High-precision gradient hole asymmetric PTFE filter membrane and preparation method thereof

Abstract

The invention provides a high-precision gradient pore asymmetric PTFE filter membrane and a preparation method and application thereof.The high-precision gradient pore asymmetric PTFE filter membrane comprises a loose layer and a compact layer, the loose layer is the outer surface of a first porous fiber assembly, and the compact layer is the outer surface of a second porous fiber assembly; the outer surface of the first porous fiber assembly comprises a filamentous network structure formed by interweaving long fibers connected by approximately elliptical nodes, the structure is loose, the outer surface of the second porous fiber assembly comprises a fiber-node interweaving structure formed by interweaving short fibers connected by circular nodes, and the structure is compact and regular; the fiber diameter, the fiber length and the node size of the outer surface of the first porous fiber assembly are all larger than the fiber diameter, the fiber length and the node size of the outer surface of the second porous fiber assembly. The PTFE filter membrane shows unique advantages in the field of semiconductor manufacture procedures, is particularly suitable for high-precision filtering operation of stripping liquid, etching liquid and the like, and can meet the strict production requirements of the semiconductor industry.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer filtration membrane materials, in particular to a high-precision gradient pore asymmetric PTFE filtration membrane and a preparation method and application thereof. Background Art

[0002] PTFE (polytetrafluoroethylene) has a highly stable molecular structure and excellent chemical inertness, making it resistant to highly corrosive cleaning agents. Filter elements made from PTFE, with their high surface area, high retention efficiency, and high filtration flux, meet the dual requirements of fluid cleanliness and processing efficiency in semiconductor wet etching and cleaning processes.

[0003] In the field of filter material performance optimization, there is an inherent contradiction between improving processing efficiency and maintaining filtration accuracy: if you need to increase the flux per unit area at a fixed pore size, you usually need to increase the pore density or reduce the thickness of the film material. However, due to the limitations of the material preparation process, there is a technical bottleneck in significantly increasing the number of pores per unit area under specific production conditions; and when reducing the mass transfer resistance by reducing the film thickness to speed up the filtration rate, it will lead to insufficient material structural strength, making it difficult to meet the mechanical performance requirements of actual working conditions. Therefore, the key to developing filter materials with both high permeability and high filtration accuracy lies in breaking through the restrictive relationship between pore size, fluid resistance and particle retention capacity, and achieving the coordinated optimization of low flow resistance, high flow rate and strong pollution holding capacity.

[0004] For example, Chinese patent publication number CN112108008A discloses a method for preparing a biaxially oriented polytetrafluoroethylene film, in which polytetrafluoroethylene resin and acetone are first mixed evenly and then aged, and then a sheet-like base tape is formed through a calendering process. The base tape after the first longitudinal stretching is stacked again, and then longitudinally stretched a second time while being stretched transversely to prepare a symmetrical porous membrane with a pore size of 0.16 to 0.22 μm.

[0005] In the semiconductor manufacturing process, symmetrically structured polytetrafluoroethylene porous membranes are difficult to meet the filtration requirements of stripping solutions and etching solutions. The pore size design of this type of membrane material is too large, resulting in a particle retention efficiency that is difficult to meet process standards. To solve this problem, the industry often uses a variety of technical means to fine-tune the pore size of polytetrafluoroethylene membranes. However, in practice, it has been found that as the pore size decreases, the flux performance of the membrane material will drop significantly. According to the laws of fluid mechanics, when the membrane pore size is reduced to 1 / 2 of the original size, the flux may drop to 1 / 4 of the original value or even lower. In addition, tiny pores are extremely prone to clogging, causing the continuous filtration process of solid-containing fluids to be interrupted, greatly shortening the service life of the membrane material. The above technical difficulties make it difficult for traditional symmetrical polytetrafluoroethylene membranes to meet the actual engineering needs of the semiconductor manufacturing field.

[0006] Of course, some researchers have studied asymmetric polytetrafluoroethylene membrane materials and tried to apply them in semiconductor filtration. For example, the Chinese patent with publication number CN117018892A discloses a method for preparing an asymmetric polytetrafluoroethylene porous membrane. The asymmetric porous membrane is formed by contacting both sides of the base tape with a cold and hot carrier to form a temperature difference. Due to the ultra-thin thickness of the stretched membrane, the ability to construct a significant temperature gradient in the thickness direction is limited, which makes it difficult to achieve the goal of precise control of the asymmetric microstructure in the membrane material. In addition, the porous membrane prepared by this method has a bubble point of only 5-100kPa, and its ability to retain nano-particles is limited.

[0007] Due to the special nature of its processes, the electronics and semiconductor industry has more stringent requirements for the filtration accuracy of filter membranes, and the filtration accuracy of some key processes must reach the 3nm level. In contrast, in traditional fields such as medicine, food, beverages, beer, and chemicals, the filter membranes used in filter elements mostly have a precision of 300-500nm. Even in a few scenarios with higher precision requirements, the 100nm level can still not meet the 3nm precision filtration needs of the semiconductor industry. Given this significant technological gap, it is of great practical significance to develop a method for preparing polytetrafluoroethylene filter membranes suitable for the semiconductor industry with a filtration accuracy of 3nm.

[0008] In view of this, it is necessary to improve the conventional method for preparing polytetrafluoroethylene membranes to solve the above problems. Summary of the Invention

[0009] The inventive concept of the present invention is: through multiple longitudinal stretching, an asymmetric porous membrane material is designed, and the asymmetric porous membrane is constructed into a gradient pore structure, which includes a loose layer and a dense layer, that is, the loose layer is the outer surface of the first porous fiber assembly, and the dense layer is the outer surface of the second porous fiber assembly. Liquid filtration needs to pass through the loose layer and the dense layer in sequence.

[0010] The first object of the present invention is to disclose a high-precision gradient pore asymmetric PTFE filter membrane. The filter membrane adopts integral molding technology to form a unique asymmetric structure. The gradient distribution design with large pore size in the upper layer and small pore size in the lower layer can effectively intercept tiny particle impurities.

[0011] To achieve the above-mentioned objectives, the present invention provides a high-precision gradient pore asymmetric PTFE filter membrane, comprising a loose layer and a dense layer, wherein the loose layer is the outer surface of a first porous fiber assembly, and the dense layer is the outer surface of a second porous fiber assembly; the outer surface of the first porous fiber assembly comprises a filamentous network structure formed by long fibers connected by approximately elliptical nodes interwoven with each other, and the structure is loose; the outer surface of the second porous fiber assembly comprises a fiber-node interwoven structure formed by short fibers connected by circular nodes interwoven with each other, and the structure is dense and regular; the fiber diameter, fiber length and node size of the outer surface of the first porous fiber assembly are all larger than the fiber diameter, fiber length and node size of the outside of the second porous fiber assembly.

[0012] In some embodiments, the outer surface of the first porous fiber assembly includes irregularly shaped block-shaped enlarged nodes as fiber connection hubs, and supports the long fibers around them. The long fibers extend from the block-shaped enlarged nodes to the surrounding areas, interweave and intertwine with each other to form a loose three-dimensional network structure.

[0013] In some embodiments, the average fiber diameter on the outer surface of the first porous fiber assembly is 0.8-1.5 μm, the average fiber length is 2-19 μm, and the average node diameter is 1.3-4 μm; the average fiber diameter on the outer surface of the second porous fiber assembly is 30-150 nm, the average fiber length is 300 nm-3 μm, and the average node diameter is 330-620 nm.

[0014] In some embodiments, the IPA bubble point value of the outer surface of the first porous fiber assembly is ≥1.3 bar, and the IPA bubble point value of the outer surface of the second porous fiber assembly is ≥4.3 bar.

[0015] In some embodiments, the longitudinal tensile strength of the asymmetric PTFE filter membrane is ≥40 MPa, and the elongation at break of the asymmetric PTFE filter membrane is 229-336%.

[0016] In some embodiments, the asymmetric PTFE filter membrane has a rejection rate of ≥84% for 30 nm particles, and a liquid flux of ≥5.5 mL / min at a pressure of -80 kPa. · cm 2 .

[0017] The second purpose of the present invention is to disclose a method for preparing a high-precision gradient pore asymmetric PTFE filter membrane. The prepared membrane has high-efficiency retention characteristics and can achieve accurate filtration of target substances. While ensuring the filtration effect, the pressure loss generated when the fluid passes through is low, which can effectively reduce energy consumption.

[0018] To achieve the above object, the present invention provides a method for preparing a high-precision gradient pore asymmetric PTFE filter membrane, comprising the following steps:

[0019] S1 mixing: adding the auxiliary oil into the polytetrafluoroethylene dispersion resin in a certain proportion;

[0020] S2 aging: Place the mixture of polytetrafluoroethylene dispersion resin and auxiliary oil in an oven for a certain period of time to allow the auxiliary oil to fully infiltrate the resin;

[0021] S3 pre-pressing: Add the matured mixture into the cylinder and maintain the pressure for a certain period of time to squeeze out the air between the resins and form a compact columnar embryo.

[0022] S4 Extrusion: The pre-pressed cylindrical embryo is placed into the extruder to obtain a strip-shaped sheet;

[0023] S5 calendering: calendering the strip sheet to obtain a base strip with uniform thickness;

[0024] S6 degreasing: At a certain temperature, the calendered base tape is placed in a degreasing machine to remove the additive oil;

[0025] S7 longitudinal stretching: The degreased base tape is stretched multiple times in the longitudinal direction at a certain stretching rate;

[0026] S8 Horizontal stretching: The longitudinally stretched base tape is stretched horizontally at a certain stretching rate to obtain an asymmetric PTFE filter membrane.

[0027] In some embodiments, in S7, the degreased base tape is stretched longitudinally at least twice: the degreased base tape is first stretched at a low temperature and a low speed for a first time to obtain a first stretched sheet, and then the first stretched sheet is quickly stretched for a second time at a high temperature and a high speed to obtain a second stretched sheet, and then the second stretched sheet is quickly heat-set.

[0028] In some embodiments, the molecular weight of the polytetrafluoroethylene dispersion resin in S1 is 4 to 9 million, the auxiliary oil is isoparaffin, and the ratio of the auxiliary oil to the polytetrafluoroethylene dispersion resin is 1:80-5:8.

[0029] In some embodiments, the mixture of the auxiliary oil and the polytetrafluoroethylene dispersion resin in S2 is placed in an oven at 45° C. and aged for more than 12 hours to fully swell the polytetrafluoroethylene dispersion resin.

[0030] In some embodiments, the matured mixture in S3 is compressed into a compact cylindrical embryo at a pressure of 40 bar and a compression ratio of 25-400.

[0031] In some embodiments, in S4, the columnar embryo is placed in an extruder with a T-shaped extrusion die to extrude a first strip-shaped sheet and a second strip-shaped sheet having the same width and a thickness ratio of 1.1 to 2.9 times.

[0032] In some embodiments, in S5, the first strip-shaped sheet and the second strip-shaped sheet are stacked from thin to thick in the thickness direction, perpendicular to the speed direction, and enter the calender roller parallel to the speed direction to press the thickness to a range of 0.06 to 2 mm to form a base belt with uniform thickness.

[0033] In some embodiments, in S6, the base tape is degreased at a temperature of 190-240° C. to remove the auxiliary oil.

[0034] In some embodiments, in S7 , the base tape is stretched several times along the speed direction (ie, longitudinally stretched) at a stretching rate of 30 to 2500% / s at 200 to 350° C. to obtain a unidirectional longitudinally drawn sheet.

[0035] In some embodiments, in S8, the longitudinally drawn sheet is stretched at a temperature of 200 to 360° C. at a stretching rate of 9 to 6000% / s in a direction perpendicular to the speed (ie, transverse stretching) to obtain an asymmetric PTFE filter membrane.

[0036] In some embodiments, the first stretching temperature is 220-290° C., the stretching rate is 50-500% / s, and the second stretching temperature is 260-350° C., the stretching rate is 800-1500% / s.

[0037] In some embodiments, the heat setting temperature is 350°C.

[0038] In some embodiments, the transverse stretching in S8 has a stretching ratio of 5-45 times; the transverse stretching includes a preheating section, a wide width section and a heat setting section, the preheating section temperature is 220-250°C, the wide width section temperature is 270-290°C, and the setting section temperature is 330-350°C.

[0039] In some embodiments, the stretching rate in the preheating section of the transverse stretching is 50-150% / s, and the stretching rate in the wide width section is 1500-3000% / s.

[0040] The present invention also provides an application of the high-precision gradient pore asymmetric PTFE filter membrane. The asymmetric PTFE filter membrane can be made into a filter element with a filtration accuracy of 3nm, which is used for filtering wet etching liquid and cleaning liquid in semiconductor manufacturing processes.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] (1) The asymmetric gradient pore structure design can show unique advantages under the same filtration and separation conditions: first, the large pore channels on the outer surface of the porous fiber assembly significantly reduce the resistance to fluid flow and can quickly pass through the liquid; second, the micropores on the outer surface of the porous fiber assembly achieve precise particle retention, which not only avoids the problem of insufficient permeation flux caused by the small pore size of the symmetrical membrane, but also overcomes the defect of decreased filtration efficiency of the large pore membrane. Thus, while ensuring the filtration accuracy, it effectively improves the processing capacity and pollution holding capacity per unit area, providing an ideal technical path for high-precision, high-throughput filtration scenarios.

[0043] (2) At least two longitudinal stretchings are used. The first longitudinal stretching can initially induce the molecular chains to orient themselves longitudinally, forming a basic fibrous structural framework. The second longitudinal stretching further strengthens the orderly arrangement of the molecular chains and refines the microstructure, effectively avoiding the stress concentration problem caused by a single high-ratio stretching and reducing the risk of material fracture or internal defects. At the same time, performing heat setting again without setting the stretching ratio can eliminate the internal residual stress accumulation caused by the forced stretching of the molecular chains during the longitudinal stretching process, promote the improvement of the crystal region, reduce lattice defects, and improve the longitudinal tensile strength.

[0044] (3) The filter membrane has excellent mechanical properties, high tensile strength, and tight bonding between layers. It will not delaminate or fall off during use, significantly extending its service life. This PTFE filter membrane has unique advantages in the field of semiconductor manufacturing, and is particularly suitable for high-precision filtration operations such as stripping solutions and etching solutions, meeting the stringent production needs of the semiconductor industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is a SEM image of the PTFE filter membrane prepared in Example 1, with a magnification of 7000 times;

[0046] Figure 2 This is a SEM image of the PTFE filter membrane prepared in Example 2, with a magnification of 7000 times;

[0047] Figure 3 This is a SEM image of the dense layer of the PTFE filter membrane prepared in Example 3, with a magnification of 7000 times;

[0048] Figure 4 This is a SEM image of the loose layer of the PTFE filter membrane prepared in Example 3, with a magnification of 1500 times;

[0049] Figure 5 This is a SEM image of the dense layer of the PTFE filter membrane prepared in Example 4, with a magnification of 7000 times;

[0050] Figure 6This is a SEM image of the loose layer of the PTFE filter membrane prepared in Example 4, with a magnification of 1500 times;

[0051] Figure 7 This is a SEM image of the dense layer of the PTFE filter membrane prepared in Example 5, with a magnification of 7000 times;

[0052] Figure 8 This is a SEM image of the loose layer of the PTFE filter membrane prepared in Example 5, with a magnification of 1500 times;

[0053] Figure 9 This is a SEM image of the dense layer of the PTFE filter membrane prepared in Example 6, with a magnification of 7000 times;

[0054] Figure 10 This is a SEM image of the loose layer of the PTFE filter membrane prepared in Example 6, with a magnification of 1500 times;

[0055] Figure 11 This is a cross-sectional SEM image of the PTFE filter membrane prepared in Example 6, with a magnification of 3500 times. DETAILED DESCRIPTION

[0056] The present invention is described in detail below with reference to the various embodiments shown in the accompanying drawings, but it should be noted that these embodiments are not limitations of the present invention, and any equivalent transformations or substitutions in functions, methods, or structures made by ordinary technicians in this field based on these embodiments are all within the scope of protection of the present invention.

[0057] The present invention provides a high-precision gradient pore asymmetric PTFE filter membrane, comprising a loose layer and a dense layer, wherein the loose layer is the outer surface of a first porous fiber assembly, and the dense layer is the outer surface of a second porous fiber assembly; the outer surface of the first porous fiber assembly comprises a filamentous network structure formed by long fibers connected by approximately elliptical nodes interwoven with each other, and the structure is loose; the outer surface of the second porous fiber assembly comprises a fiber-node interwoven structure formed by short fibers connected by circular nodes interwoven with each other, and the structure is dense and regular; the fiber diameter, fiber length and node size of the outer surface of the first porous fiber assembly are all larger than the fiber diameter, fiber length and node size of the outside of the second porous fiber assembly.

[0058] The outer surface of the first porous fiber assembly includes irregularly shaped block-shaped enlarged nodes that serve as connecting hubs for the fibers and support the long fibers around them. The long fibers extend from the block-shaped enlarged nodes to the surrounding areas, interweaving and interweaving with each other to form a loose three-dimensional network structure. The irregular block-shaped enlarged nodes are approximately elliptical, and the shape of the irregular block-shaped enlarged nodes is approximately elliptical.

[0059] The average fiber diameter on the outer surface of the first porous fiber assembly is 0.8-1.5μm, the average fiber length is 2-19μm, and the average node diameter is 1.3-4μm. The average fiber diameter on the outer surface of the second porous fiber assembly is 30-150nm, the average fiber length is 300nm-3μm, and the average node diameter is 330-620nm.

[0060] The IPA bubble point value of the outer surface of the first porous fiber assembly is ≥1.3 bar, and the IPA bubble point value of the outer surface of the second porous fiber assembly is ≥4.3 bar. The longitudinal tensile strength of the asymmetric PTFE filter membrane is ≥40 MPa, and the elongation at break of the asymmetric PTFE filter membrane is 229-336%. The retention rate of the asymmetric PTFE filter membrane for 30 nm particles is ≥84%, and the liquid flux of the asymmetric PTFE filter membrane at a pressure of -80 kPa is ≥5.5 mL / min. · cm 2 .

[0061] The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane comprises the following steps:

[0062] S1 mixing: adding the auxiliary oil into the polytetrafluoroethylene dispersion resin in a certain proportion;

[0063] S2 aging: Place the mixture of polytetrafluoroethylene dispersion resin and auxiliary oil in an oven for a certain period of time to allow the auxiliary oil to fully infiltrate the resin;

[0064] S3 pre-pressing: Add the matured mixture into the cylinder and maintain the pressure for a certain period of time to squeeze out the air between the resins and form a compact columnar embryo.

[0065] S4 Extrusion: The pre-pressed cylindrical embryo is placed into the extruder to obtain a strip-shaped sheet;

[0066] S5 calendering: calendering the strip sheet to obtain a base strip with uniform thickness;

[0067] S6 degreasing: placing the calendered base tape in a degreasing machine at a certain temperature to remove the additive oil; S7 longitudinal stretching: stretching the degreased base tape multiple times in the longitudinal direction at a certain stretching rate;

[0068] S8 Horizontal stretching: The longitudinally stretched base tape is stretched horizontally at a certain stretching rate to obtain an asymmetric PTFE filter membrane.

[0069] Among them, in S7, the degreased base tape is stretched longitudinally at least twice: the degreased base tape is first stretched at low temperature and low speed for the first time to obtain a first stretched sheet, and then the first stretched sheet is quickly stretched for the second time at high temperature and high speed to obtain a second stretched sheet, and then the second stretched sheet is quickly heat-set.

[0070] This high-precision gradient pore asymmetric PTFE filter membrane can be made into a filter element with a filtration accuracy of 3nm, which is used for filtering wet etching liquid and cleaning liquid in semiconductor manufacturing process.

[0071] Example 1

[0072] like Figure 1 As shown, a method for preparing a high-precision gradient pore asymmetric PTFE filter membrane comprises the following steps:

[0073] S1: Mixing: 0.15 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 1.13 kg of Daikin F-106 polytetrafluoroethylene dispersion resin with a molecular weight of 6.5 million are stirred uniformly by a machine to obtain a polytetrafluoroethylene dispersion resin mixture;

[0074] S2 Curing: Place the mixture of polytetrafluoroethylene dispersion resin and auxiliary oil in an oven at 45°C for more than 12 hours to allow the auxiliary oil to fully infiltrate the resin;

[0075] S3 pre-pressing: Add the matured mixture into the cylinder and maintain the pressure at 40 bar for 60 seconds to squeeze out the air between the resins and form a compact columnar embryo.

[0076] S4 Extrusion: The pre-pressed cylindrical body is placed into an extruder with a T-die and extruded at a compression ratio of 136 to obtain a strip-shaped sheet with a thickness of 0.8 mm;

[0077] S5 calendering: The strip is fed into the calender roller parallel to the speed direction for calendering, and the thickness is pressed to 0.45mm to obtain a base strip with uniform thickness;

[0078] S6 Degreasing: Place the base tape in a degreasing machine at 225°C to remove the additive oil;

[0079] S7 longitudinal stretching: The degreased base tape is longitudinally stretched for the first time at a temperature of 250°C and a stretching rate of 271% / s to obtain a first stretched sheet, and then the first stretched sheet is longitudinally stretched for the second time at a temperature of 275°C and a stretching rate of 1100% / s to obtain a second stretched sheet;

[0080] S8 horizontal stretching: The second stretched sheet is stretched transversely at a temperature of 200-360°C, i.e., transverse stretching, and the transverse stretching multiple is 17, wherein the preheating section temperature is 220°C, the stretching rate is 67% / s, the wide section temperature is 285°C, the stretching rate is 1560% / s, and the heat setting section temperature is 330°C, and finally an asymmetric PTFE filter membrane is obtained.

[0081] The figure is a 7000x scanning electron microscope image (SEM image) of the PTFE filter membrane prepared in this example. It can be clearly seen in the figure that the membrane surface is connected by several small circular nodes and short fibers, and the structure is dense and regular.

[0082] Example 2

[0083] like Figure 2 As shown, a method for preparing a high-precision gradient pore asymmetric PTFE filter membrane comprises the following steps:

[0084] S1: Mixing: 0.13 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 1.5 kg of Asahi Glass CD-126E polytetrafluoroethylene dispersion resin with a molecular weight of 9 million were stirred uniformly by a machine to obtain a polytetrafluoroethylene dispersion resin mixture;

[0085] S2 Curing: Place the mixture of polytetrafluoroethylene dispersion resin and auxiliary oil in an oven at 45°C for more than 12 hours to allow the auxiliary oil to fully infiltrate the resin;

[0086] S3 pre-pressing: Add the matured mixture into the cylinder and maintain the pressure at 40 bar for 60 seconds to squeeze out the air between the resins and form a compact columnar embryo.

[0087] S4 Extrusion: The pre-pressed cylindrical body is placed into an extruder with a T-die and extruded at a compression ratio of 145 to obtain a strip-shaped sheet with a thickness of 0.8 mm.

[0088] S5 calendering: The strip is fed into the calender roller parallel to the speed direction for calendering, and the thickness is pressed to 0.45mm to obtain a base strip with uniform thickness;

[0089] S6 Degreasing: Place the base tape in a degreasing machine at 225°C to remove the additive oil;

[0090] S7 longitudinal stretching: The degreased base tape is longitudinally stretched for the first time at 250°C at a stretching rate of 225% / s to obtain a first stretched sheet. The first stretched sheet is then longitudinally stretched for the second time at 275°C at a stretching rate of 1100% / s to obtain a second stretched sheet. Without longitudinal stretching multiples, the second unidirectional longitudinal stretched sheet is placed on a longitudinal stretching machine and heat-set at 350°C at a speed of 2 m / min.

[0091] S8 horizontal stretching: The second stretched sheet after heat setting is stretched horizontally at a temperature of 200-360°C, that is, horizontal stretching, and the horizontal stretching multiple is 15, wherein the preheating section temperature is 220°C, the stretching rate is 75% / s, the wide section temperature is 285°C, the stretching rate is 1550% / s, and the heat setting section temperature is 330°C, and finally an asymmetric PTFE filter membrane is obtained.

[0092] Figure 2 This is a 7000x electron microscope image (SEM image) of the PTFE filter membrane prepared in this example. It can be clearly seen in the image that the membrane surface is composed of a number of small circular nodes and short fibers of uniform thickness connected to each other in a radial structure, and the structure is dense and regular.

[0093] Example 3

[0094] like Figure 3-4 As shown, a method for preparing a high-precision gradient pore asymmetric PTFE filter membrane comprises the following steps:

[0095] S1 mixing: 0.12 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 1.2 kg of Daikin F-106 polytetrafluoroethylene dispersion resin with a molecular weight of 6.5 million were stirred uniformly by a machine to obtain polytetrafluoroethylene dispersion resin mixture 1;

[0096] 0.25 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 2.0 kg of Asahi Glass CD-126E polytetrafluoroethylene dispersion resin with a molecular weight of 9 million were stirred uniformly by a machine to obtain polytetrafluoroethylene dispersion resin mixture 2;

[0097] S2 aging: Place the above mixture 1 and mixture 2 in an oven at 45°C for aging for more than 12 hours to allow the additive oil to fully penetrate the resin;

[0098] S3 pre-pressing: Add the matured mixture 1 and mixture 2 into the cylinder respectively, maintain the pressure at 40 bar for 60 seconds, squeeze out the air between the resins, and form compact columnar embryos 1 and 2;

[0099] S4 Extrusion: The pre-pressed columnar embryos 1 and 2 are placed in an extruder with a T-die and extruded at a compression ratio of 148 to obtain a first strip-shaped sheet with a thickness of 0.4 mm. The pre-pressed columnar embryos 1 and 2 are extruded at a compression ratio of 63 to obtain a second strip-shaped sheet with a thickness of 0.8 mm.

[0100] S5 calendering: stack the first strip-shaped sheet and the second strip-shaped sheet in the thickness direction from thin to thick, and feed them into the calender roller parallel to the speed direction for calendering, and press the thickness to 0.8 mm to obtain an overlapping base strip with uniform thickness;

[0101] S6 Degreasing: Place the overlapped base tape in a degreasing machine at 235°C to remove the additive oil;

[0102] S7 longitudinal stretching: The degreased overlapping base tape is longitudinally stretched for the first time at a temperature of 265°C and a stretching rate of 273% / s to obtain a first stretched sheet, and then the first stretched sheet is longitudinally stretched for the second time at a temperature of 295°C and a stretching rate of 1100% / s to obtain a second stretched sheet;

[0103] S8 horizontal stretching: The second stretched sheet is stretched transversely at a temperature of 200-360°C, i.e., transverse stretching, and the transverse stretching multiple is 18, wherein the preheating section temperature is 235°C, the stretching rate is 70% / s, the wide section temperature is 285°C, the stretching rate is 1150% / s, and the heat setting section temperature is 335°C, and finally an asymmetric PTFE filter membrane is obtained.

[0104] Figure 3 This is a 7000x scanning electron microscope image (SEM image) of the dense layer of the PTFE filter membrane (i.e., the outer surface of the second porous fiber assembly) prepared in this example. Figure 4 This is a 1500x electron microscope image (SEM image) of the loose layer of the PTFE filter membrane prepared in this embodiment (i.e., the outer surface of the first porous fiber assembly). In the image, it can be clearly seen that the outer surface of the first porous fiber assembly is a filamentous network structure formed by a number of approximately elliptical nodes and filamentous fibers interconnected, and the structure is loose; the outer surface of the second porous fiber assembly is a fiber-node interwoven structure in which a number of small circular nodes and short fibers of uniform thickness are interconnected, and the structure is dense and regular.

[0105] Example 4

[0106] like Figure 5-6 As shown, a method for preparing a high-precision gradient pore asymmetric PTFE filter membrane comprises the following steps:

[0107] S1 mixing: 0.125 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 1.155 kg of Daikin F-106 polytetrafluoroethylene dispersion resin with a molecular weight of 6.5 million were stirred uniformly by a machine to obtain polytetrafluoroethylene dispersion resin mixture 1;

[0108] 0.24 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 1.95 kg of Asahi Glass CD-126E polytetrafluoroethylene dispersion resin with a molecular weight of 9 million were uniformly stirred by a machine to obtain polytetrafluoroethylene dispersion resin mixture 2;

[0109] S2 aging: Place the above mixture 1 and mixture 2 in an oven at 45°C for aging for more than 12 hours to allow the additive oil to fully penetrate the resin;

[0110] S3 pre-pressing: Add the matured mixture 1 and mixture 2 into the cylinder respectively, maintain the pressure at 40 bar for 60 seconds, squeeze out the air between the resins, and form compact columnar embryos 1 and 2;

[0111] S4 Extrusion: The pre-pressed columnar embryo 1 and columnar embryo 2 are placed in an extruder with a T-die, and extruded at a compression ratio of 145 to obtain a first strip-shaped sheet with a thickness of 0.4 mm. The pre-pressed columnar embryo 1 and columnar embryo 2 are extruded at a compression ratio of 65 to obtain a second strip-shaped sheet with a thickness of 0.8 mm.

[0112] S5 calendering: stack the first strip-shaped sheet and the second strip-shaped sheet in the thickness direction from thin to thick, and feed them into the calender roller parallel to the speed direction for calendering, and press the thickness to 0.8 mm to obtain an overlapping base strip with uniform thickness;

[0113] S6 Degreasing: Place the overlapped base tape in a degreasing machine at 235°C to remove the additive oil;

[0114] S7 longitudinal stretching: The degreased overlapping base tape is longitudinally stretched for the first time at a temperature of 265°C and a stretching rate of 275% / s to obtain a first stretched sheet, and then the first stretched sheet is longitudinally stretched for the second time at a temperature of 295°C and a stretching rate of 1100% / s to obtain a second stretched sheet;

[0115] S8 horizontal stretching: The second stretched sheet is stretched transversely at a temperature of 200-360°C, i.e., transverse stretching, and the transverse stretching multiple is 18, wherein the preheating section temperature is 235°C, the stretching rate is 80% / s, the wide section temperature is 285°C, the stretching rate is 1150% / s, and the heat setting section temperature is 335°C, and finally an asymmetric PTFE filter membrane is obtained.

[0116] Figure 5This is a 7000x scanning electron microscope image (SEM image) of the dense layer of the PTFE filter membrane (i.e., the outer surface of the second porous fiber assembly) prepared in this example. Figure 6 This is a 1500x electron microscope image (SEM image) of the loose layer of the PTFE filter membrane prepared in this embodiment (i.e., the outer surface of the first porous fiber assembly). In the image, it can be clearly seen that the outer surface of the first porous fiber assembly is a filamentous network structure formed by a number of approximately elliptical nodes and filamentous fibers interconnected, and the structure is loose; the outer surface of the second porous fiber assembly is a fiber-node interwoven structure in which a number of small circular nodes and short fibers of uniform thickness are interconnected, and the structure is dense and regular.

[0117] Example 5

[0118] like Figure 7-8 As shown, a method for preparing a high-precision gradient pore asymmetric PTFE filter membrane comprises the following steps:

[0119] S1 mixing: 0.11 kg of isoparaffin additive oil (ExxonMobil ISOPARM) and 1.15 kg of Daikin F-106 polytetrafluoroethylene dispersion resin with a molecular weight of 6.5 million were stirred uniformly by a machine to obtain polytetrafluoroethylene dispersion resin mixture 1;

[0120] 0.23 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 1.8 kg of Asahi Glass CD-126E polytetrafluoroethylene dispersion resin with a molecular weight of 9 million were uniformly stirred by a machine to obtain polytetrafluoroethylene dispersion resin mixture 2;

[0121] S2 aging: Place the above mixture 1 and mixture 2 in an oven at 45°C for aging for more than 12 hours to allow the additive oil to fully penetrate the resin;

[0122] S3 pre-pressing: Add the matured mixture 1 and mixture 2 into the cylinder respectively, maintain the pressure at 40 bar for 60 seconds, squeeze out the air between the resins, and form compact columnar embryos 1 and 2;

[0123] S4 Extrusion: The pre-pressed columnar embryo 1 and columnar embryo 2 are placed in an extruder with a T-die, and extruded at a compression ratio of 147 to obtain a first strip-shaped sheet with a thickness of 0.4 mm. The pre-pressed columnar embryo 1 and columnar embryo 2 are extruded at a compression ratio of 72 to obtain a second strip-shaped sheet with a thickness of 0.8 mm.

[0124] S5 calendering: stack the first strip-shaped sheet and the second strip-shaped sheet in the thickness direction from thin to thick, and feed them into the calender roller parallel to the speed direction for calendering, and press the thickness to 0.8 mm to obtain an overlapping base strip with uniform thickness;

[0125] S6 Degreasing: Place the overlapped base tape in a degreasing machine at 235°C to remove the additive oil;

[0126] S7 longitudinal stretching: The degreased overlapping base tape is longitudinally stretched for the first time at a temperature of 260°C and a stretching rate of 255% / s to obtain a first stretched sheet. Then, the first stretched sheet is longitudinally stretched for the second time at a temperature of 280°C and a stretching rate of 1000% / s to obtain a second stretched sheet.

[0127] As a further improvement of the present invention, the second unidirectional longitudinally drawn sheet is placed on a longitudinal drawing machine at a temperature of 350°C and a speed of 2 m / min without a longitudinal drawing multiple, thereby increasing the longitudinal tensile strength of the asymmetric porous membrane;

[0128] S8 horizontal stretching: The second stretched sheet after heat setting is stretched horizontally at a temperature of 200-360°C, that is, horizontal stretching, and the horizontal stretching multiple is 15, wherein the preheating section temperature is 230°C, the stretching rate is 75% / s, the wide section temperature is 285°C, the stretching rate is 1100% / s, and the heat setting section temperature is 330°C, and finally an asymmetric PTFE filter membrane is obtained.

[0129] Figure 7 This is a 7000x scanning electron microscope image (SEM image) of the dense layer of the PTFE filter membrane (i.e., the outer surface of the second porous fiber assembly) prepared in this example. Figure 8 This is a 1500x electron microscope image (SEM image) of the loose layer of the PTFE filter membrane prepared in this embodiment (i.e., the outer surface of the first porous fiber assembly). In the image, it can be clearly seen that the outer surface of the first porous fiber assembly is a filamentous network structure formed by a number of approximately elliptical nodes and filamentous fibers interconnected, and the structure is loose; the outer surface of the second porous fiber assembly is a fiber-node interwoven structure in which a number of small circular nodes and short fibers of uniform thickness are interconnected, and the structure is dense and regular.

[0130] Example 6

[0131] like Figure 9-11 As shown, a method for preparing a high-precision gradient pore asymmetric PTFE filter membrane comprises the following steps:

[0132] S1 mixing: 0.1 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 1.16 kg of Daikin F-106 polytetrafluoroethylene dispersion resin with a molecular weight of 6.5 million were stirred uniformly by a machine to obtain polytetrafluoroethylene dispersion resin mixture 1;

[0133] 0.22 kg of isoparaffin additive oil (ExxonMobil ISOPAR M) and 1.7 kg of Asahi Glass CD-126E polytetrafluoroethylene dispersion resin with a molecular weight of 9 million were stirred uniformly by a machine to obtain polytetrafluoroethylene dispersion resin mixture 2;

[0134] S2 aging: Place the above mixture 1 and mixture 2 in an oven at 45°C for aging for more than 12 hours to allow the additive oil to fully penetrate the resin;

[0135] S3 pre-pressing: Add the matured mixture 1 and mixture 2 into the cylinder respectively, maintain the pressure at 40 bar for 60 seconds, squeeze out the air between the resins, and form compact columnar embryos 1 and 2;

[0136] S4 Extrusion: The pre-pressed columnar embryo 1 and columnar embryo 2 are placed in an extruder with a T-die, and extruded at a compression ratio of 134 to obtain a first strip-shaped sheet with a thickness of 0.4 mm. The pre-pressed columnar embryo 1 and columnar embryo 2 are extruded at a compression ratio of 72 to obtain a second strip-shaped sheet with a thickness of 0.8 mm.

[0137] S5 calendering: stack the first strip-shaped sheet and the second strip-shaped sheet in the thickness direction from thin to thick, and feed them into the calender roller parallel to the speed direction for calendering, and press the thickness to 0.8 mm to obtain an overlapping base strip with uniform thickness;

[0138] S6 Degreasing: Place the overlapped base tape in a degreasing machine at 235°C to remove the additive oil;

[0139] S7 longitudinal stretching: The degreased overlapping base tape is longitudinally stretched for the first time at a temperature of 260°C and a stretching rate of 225% / s to obtain a first stretched sheet. Then, the first stretched sheet is longitudinally stretched for the second time at a temperature of 280°C and a stretching rate of 1000% / s to obtain a second stretched sheet.

[0140] As a further improvement of the present invention, the second unidirectional longitudinally drawn sheet is placed on a longitudinal drawing machine at a temperature of 350°C and a speed of 2 m / min without a longitudinal drawing multiple, thereby increasing the longitudinal tensile strength of the asymmetric porous membrane;

[0141] S8 horizontal stretching: The second stretched sheet after heat setting is stretched horizontally at a temperature of 200-360°C, that is, horizontal stretching, and the horizontal stretching multiple is 15, wherein the preheating section temperature is 230°C, the stretching rate is 72% / s, the wide section temperature is 285°C, the stretching rate is 1100% / s, and the heat setting section temperature is 330°C, and finally an asymmetric PTFE filter membrane is obtained.

[0142] Figure 9This is a 7000x scanning electron microscope image (SEM image) of the dense layer of the PTFE filter membrane (i.e., the outer surface of the second porous fiber assembly) prepared in this example. Figure 10 This is a 1500x electron microscope image (SEM image) of the loose layer of the PTFE filter membrane prepared in this embodiment (i.e., the outer surface of the first porous fiber assembly). In the image, it can be clearly seen that the outer surface of the first porous fiber assembly is a filamentous network structure formed by a number of approximately elliptical nodes and filamentous fibers interconnected, and the structure is loose; the outer surface of the second porous fiber assembly is a fiber-node interwoven structure in which a number of small circular nodes and short fibers of uniform thickness are interconnected, and the structure is dense and regular. Figure 11 This is a cross-sectional view of the PTFE filter membrane of this embodiment. It can be clearly seen that the gradient pore structure distribution is that the upper layer of the membrane is a loose layer and the lower layer is a dense layer. I. PTFE filter membrane material performance test

[0143] 1. Thickness measurement

[0144] The thickness of the membrane or substrate can be measured using a handheld thickness gauge (Mitutoyo 7327). First, check that the anvils of the thickness gauge are clean. Wipe them with a dust-free cloth and a small amount of ethanol. Calibrate the parts. Place the membrane or substrate flat between the two anvils of the thickness gauge to measure the thickness. Take the average of five measurements. The thickness of the porous filter membrane in the present invention is between 35 and 45 μm, with good thickness uniformity.

[0145] 2. Bubble point measurement

[0146] Place the sample membrane in the sample chamber of the Porolux500 pore size analyzer, and use an infiltration liquid with a surface tension of 16 dyne / cm to fully infiltrate the sample (IPA isopropyl alcohol solvent is used as the infiltration liquid in this application) to ensure that the liquid fills all pores and discharges bubbles. Gradually increase the gas pressure on one side of the sample and contact the other side with the infiltration liquid. When the pressure reaches the critical value, the liquid begins to be discharged from the largest pore, forming a continuous airflow. The pressure at this time corresponds to the maximum pore size. Continuing to increase the pressure, the liquid in the smaller pores is successively discharged. By detecting the change in gas flow rate, the pore size distribution data under different pressures can be obtained, and the corresponding pore size can be calculated based on the measured pressure value.

[0147]

[0148] Where: D is the pore diameter, unit is um; is the surface tension of the wetting liquid, dyne / cm; P is the pressure, Pa; C is a constant. When the pressure unit is Pa, C is 2860.

[0149] The asymmetric PTFE porous filtration membrane of the present invention has different bubble point values ​​on both sides, and the corresponding overall pore size is at the nanometer level, and is a typical mesoporous membrane material.

[0150] 3. Microstructure characterization

[0151] The surface morphology and cross-sectional structure of the asymmetric porous filter membrane were observed using a Hitachi S-5200 field emission scanning electron microscope. The test sample was first cut into 0.5 cm x 0.5 cm pieces, attached to a sample stage with conductive adhesive, and then gold-sprayed for 2 minutes using an ion sputtering instrument. The membrane surface morphology was observed at various magnifications. For cross-sectional observation, the test sample was thoroughly moistened with a low-surface-tension wetting solution, then frozen in a liquid nitrogen tank for 10 minutes. Upon removal, the sample was rapidly fractured, attached parallel to the sample stage with conductive adhesive, and gold-sprayed for 2 minutes using an ion sputtering instrument. The membrane cross-sectional morphology was then observed at various magnifications.

[0152] 4. Fiber diameter, length and node size measurement

[0153] Use Image-Pro Plus 6.0 measurement software. Draw the scale length in the software based on the actual scale length in the SEM image. To ensure more representative test results, measure multiple fibers in the image, typically 50-100 fibers, and take the average value.

[0154] 5. Liquid flux test

[0155] According to the ASTM F317-72 flow test standard, the high-precision gradient pore size asymmetric filtration membrane prepared by the present invention was subjected to a liquid flux test. The specific test method is as follows: the outer surface of the first porous fiber assembly, i.e., the loose layer, is the liquid inlet surface, and the outer surface of the second porous fiber assembly, i.e., the dense layer, is the liquid outlet surface. When 200 mL of 99.7% IPA is passed through an area of ​​15.2 cm at -80 kPa, the liquid flux is 200 mL of 99.7% IPA. 2 The time required for the membrane to pass was 101s, and the flux was 7.78mL / min.cm 2 . It shows that the asymmetric porous membrane has significant fluid processing capacity per unit area and per unit time, and its high flux characteristics can effectively shorten the operation time of the actual filtration process and reduce the time cost in actual filtration applications. The fibers on the outer surface of the first porous fiber assembly are coarse, long, and have large nodes, forming a loose support structure, which provides a low-resistance transmission channel for the fluid without reducing the interception efficiency; the fibers on the outer surface of the second porous fiber assembly have a small diameter, short length, and small nodes, and the dense interwoven structure constructs a high-precision filtration barrier, which can effectively intercept small-particle pollutants in the filtered liquid. Compared with the traditional symmetrical membrane structure, the present invention optimizes the membrane pore gradient distribution and material ratio, thereby achieving a significant reduction in fluid resistance while maintaining high filtration accuracy, thereby significantly improving filtration efficiency, and has outstanding technological advancement and engineering practical value.

[0156] 6. Strength test

[0157] The mechanical strength of filter membranes is primarily assessed through a comprehensive evaluation of two key indicators: tensile strength and elongation at break. Tensile strength reflects a material's ability to withstand parallel tensile forces. The test involves applying a progressive tensile load to the membrane sample until it breaks. This parameter is calculated by recording the maximum load at break and combining it with the sample's initial cross-sectional area. Elongation at break reflects the material's ability to deform during stretching and is determined by measuring the ratio of the change in length at break to the original length.

[0158] 7. Retention efficiency test

[0159] The retention efficiency of the asymmetric porous membrane was determined using polystyrene microspheres with a particle size of 30 nm. Test conditions: temperature 21.5°C, relative humidity: 67% RH

[0160] Test steps:

[0161] S1. Connect the empty filter membrane to the test pipe and rinse with ultrapure water for at least 30 minutes.

[0162] S2. Turn on the online liquid optical particle size analyzer and adjust the sampling flow rate to start testing the system background particle count;

[0163] S3. Soak the filter membrane to be tested in a surfactant for at least 30 minutes, remove the filter membrane, install it in the filter membrane mounting tooling, and rinse it with ultrapure water for at least 30 minutes (the air upstream of the filter membrane must be exhausted before rinsing);

[0164] S4. Turn on the online liquid optical particle size analyzer and adjust the sampling flow rate to test the system background after the filter membrane is installed and flushed;

[0165] S5. Start the filtration test, adjust the system flow, inject polystyrene particles into the system upstream of the test end, and use the online liquid optical particle size test to filter the upstream particle solubility;

[0166] S6. Maintain the filtration test and switch the online particle counter to the downstream of the filter membrane to sample and test the particle concentration after filtration. Sampling is performed until the particles after filtration are relatively stable. Sampling is performed once every minute and completed within 30 minutes. The particle concentration of the filtered liquid is based on the average value after stabilization.

[0167] S7. Maintain the filtration test and switch the online particle counter to the upstream of the filter membrane to verify the stability of the pre-filtration liquid. The stable mean value of the pre-filtration liquid is used as the particle concentration value of the pre-filtration liquid.

[0168] S8. End the test, stop the injection of particles and the sampling of the online liquid optical particle size analyzer, remove the filter membrane, and rinse the system with ultrapure water.

[0169] 2. PTFE filter membrane material performance test results

[0170] 1. PTFE filter membrane microstructure:

[0171] As shown in Table 1, the fiber diameter, fiber length, and irregularly shaped bulky nodes on the outer surface of the first porous fibrous assembly (i.e., the loose layer) are all larger than those on the outer surface of the second porous fibrous assembly (i.e., the dense layer). The fiber diameter, fiber length, and node size on the outer surface of the second porous fibrous assembly are all in the nanometer range.

[0172] Table 1 Fiber node, diameter and length measurement data

[0173]

[0174] 2. PTFE filter membrane bubble point and pore size:

[0175] As shown in Table 2, the bubble point and pore size analysis of the prepared high-precision gradient pore size asymmetric filtration membrane was performed using a Porolux 500 pore size analyzer. When testing the bubble point of the outer surface of the first porous fiber assembly, this surface was aligned with the pressurized air outlet surface, and the other surface was the contact surface with the impregnation liquid; when testing the bubble point of the outer surface of the second porous fiber assembly, this surface was aligned with the pressurized air outlet surface, and the other surface was the contact surface with the impregnation liquid. The bubble point of the outer surface of the first porous fiber assembly was smaller than that of the outer surface of the second porous fiber assembly, indicating that the pore size of the outer surface of the first porous fiber assembly was larger than that of the outer surface of the second porous fiber assembly.

[0176] Table 2 Bubble point and pore size test data

[0177]

[0178] 3. Mechanical properties of PTFE filter membrane

[0179] The longitudinal tensile strengths of Examples 2, 5, and 6, which underwent secondary heat setting without setting a stretching rate, were all greater than those of Examples 1, 3, and 4, which did not undergo secondary heat setting. This demonstrates the advantages of using two longitudinal stretches and heat setting without setting a stretching ratio: the first longitudinal stretch can initially induce the molecular chains to orient longitudinally, forming a basic fibrous structural framework; the second longitudinal stretch further strengthens the orderly arrangement of the molecular chains and refines the microstructure, effectively avoiding the stress concentration caused by a single high-rate stretch, and reducing the risk of material fracture or internal defects. Heat setting again without setting a stretching ratio can eliminate the internal residual stress accumulation caused by forced stretching of the molecular chains during the longitudinal stretching process, promote crystallization, reduce lattice defects, and improve longitudinal tensile strength.

[0180] The asymmetric PTFE porous membrane described in this invention has a tensile strength of 36-42 MPa and an elongation at break of 240%-337%. These two indicators demonstrate that the material combines high strength with good ductility, allowing it to withstand tensile loads in industrial environments while maintaining structural integrity within a certain deformation range. This combination of properties endows the membrane with excellent mechanical stability, significantly improving its reliability in practical applications. It can fully meet the stringent durability requirements of filter materials in fields such as semiconductors, and has outstanding industrial application value and market potential.

[0181] Table 3 Tensile strength and elongation at break test table

[0182] Sample Tensile strength (Mpa) Elongation at break (%) Example 1 25.92 336.56 Example 2 28.33 313.96 Example 3 34.39 322.78 Example 4 35.58 320.16 Example 5 40.26 278.54 Example 6 42.78 249.92

[0183] 4. PTFE filter membrane flux test

[0184] The test area was 15.2 cm using 200 mL of IPA (isopropyl alcohol) at -40, -60, and -80 kPa, respectively. 2 The liquid flux was calculated by measuring the time it took to hold the membrane. The liquid flux test results for Examples 1-6 are shown in Table 4. As can be seen from Table 4, the liquid flux varies when the asymmetric porous filtration membrane has different liquid inlet surfaces, demonstrating that the filtration membrane prepared by the present invention possesses both high-precision interception capabilities and significant liquid diversion capabilities. The flux with the loose layer as the liquid inlet surface is greater than the flux with the dense layer as the liquid inlet surface.

[0185] Table 4 Liquid flux test table

[0186]

[0187]

[0188] 5. PTFE filter membrane retention efficiency test

[0189] Retention efficiency tests were conducted using polystyrene particles with a particle size of 30 nm, with the first porous fibrous assembly acting as the liquid inlet surface and the second porous fibrous assembly acting as the liquid outlet surface. As shown in Table 5, when the outer surface of the first porous fibrous assembly served as the liquid inlet surface and the outer surface of the second porous fibrous assembly served as the liquid outlet surface, the asymmetric porous filtration membrane achieved a retention efficiency of ≥84% for polystyrene particles with a particle size of 30 nm, reaching a maximum of 93.95%. If the asymmetric filtration membrane prepared by the present invention is combined with other materials to form a filter element, filtration accuracy and retention efficiency can be further improved.

[0190] Table 5 Asymmetric porous filtration membrane retention efficiency test results

[0191]

[0192] In summary

[0193] When the node diameter of the membrane's internal structure is smaller, the average fiber diameter is thinner, and the fiber length is shorter, the spatial arrangement within the membrane becomes more compact and orderly. This structural feature significantly increases the apparent structural density of the interwoven fibers. This dense surface structure effectively enhances the physical interception of target substances, thereby correspondingly improving the retention rate. Filter cartridges made using the gradient pore size asymmetric membrane prepared by this invention can achieve a filtration accuracy of up to 3 nm.

[0194] The asymmetric porous membrane prepared by this invention, with its differentiated bubble point parameters and flux characteristics, significantly improves liquid flux in practical applications through its unique pore size distribution and hierarchical structure, while maintaining a stable filtration precision and avoiding the problem of decreased separation efficiency caused by increased flux. This performance advantage stems from the synergistic effect of the efficient mass transfer channels formed by the gradient pore size design within the membrane and the precise screening structure, achieving an optimal balance between fluid transfer rate and retention capacity.

[0195] The asymmetric porous membrane prepared by the present invention has a longitudinal tensile strength of ≥40 MPa. The advantage of using two longitudinal stretchings is that the first longitudinal stretching can preliminarily induce the molecular chains to orient longitudinally to form a basic fibrous structural framework; the second longitudinal stretching further strengthens the orderly arrangement of the molecular chains and refines the microstructure, effectively avoiding the stress concentration problem caused by a single high-ratio stretching, and reducing the risk of material fracture or internal defects.

[0196] The asymmetric PTFE filter membrane prepared by the present invention, due to its unique loose layer and dense layer, uses the loose layer as the liquid inlet surface, and passes through the loose layer and dense layer in sequence to complete the filtration separation process. Compared with the dense layer as the liquid inlet surface, the loose structure can make the liquid pass quickly at low pressure, and the mesh structure inside the loose layer can also achieve efficient interception of large particles. When the liquid containing only fine particles enters the dense layer, it can be intercepted again. The dense layer only needs to intercept the fine particles and can keep the effective flux from decaying. If the dense layer is used as the liquid inlet surface, although all the interception of fine particles can be achieved quickly, it is easy to cause the membrane surface to be blocked. The pore size of the dense layer is small, and high flux can be maintained under relatively high pressure. Even if the loose layer can quickly release pressure inside the dense layer, the work of intercepting particles depends only on the dense layer, and the service life is limited. Therefore, this high-precision gradient pore asymmetric PTFE filter membrane with a filtration accuracy of 3nm made of a loose layer as the liquid inlet surface can be used specifically for filtering wet etching liquid and cleaning liquid in semiconductor processes, solving the problem of insufficient accuracy in the existing technology.

[0197] The series of detailed descriptions listed above are only specific descriptions of feasible implementation methods of the present invention. They are not intended to limit the scope of protection of the present invention. Any equivalent implementation methods or changes that do not deviate from the technical spirit of the present invention should be included in the scope of protection of the present invention.

[0198] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A high-precision gradient pore asymmetric PTFE filter membrane, characterized in that: It includes a loose layer and a dense layer, the loose layer is the outer surface of the first porous fiber assembly, and the dense layer is the outer surface of the second porous fiber assembly; the outer surface of the first porous fiber assembly includes a filamentous network structure formed by long fibers connected by approximately elliptical nodes interwoven with each other, and the structure is loose, and the outer surface of the second porous fiber assembly includes a fiber-node interwoven structure formed by short fibers connected by circular nodes interwoven with each other, and the structure is dense and regular; the fiber diameter, fiber length and node size of the outer surface of the first porous fiber assembly are all larger than the fiber diameter, fiber length and node size of the outside of the second porous fiber assembly.

2. The high-precision gradient pore asymmetric PTFE filter membrane according to claim 1, characterized in that: The outer surface of the first porous fiber assembly includes irregularly shaped block-shaped enlarged nodes as fiber connection hubs, and supports the long fibers around them. The long fibers extend from the block-shaped enlarged nodes to the surrounding areas, interweave and intertwine with each other to form a loose three-dimensional network structure.

3. The high-precision gradient pore asymmetric PTFE filter membrane according to claim 1, characterized in that: The average fiber diameter on the outer surface of the first porous fiber assembly is 0.8-1.5μm, the average fiber length is 2-19μm, and the average node diameter is 1.3-4μm. The average fiber diameter on the outer surface of the second porous fiber assembly is 30-150nm, the average fiber length is 300nm-3μm, and the average node diameter is 330-620nm.

4. The high-precision gradient pore asymmetric PTFE filter membrane according to claim 1, characterized in that: The IPA bubble point value of the outer surface of the first porous fiber assembly is ≥1.3 bar, and the IPA bubble point value of the outer surface of the second porous fiber assembly is ≥4.3 bar.

5. The high-precision gradient pore asymmetric PTFE filter membrane according to claim 1, characterized in that: The longitudinal tensile strength of the asymmetric PTFE filter membrane is greater than or equal to 40 MPa, and the elongation at break of the asymmetric PTFE filter membrane is 229-336%.

6. The high-precision gradient pore asymmetric PTFE filter membrane according to claim 2, characterized in that: The asymmetric PTFE filter membrane has a retention rate of ≥84% for 30nm particles and a liquid flux of ≥5.5mL / min·cm at a pressure of -80kPa. 2 .

7. A method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1 mixing: adding the auxiliary oil into the polytetrafluoroethylene dispersion resin in a certain proportion; S2 aging: Place the mixture of polytetrafluoroethylene dispersion resin and auxiliary oil in an oven for a certain period of time to allow the auxiliary oil to fully infiltrate the resin; S3 pre-pressing: Add the matured mixture into the cylinder and maintain the pressure for a certain period of time to squeeze out the air between the resins and form a compact columnar embryo. S4 Extrusion: The pre-pressed cylindrical embryo is placed into the extruder to obtain a strip-shaped sheet; S5 calendering: calendering the strip sheet to obtain a base strip with uniform thickness; S6 degreasing: At a certain temperature, the calendered base tape is placed in a degreasing machine to remove the additive oil; S7 longitudinal stretching: The degreased base tape is stretched multiple times in the longitudinal direction at a certain stretching rate; S8 Horizontal stretching: The longitudinally stretched base tape is stretched horizontally at a certain stretching rate to obtain an asymmetric PTFE filter membrane.

8. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 7, characterized in that: In the S7, the degreased base tape is stretched longitudinally at least twice: the degreased base tape is first stretched at a low temperature and a low speed to obtain a first stretched sheet, and then the first stretched sheet is quickly stretched at a high temperature and a high speed for a second time to obtain a second stretched sheet, and then the second stretched sheet is quickly heat-set.

9. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 7, characterized in that: The molecular weight of the polytetrafluoroethylene dispersion resin in S1 is 4 to 9 million, the auxiliary oil is isoparaffin, and the ratio of the auxiliary oil to the polytetrafluoroethylene dispersion resin is 1:80 to 5:

8.

10. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 7, characterized in that: The mixture of the auxiliary oil and the polytetrafluoroethylene dispersion resin in S2 is placed in an oven at 45° C. and aged for more than 12 hours to allow the polytetrafluoroethylene dispersion resin to fully swell.

11. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 7, characterized in that: The matured mixture in S3 is compressed into a compact columnar embryo at a compression ratio of 25-400 under a pressure of 40 bar.

12. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 8, characterized in that: In the above S4, the columnar embryo is placed in an extruder with a T-shaped extrusion die to extrude a first strip-shaped sheet and a second strip-shaped sheet with the same width and a thickness ratio of 1.1 to 2.9 times.

13. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 12, characterized in that: In S5, the first strip-shaped sheet and the second strip-shaped sheet are stacked perpendicularly to the speed direction from thin to thick along the thickness direction, and enter the calender roller parallel to the speed direction to be pressed to a thickness within the range of 0.06 to 2 mm to form a base strip with uniform thickness.

14. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 13, characterized in that: In the above-mentioned S6, the base tape is degreased at a temperature of 190-240° C. to remove the auxiliary oil.

15. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 7, characterized in that: In the step S7 , the base tape is stretched several times along the speed direction (ie, longitudinally stretched) at a stretching rate of 30% to 2500% / s at a temperature of 200 to 350° C. to obtain a unidirectional longitudinally drawn sheet.

16. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 15, characterized in that: In the above S8, the longitudinally drawn sheet is stretched at a temperature of 200 to 360° C. at a stretching rate of 9 to 6000% / s in a direction perpendicular to the speed (ie, transverse stretching) to obtain an asymmetric PTFE filter membrane.

17. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 8, characterized in that: The first stretching temperature is 220-290° C., the stretching rate is 50-500% / s, the second stretching temperature is 260-350° C., the stretching rate is 800-1500% / s.

18. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 17, characterized in that: The heat setting temperature is 350°C.

19. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 16, characterized in that: The transverse stretching in S8 has a stretching ratio of 5-45 times; the transverse stretching includes a preheating section, a wide width section and a heat setting section, the preheating section temperature is 220-250°C, the wide width section temperature is 270-290°C, and the setting section temperature is 330-350°C.

20. The method for preparing a high-precision gradient pore asymmetric PTFE filter membrane according to claim 19, characterized in that: The stretching rate of the preheating section of the transverse stretching is 50-150% / s, and the stretching rate of the wide section is 1500-3000% / s.

21. Use of the high-precision gradient pore asymmetric PTFE filter membrane according to any one of claims 1 to 6, characterized in that: The asymmetric PTFE filter membrane can be made into a filter element with a filtration accuracy of 3nm, which is used for filtering wet etching liquid and cleaning liquid in semiconductor manufacturing processes.

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