0047] The fiber in Figure 3 was measured geometrically using Solidworks 2014 software, and the fiber sizes are shown in Table 4 below.

[0048] [Table 4]

[0049] The bubble points were measured using a pore size distribution porometer under two test conditions: in the first test, the first porous outer surface of the PTFE membrane faced the metal mesh, and in the second test, the second porous outer surface of the PTFE membrane faced the metal mesh, resulting in bubble points of 1.97 bar and 1.62 bar, respectively.

[0050] Example 2 Daikin's F-106 polytetrafluoroethylene dispersion resin was uniformly mixed with 19.5% by weight of isoalkane (ExxonMobil ISOPAR M), and then left in a thermostatic chamber at 25°C for 20 hours. The PTFE resin mixture was pressed into a cylindrical preform, and the cylindrical preform was extruded using a T-shaped die at a compression ratio of 195 to obtain a continuous first sheet with a rectangular cross section and a thickness of 0.4 mm.

[0051] AGC's CD-126E polytetrafluoroethylene dispersion resin was uniformly mixed with 23.5% by weight of isoalkane (ExxonMobil ISOPAR K), and then left in a 25°C thermostatic chamber for 20 hours. The PTFE resin mixture was pressed into a cylindrical preform, which was then extruded through a T-shaped die with different specifications at a compression ratio of 56 to obtain a continuous second sheet with a rectangular cross section and a thickness of 0.8 mm, and then extruded at a compression ratio of 45 to obtain a continuous third sheet with a rectangular cross section and a thickness of 1.0 mm.

[0052] Three sheets with different specifications were stacked in the same width direction in order of increasing or decreasing thickness, and then rolled longitudinally between a pair of metal rolls to a thickness of 0.28 mm to form a laminate. The dried laminate, from which the lubricant had been removed, was stretched longitudinally using a longitudinal stretching mechanism at a roll temperature of 300°C, a stretching speed of 150% / s, and a stretching expansion ratio of 250%. The longitudinally stretched unidirectionally stretched product (base band) was then stretched transversely using a transverse expander at a stretching temperature of 250°C, a stretching speed of 22% / s, and a stretching expansion ratio of 2000%, and finally heat-set to form a film.

[0053] The resulting polytetrafluoroethylene (PTFE) microporous membrane has a first porous outer surface and a second porous outer surface with different microstructures, as described above. As shown in Figures 4 and 5, Figure 4 is a scanning electron micrograph (SEM) of the first porous outer surface of the PTFE membrane taken at 2000x magnification, showing an island-like microstructure constructed from several interconnected, relatively small nodes. Figure 5 is a scanning electron micrograph (SEM) of the second porous outer surface of the PTFE membrane taken at 2000x magnification, showing an H-shaped ladder-like microstructure constructed from several interconnected, larger nodes.

[0054] The nodes in Figure 4 were geometrically measured using Solidworks 2014 software, and the node sizes are shown in Table 5 below.

[0055] [Table 5]

[0056] The fiber in Figure 4 was measured geometrically using Solidworks 2014 software, and the fiber sizes are shown in Table 6 below.

[0057] [Table 6]

[0058] The nodes in Figure 5 were geometrically measured using Solidworks 2014 software, and the node sizes are shown in Table 7 below.

[0059] [Table 7]

[0060] The fiber in Figure 5 was measured geometrically using Solidworks 2014 software, and the fiber sizes are shown in Table 8 below.

[0061] [Table 8]

[0062] The bubble points were measured using a pore size distribution porometer under two test conditions: in the first test, the first porous outer surface of the PTFE membrane faced the metal mesh, and in the second test, the second porous outer surface of the PTFE membrane faced the metal mesh, resulting in bubble points of 1.78 bar and 1.53 bar, respectively.

[0063] Example 3 Daikin's F-106 polytetrafluoroethylene dispersion resin was uniformly mixed with 21.3% by weight of isoalkane (ExxonMobil ISOPAR M), and then left in a thermostatic chamber at 25°C for 22 hours. The PTFE resin mixture was pressed into a cylindrical preform, and the cylindrical preform was extruded using a T-shaped die at a compression ratio of 195 to obtain a continuous first sheet with a rectangular cross section and a thickness of 0.4 mm.

[0064] AGC's CD-126E polytetrafluoroethylene dispersion resin was uniformly mixed with 25.5% by weight of isoalkane (ExxonMobil ISOPAR K), and then left in a 25°C thermostatic chamber for 22 hours. The PTFE resin mixture was pressed into a cylindrical preform, which was then extruded through a T-shaped die with different specifications at a compression ratio of 56 to obtain a continuous second sheet with a rectangular cross section and a thickness of 0.8 mm, and then extruded at a compression ratio of 37 to obtain a continuous third sheet with a rectangular cross section and a thickness of 1.2 mm.

[0065] Three sheets with different specifications were stacked in the same width direction in order of increasing or decreasing thickness, and then rolled longitudinally between a pair of metal rolls to a thickness of 0.32 mm to form a laminate. The dried laminate, from which the lubricant had been removed, was stretched longitudinally using a longitudinal stretching mechanism at a roll temperature of 300°C, a stretching speed of 66.7% / s, and an expansion ratio of 200%. The longitudinally stretched unidirectionally stretched product (base band) was then stretched transversely using a transverse expander at a stretching temperature of 250°C, a stretching speed of 22% / s, and an expansion ratio of 2000%, and finally heat-set to form a film.

[0066] The resulting polytetrafluoroethylene (PTFE) microporous membrane has a first porous outer surface and a second porous outer surface with different microstructures, as described above. As shown in Figures 6 and 7, Figure 6 is a scanning electron micrograph (SEM) of the first porous outer surface of the PTFE membrane taken at 2000x magnification, showing an island-like microstructure constructed from several interconnected, relatively small nodes. Figure 7 is a scanning electron micrograph (SEM) of the second porous outer surface of the PTFE membrane taken at 2000x magnification, showing an H-shaped ladder-like microstructure constructed from several interconnected, larger nodes.

[0067] The nodes in Figure 6 were geometrically measured using Solidworks 2014 software, and the node sizes are shown in Table 9 below.

[0068] [Table 9]

[0069] The fiber in Figure 6 was measured geometrically using Solidworks 2014 software, and the fiber sizes are shown in Table 10 below.

[0070] [Table 10]

[0071] The nodes in Figure 7 were geometrically measured using Solidworks 2014 software, and the node sizes are shown in Table 11 below.

[0072] [Table 11]

[0073] The fiber in Figure 7 was measured geometrically using Solidworks 2014 software, and the fiber sizes are shown in Table 12 below.

[0074] [Table 12]

[0075] The bubble points were measured using a pore size distribution porometer under two test conditions: in the first test, the first porous outer surface of the PTFE membrane faced the metal mesh, and in the second test, the second porous outer surface of the PTFE membrane faced the metal mesh, resulting in bubble points of 2.16 bar and 1.51 bar, respectively.

[0076] Example 4 The polytetrafluoroethylene resin dispersion was uniformly mixed with 19.5% by weight of isoalkane, and then left in a thermostatic chamber at 25°C for 20 hours. The PTFE resin mixture was pressed into a cylindrical preform, and the cylindrical preform was extruded using a T-shaped die at a compression ratio of 190 to obtain a continuous first sheet with a rectangular cross section and a thickness of 0.5 mm.

[0077] The polytetrafluoroethylene resin dispersion was uniformly mixed with 23.5% by weight of isoalkane and then left in a constant temperature bath at 25°C for 20 hours. The PTFE resin mixture was pressed into a cylindrical preform, which was then extruded through T-shaped dies with different specifications at a compression ratio of 56 to obtain a continuous second sheet with a rectangular cross section and a thickness of 0.8 mm, and extruded at a compression ratio of 45 to obtain a continuous third sheet with a rectangular cross section and a thickness of 1.0 mm.

[0078] Three sheets with different specifications were stacked in the same width direction in order of increasing or decreasing thickness, and then rolled longitudinally between a pair of metal rolls to a thickness of 0.28 mm to form a laminate. The dried laminate, from which the lubricant had been removed, was stretched longitudinally using a longitudinal stretching mechanism at a roll temperature of 300°C, a stretching speed of 150% / s, and a stretching expansion ratio of 250%. The longitudinally stretched unidirectionally stretched product (base band) was then stretched transversely using a transverse expander at a stretching temperature of 250°C, a stretching speed of 22% / s, and a stretching expansion ratio of 2000%, and finally heat-set to form a film.

[0079] The resulting polytetrafluoroethylene (PTFE) microporous membrane has a first porous outer surface and a second porous outer surface with different microstructures, as described above. As shown in Figures 8 and 9, Figure 8 is a scanning electron micrograph (SEM) of the first porous outer surface of the PTFE membrane taken at 2000x magnification, showing an island-like microstructure constructed from several interconnected, relatively small nodes. Figure 9 is a scanning electron micrograph (SEM) of the second porous outer surface of the PTFE membrane taken at 2000x magnification, showing an H-shaped ladder-like microstructure constructed from several interconnected, larger nodes.

[0080] The bubble points were measured using a pore size distribution porometer under two test conditions: in the first test, the first porous outer surface of the PTFE membrane faced a metal mesh, and in the second test, the second porous outer surface of the PTFE membrane faced a metal mesh, resulting in bubble points of 2.89 bar and 1.61 bar, respectively.

[0081] Example 5 Daikin's F-106 polytetrafluoroethylene dispersion resin was uniformly mixed with 19.5% by weight of isoalkane (ExxonMobil ISOPAR M), and then left in a thermostatic chamber at 25°C for 18 hours. The PTFE resin mixture was pressed into a cylindrical preform, and the cylindrical preform was extruded using a T-shaped die at a compression ratio of 190 to obtain a continuous first sheet with a rectangular cross section and a thickness of 0.5 mm.

[0082] AGC's CD-126E polytetrafluoroethylene dispersion resin was uniformly mixed with 21.6% by weight of isoalkane (ExxonMobil ISOPAR K), and then left in a 25°C thermostatic chamber for 18 hours. The PTFE resin mixture was pressed into a cylindrical preform, which was then extruded through a T-shaped die with different specifications at a compression ratio of 49 to obtain a continuous second sheet with a rectangular cross section and a thickness of 0.9 mm, and then extruded at a compression ratio of 37 to obtain a continuous third sheet with a rectangular cross section and a thickness of 1.2 mm.

[0083] Three sheets with different specifications were stacked in the same width direction in order of increasing or decreasing thickness, and then rolled longitudinally between a pair of metal rolls to a thickness of 0.32 mm to form a laminate. The dried laminate, from which the lubricant had been removed, was stretched longitudinally using a longitudinal stretching mechanism at a roll temperature of 300°C, a stretching speed of 66.7% / s, and an expansion ratio of 200%. The longitudinally stretched unidirectionally stretched product (base band) was then stretched transversely using a transverse expander at a stretching temperature of 250°C, a stretching speed of 22% / s, and an expansion ratio of 2000%, and finally heat-set to form a film.

[0084] The resulting polytetrafluoroethylene (PTFE) microporous membrane has a first porous outer surface and a second porous outer surface with different microstructures, as described above. As shown in Figures 10 and 11, Figure 10 is a scanning electron micrograph (SEM) of the first porous outer surface of the PTFE membrane taken at 2000x magnification, showing an island-like microstructure constructed from several interconnected, relatively small nodes. Figure 11 is a scanning electron micrograph (SEM) of the second porous outer surface of the PTFE membrane taken at 2000x magnification, showing an H-shaped ladder-like microstructure constructed from several interconnected, larger nodes.

[0085] The bubble points were measured using a pore size distribution porometer under two test conditions: in the first test, the first porous outer surface of the PTFE membrane faced the metal mesh, and in the second test, the second porous outer surface of the PTFE membrane faced the metal mesh, resulting in bubble points of 2.35 bar and 1.77 bar, respectively.

[0086] Example 6 The polytetrafluoroethylene dispersion resin was uniformly mixed with 21.3% by weight of isoalkane (ISOPAR M from ExxonMobil) and then left in a thermostatic chamber at 30°C for 15 hours. The PTFE resin mixture was pressed into a cylindrical preform, which was then extruded using a T-shaped die at a compression ratio of 190 to obtain a continuous first sheet with a rectangular cross section and a thickness of 0.5 mm.

[0087] AGC's CD-126E polytetrafluoroethylene dispersion resin was uniformly mixed with 26.2% by weight of isoalkane (ExxonMobil ISOPAR K), and then left in a 30°C thermostatic chamber for 15 hours. The PTFE resin mixture was pressed into a cylindrical preform, which was then extruded through a T-shaped die with different specifications at a compression ratio of 56 to obtain a continuous second sheet with a rectangular cross section and a thickness of 0.8 mm, and then extruded at a compression ratio of 37 to obtain a continuous third sheet with a rectangular cross section and a thickness of 1.2 mm.

[0088] Three sheets with different specifications were stacked in the same width direction in order of increasing or decreasing thickness, and then rolled longitudinally between a pair of metal rolls to a thickness of 0.3 mm to form a laminate. The dried laminate, from which the lubricant had been removed, was stretched longitudinally using a longitudinal stretching mechanism at a roll temperature of 300°C, a stretching speed of 117% / s, and an expansion ratio of 230%. The longitudinally stretched unidirectionally stretched product (base band) was then stretched transversely using a transverse expander at a stretching temperature of 250°C, a stretching speed of 22% / s, and an expansion ratio of 2000%, and finally heat-set to form a film.

[0089] The resulting polytetrafluoroethylene (PTFE) microporous membrane has a first porous outer surface and a second porous outer surface with different microstructures, as described above. As shown in Figures 12-13, Figure 12 is a scanning electron micrograph (SEM) of the first porous outer surface of the PTFE membrane taken at 2000x magnification, showing an island-like microstructure constructed from several interconnected, relatively small nodes. Figure 13 is a scanning electron micrograph (SEM) of the second porous outer surface of the PTFE membrane taken at 2000x magnification, showing an H-shaped ladder-like microstructure constructed from several interconnected, larger nodes.

[0090] The bubble points were measured using a pore size distribution porometer under two test conditions: in the first test, the first porous outer surface of the PTFE membrane faced the metal mesh, and in the second test, the second porous outer surface of the PTFE membrane faced the metal mesh, resulting in bubble points of 2.12 bar and 1.68 bar, respectively.

[0091] The above series of detailed descriptions are only specific descriptions of possible embodiments of the present invention, and are not used to limit the protection scope of the present invention; any equivalent embodiments or modifications that do not deviate from the technical spirit of the present invention should be included in the protection scope of the present invention.

[0092] Furthermore, although this specification is described according to embodiments, each embodiment does not include only one independent technical solution, and such description form of this specification is merely for clarity, and those skilled in the art should understand that the specification must be considered as a whole, and the technical solutions of each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A polytetrafluoroethylene membrane having a gradient pore structure, A polytetrafluoroethylene membrane having a gradient pore structure, characterized in that the cross section of the polytetrafluoroethylene membrane has a gradient pore structure, and the first porous outer surface and the second porous outer surface of the polytetrafluoroethylene membrane have fibers and nodes with different microstructures, the first porous outer surface has an island-like microstructure constructed from a plurality of interconnected, relatively small nodes, and the second porous outer surface has an H-shaped ladder-type microstructure constructed from a plurality of interconnected, relatively large nodes.

2. The polytetrafluoroethylene membrane with gradient pore structure according to claim 1, characterized in that the relatively small nodes have a circular structure and an average diameter of 0.30 to 1.50 μm.

3. The polytetrafluoroethylene membrane with a gradient pore structure according to claim 2, wherein the relatively small nodes are formed by connecting a plurality of thick and short microfibers having an average length range of 0.2 to 2.5 μm, an average diameter range of 0.05 to 0.2 μm, and an aspect ratio of 2 to 20.

4. The polytetrafluoroethylene membrane with gradient pore structure according to claim 1, characterized in that the relatively large nodes are elongated elliptical structures, with an average length of 0.5-10 μm and an average width of 0.5-2 μm.

5. The polytetrafluoroethylene membrane having a gradient pore structure according to claim 4, wherein the relatively large nodes are formed by connecting a plurality of elongated microfibers having an average length range of 1 to 10 μm, an average diameter range of 5 to 200 nm, and an aspect ratio of 10 to 200.

6. First, a step (1) of uniformly mixing an isoalkane and a PTFE-dispersed resin to obtain a PTFE resin mixture, and then pressing the PTFE resin mixture into a cylindrical preform; Step (2) using an extruder to extrude the cylindrical preform into three continuous strip-shaped sheets of equal width and rectangular cross section with different thicknesses through T-shaped extrusion dies of different specifications, which are designated as a first sheet, a second sheet, and a third sheet, respectively; Step (3) of stacking the first sheet, the second sheet, and the third sheet in the same direction along the width in order of gradually increasing or decreasing thickness, and rolling them into a pair of metal rolls along the longitudinal direction in the thickness direction to a range of 0.05 to 1 mm to form a laminate; (4) drying the laminate in an environment of 200-250°C, and then stretching the laminate along the speed direction at a temperature of 250-350°C with a stretching speed of 20% / sec-3000% / sec and an expansion ratio of 50%-900% to form a unidirectionally stretched product; and (5) stretching the unidirectionally stretched product in a direction perpendicular to the stretching direction at a temperature of 200-400°C, a stretching rate of 5% / sec-500% / sec, and a stretching ratio of 300%-3000%, thereby finally forming a PTFE membrane material with a gradient pore structure.

7. The method for producing a polytetrafluoroethylene membrane with a gradient pore structure according to claim 6, characterized in that in step (1), isoalkane and PTFE-dispersed resin are uniformly mixed in a weight ratio of 15 to 30% by weight to obtain a PTFE resin mixture, which is dried at 20°C or higher for 12 hours or more, and the PTFE resin mixture is pressed into a cylindrical preform.

8. 8. The method for producing a polytetrafluoroethylene membrane with a gradient pore structure according to claim 7, wherein in step (2), the cylindrical preform is extruded through a T-shaped extrusion die with different specifications at a compression ratio of 20 to 500 to form three continuous strip-shaped sheets with rectangular cross-sections and equal widths and different thicknesses, which are designated as the first sheet, the second sheet, and the third sheet, respectively, wherein the thickness of the second sheet is at least 1.2 times that of the first sheet, and the thickness of the third sheet is at least 1.2 times that of the second sheet, and the compression ratio of the first sheet is at least 2.5 times that of the second sheet and 3 times that of the third sheet.

9. 9. The method for producing a polytetrafluoroethylene membrane with a gradient pore structure according to claim 8, wherein in step (4), the laminate is stretched along a speed direction at a stretching speed of 50% / sec to 2000% / sec and an expansion ratio of 80% to 800% to form a unidirectionally stretched product.

10. 10. The method for producing a polytetrafluoroethylene membrane with a gradient pore structure according to claim 9, wherein in step (5), the stretching is performed in a direction perpendicular to the stretching direction at a stretching rate of 20% / s-300% / s and a stretching ratio of 500%-2500%, thereby finally forming a PTFE membrane material with a gradient pore structure.

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