A polytetrafluoroethylene micro-nano multi-stage pore membrane material and a preparation method thereof

Abstract

The application provides a polytetrafluoroethylene micro-nano multi-stage pore membrane material and a preparation method thereof, and relates to the technical field of polymer materials. The preparation method comprises the following steps: firstly, mixing a polytetrafluoroethylene base body with a soluble template material, adding an auxiliary oil, and aging; then, pre-pressing, extruding, calendering, degreasing, and bidirectional asynchronous stretching to obtain a primary film; secondly, placing the primary film in a three-dimensional micron mold, hot-pressing, cooling, and demolding to obtain a shaped body; and thirdly, contacting the shaped body with an etching solution to obtain the material. According to the application, the cross-linked skeleton is prepared through bidirectional stretching, the hot-pressing is performed by using a mold with a three-dimensional micron structure, the base body forms the corresponding morphology characteristics in the molding process, the template is removed through the etching solution, the micro-pores in the base body are constructed according to the size and morphology of the template, and the multi-stage pore structure with the micron structure characteristics given by the mold and the nano and / or sub-micron pore characteristics determined by the template is simultaneously obtained.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a polytetrafluoroethylene micro / nano hierarchical porous membrane material and its preparation method. Background Technology

[0002] Polytetrafluoroethylene (PTFE) is widely used in high-efficiency filtration, waterproofing and soundproofing, medical devices, aerospace, and other fields due to its excellent chemical stability, resistance to high and low temperatures, low surface energy, and good biocompatibility. PTFE materials with micro- and nano-porous structures have attracted particular attention. However, PTFE is difficult to dissolve and melt, resulting in extremely poor processing performance, making it difficult to prepare high-performance microporous materials using conventional polymer processing techniques. ePTFE membranes further extend the excellent comprehensive properties of PTFE, maintaining its inherent wide operating temperature range, higher mechanical strength, and also possessing unique properties such as porosity, air permeability, hydrophobicity, and flexibility. It is widely used in numerous industries including automotive, consumer electronics, new energy, security, aerospace, cables, packaging, medical, apparel, and chemicals. For a long time, the stretching method (ePTFE technology) developed by Gore & Co. has been the method for large-scale commercialization of PTFE microporous products. This method prepares porous PTFE materials through steps such as mixing PTFE resin with lubricant, pressing, extrusion, degreasing, and stretching. Its microstructure consists of fibrils and nodes, and the pore size is roughly uniform inside the membrane and on both surfaces, forming a symmetrical pore structure.

[0003] Although the stretching method for preparing PTFE membranes has been successfully industrialized, key technological barriers such as controlling the "bowing effect" and precisely regulating the microporous structure in the biaxial stretching process still need further breakthroughs, and the following problems remain: There is a dilemma between filtration efficiency and permeation flux: high filtration precision requires small pore size, but small pore size will significantly reduce permeation flux; increasing pore size will reduce filtration efficiency.

[0004] It is difficult to achieve simultaneous improvement in high porosity and ultra-fine pore size: In conventional stretching processes, high porosity is often accompanied by a significant increase in pore size.

[0005] Lack of multi-level, programmable construction capability for PTFE micro / nano structures: unable to form three-dimensional multi-level nested channels from nanoscale to microscale.

[0006] In view of this, the present invention proposes a novel polytetrafluoroethylene micro-nano hierarchical porous membrane material and its preparation method. Summary of the Invention

[0007] One of the objectives of this invention is to provide a method for preparing polytetrafluoroethylene micro-nano hierarchical porous membrane materials, so as to at least solve one of the technical problems existing in the prior art.

[0008] The second objective of this invention is to provide a polytetrafluoroethylene micro-nano hierarchical porous membrane material.

[0009] In order to achieve the above-mentioned objectives of the present invention, the following technical solution is adopted: In a first aspect, the present invention provides a method for preparing a polytetrafluoroethylene micro / nano hierarchical porous membrane material, comprising the following steps: (a) A polytetrafluoroethylene (PTFE) emulsion is mixed with a soluble template material, an auxiliary oil is added and mixed evenly and cured, and then subjected to pre-pressing, extrusion, calendering, and degreasing treatment in sequence, followed by bidirectional asynchronous stretching to obtain a PTFE primary film with uniformly dispersed template particles inside; the soluble template material can be completely dissolved and removed by an alkaline etching solution; the PTFE primary film containing the template material is placed in a mold with a three-dimensional micron structure and hot-pressed, and then demolded after cooling to obtain a molded body with mold structure imprints; (b) The molded body is immersed in an etching solution to remove the template material and obtain the polytetrafluoroethylene micro-nano hierarchical porous membrane material.

[0010] This invention employs a three-pronged approach: biaxial stretching to create a pre-crosslinked framework, hot pressing with a mold to replicate micron-sized primary pores, and alkali-dissolving templates to construct nano-sized secondary pores. Biaxial stretching forms a rich and dense PTFE primary network, providing a foundation for high mechanical strength in the membrane. Hot pressing with a mold allows the PTFE melt to completely replicate the mold's three-dimensional micron-sized uneven structure, forming a micron-sized fluid transport main channel that penetrates the membrane layer. Alkaline etching removes template particles dispersed within the matrix, uniformly generating numerous nano / submicron secondary pores on the walls of the micron-sized channels. These two levels of pores are interconnected, forming a three-dimensional nested multi-level pore network.

[0011] Furthermore, the mass ratio of polytetrafluoroethylene body in the primary membrane to the template material is 1:0.2-1:3.

[0012] Furthermore, the template material includes one or both of silica microspheres and diatomaceous earth nanoparticles; The template material has a particle size of 20 nm-50 μm.

[0013] Furthermore, the polytetrafluoroethylene body in the primary membrane comprises a polytetrafluoroethylene dispersion resin; The molecular weight of the polytetrafluoroethylene dispersion resin is 2 million to 10 million.

[0014] Furthermore, the hot pressing temperature is 340-380℃, the pressure is 0.1-0.4 MPa, and the holding time is 10-120 minutes; The hot pressing process employs isostatic pressing or uniaxial pressure.

[0015] Furthermore, the mold is made of one or more materials selected from stainless steel, nickel-based alloys, and zirconium oxide ceramics; The three-dimensional micron structure on the surface of the mold is either a positive texture or a negative texture. The positive texture structure is selected from one or more of column array, prism array and convex dot array; The negative texture structure is selected from one or more of the following: grid, groove, or honeycomb structure; The protrusions or depressions of the three-dimensional microstructure have a depth of 10μm-500μm.

[0016] Furthermore, the etching solution includes an alkaline solution; The alkaline solution includes an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide; The concentration of the etching solution is 1-10 mol / L.

[0017] Furthermore, in step (b), the soaking temperature is 60-95°C and the soaking time is 2-48 hours.

[0018] Furthermore, the preparation method further includes: adjusting the surface wettability of the material obtained in step (b); the surface wettability adjustment includes hydrophobic modification or hydrophilic modification; The hydrophobic modification involves mixing the obtained material with a fluorinated silane compound and treating it at 40-100°C for 1-12 hours; the fluorinated silane compound includes one or both of perfluorodecyltrichlorosilane and perfluorooctyltriethoxysilane. The hydrophilic modification method includes plasma treatment; the plasma treatment voltage is 100-220V, the current is 0.5-5A, and the power is 50-1100 W; the plasma treatment time is 30-300 s; the protective gas used in the plasma treatment includes one or both nitrogen and argon, and the gas flow rate is 10-50 cm³. 3 / min.

[0019] Secondly, the present invention provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which is prepared by the aforementioned preparation method.

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention utilizes three collaborative processes: biaxial stretching to prefabricate a PTFE fibril crosslinked framework, hot pressing with a mold to replicate micron-sized through-holes, and alkali-soluble templates to construct nano-sized secondary pores. This process simultaneously builds a micro-nano three-dimensional nested multi-level pore network, overcoming the limitations of traditional stretched ePTFE single-scale pores. It also solves the industry challenges of the mutual constraints between filtration throughput and sieving accuracy, and the inability to simultaneously achieve high porosity and ultra-fine pore size. The micron-sized through-holes ensure high-throughput fluid transport, while the embedded nano-sized secondary pores in the pore walls provide high specific surface area and precise sieving capabilities.

[0021] 2. By changing the micron-level topology of the mold surface and adjusting the particle size and addition ratio of the template particles, the size, morphology and distribution of the micron-level primary pores and nano-level secondary pores can be independently and separately controlled, adapting to different application scenarios such as precision filtration, waterproof and breathable, and medical separation.

[0022] 3. The preparation process relies solely on hot pressing and melting for shaping, and selective etching with alkaline solution. There is no chemical cross-linking or degradation of the PTFE matrix, thus fully preserving the inherent properties of PTFE, such as acid and alkali resistance, high and low temperature resistance, low surface energy, and biocompatibility. Furthermore, the alkaline etching waste liquid only needs to be neutralized, making the process safe, environmentally friendly, and suitable for industrial mass production.

[0023] 4. The multi-level porous structure formed by biaxial stretching can uniformly disperse external forces. Combined with the stress buffering effect of the multi-level porous structure, the mechanical properties of the resulting membrane material are far superior to those of conventional single template method, single mold hot pressing method and traditional stretched ePTFE membrane. Attached Figure Description

[0024] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0025] Figure 1 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 1; Figure 2 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 2; Figure 3 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 3; Figure 4 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 4; Figure 5 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 5; Figure 6 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 6; Figure 7 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 7; Figure 8 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 8; Figure 9SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 9; Figure 10 SEM image of the polytetrafluoroethylene micro / nano hierarchical porous membrane material prepared in Example 10; Figure 11 Here is a SEM image of the polytetrafluoroethylene membrane material prepared in Comparative Example 1; Figure 12 Here is a SEM image of the polytetrafluoroethylene membrane material prepared in Comparative Example 2; Figure 13 SEM image of the polytetrafluoroethylene membrane material prepared in Comparative Example 3; Figure 14 This is a schematic flowchart illustrating the method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane materials provided by the present invention. Detailed Implementation

[0026] Unless otherwise defined herein, the scientific and technical terms used in conjunction with this invention shall have the meanings commonly understood by one of ordinary skill in the art. The meaning and scope of terms shall be clear; however, in any case of potential ambiguity, the definitions provided herein shall prevail over any dictionary or foreign definitions. In this application, unless otherwise stated, the use of "or" means "and / or". Furthermore, the use of the term "comprising" and other forms is non-limiting.

[0027] The nomenclature used in analytical chemistry, synthetic organic chemistry, and medical and medicinal chemistry, along with their laboratory procedures and techniques, as described herein, are those well-known and commonly used in the field.

[0028] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] The first aspect of this invention provides a method for preparing a polytetrafluoroethylene micro / nano hierarchical porous membrane material, comprising the following steps: (a) A polytetrafluoroethylene (PTFE) emulsion is mixed with a soluble template material, an auxiliary oil is added and mixed evenly and cured, and then subjected to pre-pressing, extrusion, calendering and degreasing treatment in sequence, followed by bidirectional asynchronous stretching to obtain a PTFE primary film with uniformly dispersed template particles inside; the soluble template material can be dissolved and removed by an alkaline etching solution; the PTFE primary film containing the template material is placed in a mold with a three-dimensional micron-shaped concave-convex structure, and hot-pressed, and then demolded after cooling to obtain a molded body with mold structure imprints; (b) The molded body is immersed in an etching solution to remove the template material and obtain clean, complete nano and / or submicron level pores, ultimately obtaining a three-dimensional interconnected nested polytetrafluoroethylene micro-nano hierarchical porous membrane material that combines the micron-through main pores formed by the mold and the nano-secondary pores etched by the template.

[0030] The method for preparing polytetrafluoroethylene (PTFE) micro-nano hierarchical porous membrane materials provided by this invention involves biaxially stretching a pre-crosslinked framework and hot-pressing it using a mold with a three-dimensional micron structure. This allows the PTFE body to form morphological features corresponding to the mold structure during the molding process. Then, the template material is removed using an etching solution, constructing micropores within the PTFE body that are determined by the size and morphology of the template. Therefore, the method for preparing PTFE micro-nano hierarchical porous membrane materials provided by this invention simultaneously obtains a hierarchical porous structure that combines the micron-level structural features imparted by the mold with the nano- and / or submicron-level pore features determined by the template.

[0031] Specifically, this invention utilizes template-assisted hot pressing to construct multi-level pores in PTFE, overcoming the limitations of traditional stretching methods with their single pore size. It can precisely replicate the mold's microstructure (micrometer level) and superimpose template-etched pores (nanometer and / or submicrometer level) to form a three-dimensional, interconnected multi-level pore network. Simultaneously, the preparation method provided by this invention maintains the excellent properties of PTFE. During the hot pressing process, PTFE only undergoes melting and resolidification, without introducing chemical cross-linking or degradation. The resulting material retains PTFE's inherent corrosion resistance, high and low temperature resistance, and low surface energy characteristics.

[0032] Furthermore, this invention can also realize programmable channel design. By changing the mold topology and adjusting the particle size of the template, the size, shape and distribution of micron-level macro channels and nano-level micro channels can be independently controlled.

[0033] In some preferred embodiments, the mass ratio of polytetrafluoroethylene vinyl body in the primary membrane to the template material is 1:0.2-1:3, for example, it can be 1:0.2, 1:0.5, 1:1, 1:1.5, 1:2, 1:2.5, 1:3, etc.

[0034] In some preferred embodiments, the template material is a soluble template material.

[0035] In some preferred embodiments, the template material includes one or both of silica microspheres and diatomaceous earth nanoparticles.

[0036] In some preferred embodiments, the particle size of the template material is 20 nm-50 μm, for example, it can be 20 nm, 50 nm, 100 nm, 500 nm, 1 μm, 5 μm, 10 μm, 20 μm, 30 μm, 50 μm, etc.

[0037] In some preferred embodiments, the polytetrafluoroethylene body comprises a polytetrafluoroethylene dispersion resin.

[0038] In some preferred embodiments, the polytetrafluoroethylene dispersion resin has a molecular weight of 200-1000 million.

[0039] In some preferred embodiments, the hot pressing temperature is 340-380℃, for example, 340℃, 350℃, 360℃, 370℃, 380℃, etc.; the pressure is 0.1-0.4 MPa, for example, 0.1MPa, 0.2MPa, 0.3MPa, 0.4MPa, etc., more preferably 0.15-0.25MPa; the holding time is 10-120 minutes, for example, 10 minutes, 50 minutes, 60 minutes, 80 minutes, 100 minutes, 120 minutes, etc., to allow PTFE to fully creep and flow.

[0040] In some preferred embodiments, the hot pressing process employs isostatic pressing or uniaxial pressure. Isostatic pressing, in particular, ensures uniform pressure distribution.

[0041] In some preferred embodiments, the mold material is selected from one or more of stainless steel, nickel-based alloys, and zirconia ceramics. The three-dimensional micron structure on the mold surface is a positive or negative texture structure. The positive texture structure is selected from one or more of column arrays, prism arrays, and bump arrays. The negative texture structure is selected from one or more of mesh, groove, or honeycomb structures. The mold is a mold with a circular plate (10 cm in diameter) on which positive and negative textures are provided, and the depth of the protrusions or depressions of the positive and negative textures is 10 μm-500 μm.

[0042] In some preferred embodiments, the etching solution comprises an alkaline solution.

[0043] This invention utilizes alkaline etching, significantly improving process safety and environmental friendliness. High-temperature concentrated alkaline solutions can efficiently etch templates such as silica and diatomaceous earth without damaging the PTFE substrate. Waste liquid treatment is simple (neutralization is sufficient), making it suitable for industrial production.

[0044] In some preferred embodiments, the alkaline solution comprises an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide.

[0045] In some preferred embodiments, the concentration of the etching solution is 1-10 mol / L, for example, it can be 1 mol / L, 2 mol / L, 3 mol / L, 4 mol / L, 5 mol / L, 6 mol / L, 7 mol / L, 8 mol / L, 9 mol / L, 10 mol / L, etc.

[0046] In some preferred embodiments, in step (b), the etching immersion temperature is 60-95°C, for example, it can be 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, etc., and the etching time is 2-48 hours, for example, it can be 2 hours, 5 hours, 12 hours, 18 hours, 24 hours, 30 hours, 36 hours, etc.

[0047] In some preferred embodiments, the preparation method further includes: adjusting the surface wettability of the material obtained in step (b); the surface wettability adjustment includes hydrophobic modification or hydrophilic modification.

[0048] In some preferred embodiments, the hydrophobic modification involves mixing the resulting material with a fluorinated silane compound and treating it at 40-100°C (e.g., 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, etc.) for 1-12 hours, such as 1 hour, 3 hours, 5 hours, 8 hours, 10 hours, 12 hours, etc.; the fluorinated silane compound includes one or both of perfluorodecyltrichlorosilane and perfluorooctyltriethoxysilane.

[0049] In some preferred embodiments, the hydrophilic modification includes plasma treatment; the plasma treatment voltage is 100-220V, the current is 0.5-5A, the power is 50-1100 W, the plasma treatment time is 30-300 s, and the protective gas used for the plasma treatment includes inert gases such as nitrogen or argon, with a gas flow rate of 10-50 cm³. 3 / min.

[0050] like Figure 14 As shown, in the optional embodiments of the present invention, a more preferred method for preparing the polytetrafluoroethylene micro / nano hierarchical porous membrane material includes the following steps: Step 1 (Pretreatment): Preparation of PTFE Composite Preform Daikin's F104 dispersion resin was added to a mixing device and uniformly mixed with a certain proportion of soluble template material. ExxonMobil's Isopar G additive oil was then sprayed and mixed evenly. After curing, the mixture was loaded into a φ150mm pre-compression molding machine to obtain a cylindrical rod blank, resulting in a PTFE composite blank with uniformly dispersed template particles. The main purpose of this step is to introduce nano- and / or submicron-level templates to form micropores during subsequent etching.

[0051] Step 2: Extrusion-calendering-degreasing-biaxial co-stretching to form a film The composite preform is extruded using horizontal or vertical extrusion equipment to obtain rod-shaped or cylindrical PTFE preforms.

[0052] Calendering: The base material strip is produced by rolling a rod-shaped or cylindrical PTFE billet into a base material strip using a twin-roll calender at a certain calendering speed.

[0053] Degreasing treatment: The prepared base material strip is sent into a high-temperature chamber for heating treatment to remove the auxiliary oil in the mold and obtain a degreased base strip.

[0054] The degreased substrate was then subjected to bidirectional asynchronous stretching to obtain a PTFE primary membrane with a porous structure. The bidirectional asynchronous stretching process in this step, by stretching the degreased substrate longitudinally and laterally, promotes the directional growth and cross-linking of PTFE fibrils along the stress direction, forming a dense network skeleton structure. This significantly improves the elastic modulus, anti-delamination ability, and dimensional stability of the primary membrane.

[0055] Step 3: Template-assisted hot pressing molding The PTFE primary film obtained in step 2 is placed in a mold with a three-dimensional micron structure. Isostatic pressure or uniaxial pressure is applied at a temperature higher than the melting point of PTFE to melt the PTFE and completely fill the micro-nano structure cavity of the mold. After cooling, the mold is demolded to obtain a PTFE molded body with mold structure imprints on the surface or inside.

[0056] The cooling methods include pressure holding and cooling down to room temperature, or quenching and rapid cooling to fix the structure.

[0057] Step 4: Selective removal of template by alkaline etching The hot-pressed PTFE and template composite is immersed in an alkaline etching solution (alkali tank). Under specific temperature and time conditions, the soluble template is completely dissolved, leaving pores in the PTFE matrix corresponding to the shape and size of the template. After cleaning and drying, a three-dimensional multi-level porous PTFE material is obtained.

[0058] Optionally, ultrasonic oscillation or mechanical stirring can be used to accelerate the entry of the etching solution into the internal pores of the PTFE.

[0059] The etching process then proceeds with post-etching treatment: repeated rinsing with deionized water until neutral, followed by ethanol replacement and vacuum drying.

[0060] Step 5: Surface wettability control The obtained multi-porous PTFE material is immersed in a fluorosilane solution for hydrophobic modification, or a hydrophilic surface is obtained through plasma treatment.

[0061] The second aspect of the present invention provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which is prepared by the preparation method described in the first aspect.

[0062] The present invention will be further illustrated by the following examples. Unless otherwise specified, the materials in the examples are prepared according to existing methods or purchased directly from the market.

[0063] Example 1 This embodiment provides a polytetrafluoroethylene micro / nano hierarchical porous membrane material, the preparation process of which is as follows: Step 1: Preparation of PTFE composite material Daikin F104 polytetrafluoroethylene dispersion resin was added to a mixing device and uniformly mixed with silica microspheres (particle size 35μm). The mass ratio of polytetrafluoroethylene dispersion resin to silica microspheres was 1:1.5. ExxonMobil Isopar G additive oil was mixed evenly by spraying (the mass ratio of polytetrafluoroethylene dispersion resin to additive oil was 4:1). After curing at 50°C for 24 hours, the mixture was filled into a φ150mm pre-compression molding equipment with a pressure set at 3.5MPa and held for 10 minutes to obtain a cylindrical rod blank.

[0064] Step 2: Extrusion-calendering-degreasing-biaxial co-stretching to form a film The composite preform is extruded using a vertical extrusion device with an extrusion die size of φ15mm and an extrusion speed of 50mm / min to obtain a rod-shaped PTFE preform.

[0065] Calendering: The bar-shaped PTFE billet is rolled into a base strip using a twin-roll calender with a roll diameter of 600 mm. The calendering speed is 20 m / min and the thickness of the base strip is 60 μm.

[0066] Degreasing treatment: The prepared base material strip is sent into a high-temperature chamber for heating treatment to remove the auxiliary oil in the mold. The treatment temperature is 230℃ and the treatment time is 30min to obtain the degreased base strip.

[0067] The degreased substrate was then subjected to biaxial asynchronous stretching. First, longitudinal stretching was performed at 260℃ with a stretching ratio of 200%. The longitudinally stretched membrane sample was then subjected to transverse stretching at 200℃ with a stretching ratio of 300%, resulting in a PTFE primary membrane with a porous structure. The biaxial asynchronous stretching was performed using a high-temperature biaxial film stretching apparatus (capable of synchronous / asynchronous stretching).

[0068] Step 3: Template-assisted hot pressing molding The composite material obtained in step 2 is placed in a mold with a three-dimensional micron structure (the depth of the raised or recessed textures is 10 μm-500 μm). Isostatic pressure is applied at 360°C with a pressure of 0.2 MPa and a holding time of 65 minutes to melt the PTFE and completely fill the micro-nano structure cavity of the mold. After cooling to room temperature under holding pressure, the material is demolded to obtain a PTFE molded body with mold structure imprints.

[0069] Step 4: Selective removal of template by alkaline etching The hot-pressed PTFE and template composite were immersed in an alkaline bath (8 mol / L sodium hydroxide aqueous solution) for etching at 80°C for 24 hours, with mechanical stirring as a supplement. After cleaning and drying, a three-dimensional hierarchical porous PTFE material was obtained.

[0070] The etching process then proceeds with post-etching treatment: repeated rinsing with deionized water until neutral, followed by ethanol replacement and vacuum drying.

[0071] Step 5: Surface wettability control The obtained multi-porous PTFE material was immersed in a perfluorodecyltrichlorosilane solution (the concentration of the perfluorodecyltrichlorosilane solution prepared by dissolving perfluorodecyltrichlorosilane in n-hexane solvent was 1.5 wt%) for hydrophobic modification for 6 hours at a temperature of 70°C.

[0072] Example 2 This embodiment provides a polytetrafluoroethylene micro / nano-porous membrane material, which differs from Embodiment 1 in that: in step 5, a rough surface is obtained through plasma treatment to increase wettability. Specific process parameters are: the plasma treatment voltage is 220V, the current is 4A, the power is 800W, the plasma treatment time is 200s, and the protective gas used in the plasma treatment is nitrogen with a flow rate of 30 cm³. 3 / min.

[0073] Example 3 This embodiment provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which differs from Embodiment 1 in that: in step 1, the template material is diatomaceous earth (50 μm), and the mass ratio of polytetrafluoroethylene dispersion resin to diatomaceous earth is 1:0.2; In step 4, the hot-pressed PTFE and template composite are immersed in an alkaline bath (a 1 mol / L sodium hydroxide aqueous solution) for etching at a temperature of 60°C for 48 hours.

[0074] Example 4 This embodiment provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which differs from Embodiment 1 in that: in step 3, isostatic pressure is applied at 340°C.

[0075] Example 5 This embodiment provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which differs from Embodiment 1 in that: in step 3, isostatic pressure is applied at 380°C.

[0076] Example 6 This embodiment provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which differs from Embodiment 1 in that: in step 3, the pressure holding time is 10 minutes.

[0077] Example 7 This embodiment provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which differs from Embodiment 1 in that the pressure in step 3 is 0.1 MPa.

[0078] Example 8 This embodiment provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which differs from Embodiment 1 in that the pressure in step 3 is 0.3 MPa.

[0079] Example 9 This embodiment provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which differs from Embodiment 1 in that the pressure in step 3 is 0.4 MPa.

[0080] Example 10 This embodiment provides a polytetrafluoroethylene micro-nano hierarchical porous membrane material, which differs from Embodiment 1 in that the pressure in step 3 is 0.5 MPa.

[0081] Comparative Example 1 This comparative example provides a polytetrafluoroethylene membrane material, which differs from Example 1 in that: no template material is added and step 4 is omitted.

[0082] Comparative Example 2 This comparative example provides a polytetrafluoroethylene membrane material, which differs from Example 1 in that step 3 is omitted.

[0083] Comparative Example 3 This comparative example provides a polytetrafluoroethylene membrane material, which differs from Example 1 in that: a template-assisted hot pressing process is not used (i.e., no template material is added, and steps 3 and 4 are omitted), and a conventional process membrane sample is obtained.

[0084] The polytetrafluoroethylene micro-nano hierarchical porous membrane materials prepared in the examples and comparative examples were used as samples for testing.

[0085] Test method: The tensile breaking strength of the polytetrafluoroethylene micro-nano hierarchical porous membrane material of the present invention was tested using an electronic universal tensile testing machine (Instron 5965). The method was in accordance with GB / T 1040.3-2006. The ambient temperature was 20℃, the humidity was 45%RH, the sample width was 20mm, the tensile spacing was 150mm, and the tensile speed was 150mm / min.

[0086] The porous morphology and structural characteristics of the membrane were characterized using a cryo-scanning electron microscope (HITACHIE S8200). Before testing, the PTFE porous membrane was subjected to vacuum ion sputtering gold sputtering treatment.

[0087] The pore size of the PTFE porous membrane was measured using a capillary flow pore size analyzer (POROMETER POROLUX1000), and the average pore size was determined by measuring the wet-dry curve using the gas-liquid displacement principle.

[0088] The test results are shown in Table 1.

[0089] Table 1

[0090] As shown in Table 1, the tensile strength and average pore size of Examples 1-10 of this invention are superior to all comparative examples. In Example 10, although the tensile strength briefly increased to 25 MPa due to excessive densification of the PTFE melt caused by excessively high hot-pressing pressure (0.5 MPa), the micron-level mold imprint completely disappeared, the nano-level template etched channels were compacted and closed, the multi-level pore structure collapsed and failed, resulting in the loss of effective sieving ability. The tensile strength of Examples 1-9 is ≥23 MPa, and the average pore size of these examples is concentrated in the 47-85 nm range, while the average pore sizes of Comparative Examples 1-3 are as high as 196 nm, 153 nm, and 335 nm, respectively. Comparative Example 1 did not introduce template material and skipped the etching step, retaining only the primary film structure, resulting in the absence of nano-level pores and an irregular pore size distribution. Figure 11 The strength is relatively low; in Comparative Example 2, although the hot pressing process of the mold was eliminated and the nanopores formed by template etching were retained, the micron-level morphology was lost and the channel connectivity was poor. Figure 12 Insufficient mechanical support; Comparative Example 3, using conventional processes (no template, no mold, no etching), resulted in samples with thin and weak fiber diameters, loose structures, and poor strength. Figure 13 It is evident that both micron-level mold imprinting and nano-level template etching are indispensable; only through their synergy can a PTFE porous structure with both high strength and high regularity be constructed.

[0091] The key parameters of Examples 1-9 all fall within the preferred range of the present invention, and their mechanical and structural properties are generally stable and excellent; in particular, Example 1 achieves optimal matching, exhibiting the strongest structural conformity and duct integrity.

[0092] Depend on Figure 1As can be seen, Example 1 successfully constructed a polytetrafluoroethylene (PTFE) micro / nano-level porous membrane structure by combining biaxially stretched pre-fabricated porous primary membrane with mold hot pressing and template etching synergistic processes. The membrane as a whole forms a PTFE fibril-node cross-linked network framework. After being shaped by structured mold hot pressing, a regular micron-level interconnected channel structure is formed on the membrane surface. Within the pore walls of the micron-level channels and inside the cross-linked matrix between the pores, nano-level secondary pores formed by silica template etching are uniformly and densely distributed, ultimately forming a three-dimensional interconnected micro / nano-level nested structure of mold-derived micron-level primary channels and template-etched nano-level secondary channels. This structure realizes a hierarchical mass transfer pathway: micron-sized macropores ensure high-throughput fluid transfer, while nanopores embedded in the pore walls provide high specific surface area and precise sieving capabilities; at the same time, the fibrillary nodes between the membrane pores are densely cross-linked and the skeleton has excellent continuity, so external forces can be uniformly distributed and borne through the overall cross-linked network, effectively avoiding stress concentration defects. This enables Example 1 to achieve a high tensile strength of 41 MPa and a narrow, uniformly distributed pore size of 65 nm, achieving a synergistic match between high strength and high-precision pore size.

[0093] Figure 1 and Figure 2 Both exhibit highly consistent macroscopic hierarchical porous structures, achieving a hierarchical composite structure of micron-sized interconnected main channels and nanometer-sized secondary pores. The subtle differences in their microscopic morphology stem from the different surface modification methods employed. Figure 1 (Example 1) By using hydrophobic modification with fluorosilane, the PTFE melt fully creeps and rearranges under the optimal hot pressing process, resulting in tight cross-linking of stretched fibrils, dense and regular pore wall structure, no microscopic defects, optimal mechanical load-bearing capacity of the skeleton, and best stress dispersion effect, thus obtaining the highest tensile strength and the smallest average pore size. Figure 2 (Example 2) Plasma hydrophilic modification treatment was used. The basic multi-level porous structure of the membrane was not destroyed, but the surface fibrillary network underwent moderate roughening and reconstruction. The pore walls showed fine fiber interweaving, and the pore size was slightly widened to 85 nm. Thanks to the stable three-dimensional fiber node cross-linking network formed by hot pressing sintering, the integrity of the overall skeleton of the membrane was not damaged. It can still achieve multi-directional stress dispersion and maintain an excellent tensile strength of 36 MPa, taking into account both the hydrophilic modification requirements and good mechanical properties.

[0094] Figure 3 In Example 3, under the boundary conditions of the corresponding process parameters, the deviation of the template type and etching process parameters led to a decrease in structural integrity. Only shallow circular imprints formed by hot pressing of the mold could be observed on the film surface, without forming micron-sized channels that penetrated the film thickness. At the same time, due to the poor uniformity of template dispersion and incomplete etching at low temperature and low concentration, the number of nano-secondary pores in the micron-sized pore region was small and the connectivity was poor. The multi-level pore mass transfer pathways inside the film were cut off, making it impossible to achieve efficient mass transfer. The structural integrity and functionality were slightly lower than those of the optimal example.

[0095] Figure 4 , Figure 5Examples 4 and 5 correspond to the temperature process boundary, respectively. Figure 4 The number density of micron-sized imprinted pores on the film surface is reduced to a certain extent, resulting in a decrease in the effective open area. Figure 5 Due to the imbalance between temperature and pressure during hot pressing and the uneven distribution of residual stress after demolding and cooling, the regular circular micron-sized pores undergo tensile distortion, exhibiting anisotropic pore structures in the form of ellipsoids and elongated shapes. Pore orientation distortion and structural defects exacerbate stress concentration in the membrane, reducing the uniform connectivity of multi-level pores and making them more susceptible to microcrack propagation under stress. Ultimately, this leads to a gradual decrease in the mechanical strength of the membrane material and a widening of the pore size distribution.

[0096] Figure 6 , Figure 7 In Examples 6-7, which correspond to extreme process parameters of short holding pressure and low pressure, the ability to replicate the microstructure of the mold is significantly compromised. Only scattered and irregular mold imprints remain on the membrane surface, large areas of the membrane tend to be dense and flat, the micron-level main channels have basically disappeared, only a small number of template-etched nanopores remain inside the substrate, the hierarchical pore structure has basically degraded, the effective permeability porosity of the membrane has dropped to a critical value, the mass transfer performance has been greatly reduced, and the core function of the porous membrane has been basically lost.

[0097] Figures 8 to 10 The gradient morphological changes as the hot-pressing pressure increases visually demonstrate the upper limit of the pressure parameter's control over the hierarchical porous structure. As the hot-pressing pressure continues to increase, the PTFE melt undergoes forced viscous rheology, compressing and compacting the fibrillary network formed during the initial stretching. The micron-sized pore structure replicated by the mold is gradually crushed and flattened. Simultaneously, excessive pressure causes the nanotemplate particles to agglomerate and become deeply embedded within the matrix, preventing the formation of effectively interconnected nanopores after etching. A clear densification gradient is observed in the morphology. Figure 8 Only a very small number of isolated closed pores remain, and the hierarchical pore structure is significantly damaged; as the pressure continues to rise, Figure 9 , Figure 10 The membrane surface gradually becomes flat and dense, the original micro-nano hierarchical pore structure gradually disappears, and the porous characteristics are completely reduced to zero.

[0098] Although the dense sample in Example 10 had increased matrix density and higher short-term fracture force, it completely deviated from the original design intent of the micro-nano hierarchical porous membrane of this invention. It was classified as the worst control group along with Comparative Examples 1-3 (random fiber loose network or ultra-large coarse pore structure). Figures 8 to 10 The absolute safe upper limit of the hot-pressing pressure of this invention is marked by intuitive images—beyond this critical point, the microstructure collapses irreversibly, and even if some nanoscale etching traces are retained, the loss of macroscopic porosity cannot be recovered.

[0099] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a polytetrafluoroethylene micro / nano hierarchical porous membrane material, characterized in that, Includes the following steps: (a) The polytetrafluoroethylene body is mixed with a soluble template material, and an auxiliary oil is added to mix evenly and mature. After being pre-pressed, extruded, calendered, and degreased, it is subjected to biaxial asynchronous stretching to obtain a polytetrafluoroethylene primary film with uniformly dispersed template particles inside. The polytetrafluoroethylene primary film containing the template material is placed in a mold with a three-dimensional micron structure and hot-pressed. After cooling, it is demolded to obtain a molded body with mold structure imprints. (b) The molded body is immersed in an etching solution to remove the template material and obtain the polytetrafluoroethylene micro-nano hierarchical porous membrane material.

2. The method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane material according to claim 1, characterized in that, The mass ratio of polytetrafluoroethylene body in the primary membrane to the template material is 1:0.2-1:

3.

3. The method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane material according to claim 1, characterized in that, The template material includes one or both of silica microspheres and diatomaceous earth nanoparticles; The template material has a particle size of 20 nm-50 μm.

4. The method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane material according to claim 1, characterized in that, The polytetrafluoroethylene body in the primary membrane includes a polytetrafluoroethylene dispersion resin; The molecular weight of the polytetrafluoroethylene dispersion resin is 2 million to 10 million.

5. The method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane material according to claim 1, characterized in that, The hot pressing temperature is 340-380℃, the pressure is 0.1-0.4 MPa, and the holding time is 10-120 minutes; The hot pressing process employs isostatic pressing or uniaxial pressure.

6. The method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane material according to claim 1, characterized in that, The mold is made of one or more of stainless steel, nickel-based alloys, and zirconium oxide ceramics. The three-dimensional micron structure on the surface of the mold is either a positive texture or a negative texture. The positive texture structure is selected from one or more of column array, prism array and convex dot array; The negative texture structure is selected from one or more of the following: grid, groove, or honeycomb structure; The protrusions or depressions of the three-dimensional microstructure have a depth of 10μm-500μm.

7. The method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane material according to claim 1, characterized in that, The etching solution includes an alkaline solution; The alkaline solution includes an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide; The concentration of the etching solution is 1-10 mol / L.

8. The method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane material according to claim 1, characterized in that, In step (b), the soaking temperature is 60-95℃ and the soaking time is 2-48 hours.

9. The method for preparing polytetrafluoroethylene micro / nano hierarchical porous membrane material according to claim 1, characterized in that, The preparation method further includes: adjusting the surface wettability of the material obtained in step (b); the surface wettability adjustment includes hydrophobic modification or hydrophilic modification; The hydrophobic modification involves mixing the obtained material with a fluorinated silane compound and treating it at 40-100°C for 1-12 hours; the fluorinated silane compound includes one or both of perfluorodecyltrichlorosilane and perfluorooctyltriethoxysilane. The hydrophilic modification method includes plasma treatment; the plasma treatment voltage is 100-220V, the current is 0.5-5A, and the power is 50-1100 W; the plasma treatment time is 30-300 s; the protective gas used in the plasma treatment includes one or both nitrogen and argon, and the gas flow rate is 10-50 cm³. 3 / min.

10. A polytetrafluoroethylene micro / nano hierarchical porous membrane material, characterized in that, It is prepared by the preparation method according to any one of claims 1-9.

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