A polytetrafluoroethylene composite aerogel material and its preparation method and application

Abstract

The present invention provides a polytetrafluoroethylene (PTFE) composite aerogel material, its preparation method, and its application, belonging to the technical field of aerogel materials. Using a polytetrafluoroethylene (PTFE) dispersion as a raw material, the present invention compounded a skeleton-reinforced phase with fiber-reinforced fillers of varying types, morphologies, and functions to produce a functionally reinforced filler. The polytetrafluoroethylene (PTFE) dispersion and the functionally reinforced filler are then compounded and prepared through directional freezing, freeze-drying, and heat treatment to produce the polytetrafluoroethylene (PTFE) composite aerogel material. The resulting polytetrafluoroethylene (PTFE) composite aerogel material exhibits low thermal conductivity, excellent mechanical properties, good chemical stability, a wide temperature range, superhydrophobicity, and strong weather resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogel materials, and in particular to a polytetrafluoroethylene composite aerogel material and a preparation method and application thereof. Background Art

[0002] Aerogel has low density (0.01~0.3g / cm 3 ), high specific surface area (100~2000m 2 / g) and low thermal conductivity (less than 0.02W / mK) and has great potential as a thermal insulation material. The extremely low thermal conductivity shows the competitiveness of aerogel in the field of thermal insulation. Aerogels can be divided into two categories: inorganic aerogels and organic aerogels. Inorganic aerogels represented by silica-based aerogels have a porosity of up to 99.8% and a specific surface area of up to 1000m 2 / g or above, and recent research has even achieved thermal conductivity as low as 14mW / m·K. However, in practical applications, these aerogels suffer from dusting due to their brittleness. Consequently, organic aerogels have gradually gained research attention. Compared to brittle inorganic aerogels, organic aerogels often possess superior overall mechanical properties. Furthermore, due to their flexible molecular designability, they have potential for application in a wide range of fields. To date, the main organic aerogel materials reported include cellulose, polyacrylonitrile, polyvinyl alcohol, polyimide, and polyurethane. However, while these aerogels possess excellent processing properties and rich design possibilities, they also have relatively fragile molecular segments, and their high-temperature and chemical resistance are significantly lower than that of inorganic aerogels. This significantly limits their application in complex environments such as aerospace, chemical industry, and cold climates. To ensure the service life of aerogels in complex environments such as aerospace, chemical industry, construction, and environmental protection, achieving long-term stability in aerogel performance is crucial.

[0003] Polytetrafluoroethylene (PTFE) is widely used in chemical, electronic and medical fields due to its excellent chemical inertness, high and low temperature resistance and low friction coefficient. Although PTFE materials have many performance advantages, few people have studied PTFE-based aerogels. Guo et al. (Guo, X., Yao, Y., Zhu, P., Zhou, M. and Zhou, T., Preparation of porous PTFE / C composite foa m and its application in gravity-driven oil-water separation. Polym Int, (2022) 71: 874-883.) used polytetrafluoroethylene and PVFG as raw materials and low-temperature in-situ carbonization to design and prepare porous polytetrafluoroethylene (PTFE) / C composite foam materials. The prepared porous PTFE / C composite foam has a connected macroporous structure and superhydrophobicity, and the oil flux reaches 73242 Lh -1 m -2 Baskakov et al. (Baskakov, SA; Baskakova, YV; Kabachkov, EN; Dremova, NN; Michtchenko, A.; Shulga, YM, Novel Superhydrophobic Aerogel on the Base of Polytetrafluoroethylene. ACS Applied Materials & Interfaces (2019), 11(35), 32517-32522.) composited graphene with PTFE to obtain a superhydrophobic aerogel with a contact angle of 161.9-163.7°. Isopropyl alcohol, acetone and hexane can almost completely fill the free volume of the aerogel; at the same time, the aerogel also achieved high resistance to solvent cyclic loading.

[0004] However, while the aforementioned studies have successfully prepared PTFE aerogels and demonstrated their excellent performance in oil-water separation, they have focused solely on constructing superhydrophobic aerogel surfaces to achieve enhanced hydrophobicity and oleophilicity. Numerous unresolved issues remain, such as the lack of understanding of the relationship between aerogel microstructure and its macroscopic properties. First, strength and durability are critical factors for any material used in industrial applications, yet relevant research has not focused on the mechanical properties and recovery of PTFE aerogel composites. It is well known that PTFE aerogels are difficult to prepare due to their weak intermolecular forces, resulting in difficulty in molding and poor mechanical strength. Furthermore, PTFE molecular chains are susceptible to creep under stress, and even when aerogels are formed, they are prone to deformation under stress and cannot recover. These issues have been circumvented in existing research. Second, while relevant studies have successfully produced PTFE aerogels using reinforcing fillers, most have chosen to use high-volume, large-scale carbon fillers, failing to consider the potential degradation of aerogel performance due to aerogel density and filler interface issues. Finally, relevant research lacks research on structural regulation. Current PTFE aerogel research focuses solely on oil-water separation, failing to fully exploit the advantages of PTFE. It is crucial to understand how to manipulate the structure and properties of fillers, thereby broadening the application of PTFE aerogels. To achieve even higher-performance PTFE aerogels and achieve widespread application, these issues must be addressed.

[0005] The main reason for the problem in constructing PTFE aerogels lies in the structural characteristics of the material itself. On the one hand, the F atoms on the PTFE molecular chain make its interaction with most materials poor, which makes it difficult to construct PTFE aerogel materials. On the other hand, the unique chain structure also makes it have a complex preparation process. PTFE itself is an inert polymer and difficult to mix with other substances, which makes it very difficult to achieve uniform dispersion during the preparation of aerogels. Traditional aerogel preparation methods, such as sol-gel method and supercritical drying method, require precise control of reaction conditions, which places higher requirements on the processing of PTFE. In addition, PTFE has a high melting point and needs to be processed at a higher temperature, which increases the complexity and cost of experimental equipment and processes. Therefore, how to prepare PTFE-based aerogels is still a difficult problem in academia, and there is an urgent need to develop a new method to prepare PTFE aerogels with good mechanical properties and structural stability. Summary of the Invention

[0006] The purpose of the present invention is to provide a polytetrafluoroethylene composite aerogel material and its preparation method and application, which can obtain a low thermal conductivity polytetrafluoroethylene composite aerogel with excellent mechanical properties, good chemical stability, a wide temperature resistance range, super hydrophobicity, and strong weather resistance.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] The present invention provides a method for preparing a polytetrafluoroethylene composite aerogel material, comprising the following steps:

[0009] Mixing a dianhydride monomer and a diamine monomer with an organic solvent to carry out a polymerization reaction, and mixing the obtained polyimide precursor with an amino-containing organic base hydrophilic modifier to carry out modification to obtain a polyimide precursor salt as a skeleton reinforcement phase;

[0010] Mixing the skeleton reinforcement phase with a fiber-reinforced filler to obtain a functional reinforcement filler;

[0011] Mixing PTFE dispersion, functional strengthening filler and water to obtain a composite dispersion;

[0012] The composite dispersion liquid is placed in a mold and placed on a substrate. Liquid nitrogen is added to a metal container below the substrate to cool the substrate. The composite dispersion liquid is directionally frozen vertically upward from the bottom in contact with the substrate by cooling, and then freeze-dried to obtain an aerogel.

[0013] After the aerogel is demoulded, it is heat-treated by programmed temperature increase to obtain a polytetrafluoroethylene composite aerogel material.

[0014] Preferably, the dianhydride monomer includes one or more of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride and 3,3'4,4'-benzophenone tetracarboxylic dianhydride;

[0015] The diamine monomer includes one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminobenzanilide, 4,4'-diaminobenzophenone and 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl;

[0016] The amino-containing organic base hydrophilic modifier includes one or more of triethylamine, triethylenediamine, N,N-dimethylethanolamine and pyridine;

[0017] The molar ratio of the dianhydride monomer, the diamine monomer and the amino-containing organic base hydrophilic modifier is 100:98 to 100:200;

[0018] The polymerization reaction time is 4 to 16 hours; the modification time is 2 to 8 hours.

[0019] Preferably, the fiber-reinforced filler is a fiber filler with a hydrophilic contact angle of less than 90°, and the fiber-reinforced filler includes one or more of glass fiber, carbon fiber, cellulose and polyimide fiber; and the cellulose includes nanofibrillated cellulose or cellulose nanocrystals.

[0020] Preferably, the mass of the skeleton reinforcement phase accounts for 40-60% of the total mass of the functional reinforcement filler; the mass of the fiber reinforcement filler accounts for 40-60% of the total mass of the functional reinforcement filler, and the average length of the fiber reinforcement filler is 20-300 μm, and the fiber diameter is 0.2-1 μm.

[0021] Preferably, the solid content of polytetrafluoroethylene in the PTFE dispersion is 40-80 wt %, and the particle size is 0.1-0.5 μm; the mass ratio of the PTFE dispersion, functional strengthening filler and water is 2-10:1-10:80-95.

[0022] Preferably, the composite dispersion further includes a thickener, and the mass ratio of the PTFE dispersion to the thickener is 4:0-2; the thickener includes one or more of polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, sodium polyacrylate and water-based polyurethane.

[0023] Preferably, the thickness of the substrate is 0.5 mm to 5 mm, the temperature of liquid nitrogen during the directional freezing is -196° C., and the freeze-drying time is 48 to 96 hours.

[0024] Preferably, the programmed temperature rise includes: the first section temperature is 70-120°C, and the sintering time is 40-80 min; the second section temperature is 190-240°C, and the sintering time is 40-80 min; the third section temperature is 280-330°C, and the sintering time is 40-80 min; the fourth section temperature is 370-390°C, and the sintering time is 20-40 min.

[0025] The present invention provides a polytetrafluoroethylene composite aerogel material prepared by the preparation method described in the above technical solution.

[0026] The present invention provides applications of the polytetrafluoroethylene composite aerogel material described in the above technical solution in the fields of aerospace, chemical industry, construction or environmental protection.

[0027] The present invention uses a polytetrafluoroethylene (PTFE) dispersion as a raw material. A water-soluble skeleton reinforcement phase that can be in-situ cured and cross-linked during a subsequent temperature-programmed sintering process is compounded with fiber-reinforced fillers of varying types, morphologies, and functions, each with a high aspect ratio and a hydrophilic contact angle of less than 90°, to produce a functionally reinforced filler. The polytetrafluoroethylene dispersion and the functionally reinforced filler are then compounded and subjected to directional freezing, freeze-drying, and heat treatment to produce a polytetrafluoroethylene composite aerogel material. The polytetrafluoroethylene composite aerogel material prepared by the present invention exhibits low thermal conductivity, excellent mechanical properties, good chemical stability, a wide temperature range, superhydrophobicity, and strong weather resistance.

[0028] The present invention introduces a first reinforcing phase (skeleton reinforcing phase) that can promote the attachment and growth of PTFE colloid particles into the PTFE dispersion, which is formed into a support structure by freeze drying, and then in situ solidified and cross-linked to form a skeleton reinforcing phase, i.e., a polyimide microphase, during a subsequent programmed temperature sintering process. Since the skeleton reinforcing phase is formed by in situ solidification and cross-linking in the composite dispersion, it will be embedded in the PTFE matrix, significantly improving the strength and stability of the material, thereby preparing a high-strength PTFE aerogel; while introducing a second reinforcing phase (fiber reinforced filler) of high aspect ratio fiber, the fiber reinforced filler high aspect ratio and good hydrophilicity (hydrophilic contact angle is less than 90 °) are used to form a three-dimensional support network in the PTFE dispersion, thereby improving the structural stability of the aerogel. Moreover, the mesh structure of the high aspect ratio fiber reinforced filler can effectively disperse stress, thereby improving the mechanical strength of the aerogel material, and reducing the volume shrinkage of the aerogel to a certain extent when the aerogel is heat treated, thereby achieving the maintenance of high porosity and the regulation of pore size. At the same time, after heat treatment, the PTFE particles in the PTFE dispersion overlap and entangle with the skeleton reinforcement phase and fiber-reinforced filler. The in-situ curing and crosslinking of the skeleton reinforcement phase and the melting and interfacial diffusion of the PTFE particles occur simultaneously, maintaining a good interfacial interaction between the skeleton reinforcement phase and PTFE. At the same time, the strong polarity of the skeleton reinforcement phase also enables it to have a good interfacial interaction with the fiber-reinforced filler. The good hydrophilicity of the fiber-reinforced filler (hydrophilic contact angle less than 90°) enables it to disperse evenly in the aqueous system and also has good interfacial wettability with the PTFE dispersion. This interfacial reinforcement effect of the three-phase system will further improve the mechanical properties of the aerogel, thereby obtaining a low thermal conductivity polytetrafluoroethylene composite aerogel material with excellent comprehensive performance.

[0029] Compared with the traditional polytetrafluoroethylene (PTFE) aerogel preparation method and material characteristics, the present invention has the following significant advantages:

[0030] 1. This invention overcomes the cracking and shrinkage that often occurs during PTFE aerogel processing by adding a skeleton-reinforced phase and fiber-reinforced filler to a PTFE dispersion, followed by freeze-drying and a gradual temperature increase process, thereby resolving the aerogel molding issue. The addition of the skeleton-reinforced phase enhances the aerogel's high-temperature resistance and toughness, preventing the formation of microcracks. The fiber-reinforced filler provides a network support structure, dispersing internal stress and significantly improving the aerogel's mechanical behavior and pore structure.

[0031] The freeze-drying process avoids changes in the material's surface tension and physical stress that may be caused by water evaporation. The gradual heating process ensures that the internal and external temperatures of the material rise evenly, reducing the formation of temperature gradients and thus preventing internal stress and defects caused by uneven thermal expansion.

[0032] 2. This invention employs directional freezing technology, leveraging the different behaviors of the skeleton-reinforced phase and fiber-reinforced filler during directional ice crystal growth to construct a network-supported, layered pore structure. This design optimizes the heat conduction path, resulting in a low thermal conductivity in the radial direction perpendicular to the freezing direction, ensuring the material's excellent thermal insulation properties. Furthermore, due to its larger size, the fiber-reinforced filler is less susceptible to orientation effects, resulting in a network-like distribution, which balances the aerogel's mechanical properties in both directions.

[0033] 3. The present invention adopts PTFE dispersion as raw material, and this raw material inside contains the surfactant that assists PTFE particle dispersion. Utilize the charged surfactant molecule adsorbed on the surface of PTFE particles to produce charge interaction with polar molecules or ionic salts (corresponding to polyimide precursor salt in the present invention). This interaction can form a more stable combination with polar molecules or ionic salts, significantly enhancing the interfacial interaction between PTFE and filler. Compared to directly using PTFE powder, the surfactant of PTFE dispersion not only improves the dispersibility between particles, but also promotes the good compounding of PTFE matrix and reinforcement material. This improvement significantly improves the uniformity of material and the overall performance of composite material.

[0034] 4. The skeleton reinforcement phase in the composite dispersion of the present invention is formed by in-situ curing and cross-linking. During the subsequent molding and heating sintering process, it will be embedded in the PTFE matrix, significantly improving the strength and stability of the material, thereby preparing a high-strength PTFE aerogel. At the same time, a second reinforcement phase (fiber-reinforced filler) of high aspect ratio fibers is introduced. The high aspect ratio and good hydrophilicity (hydrophilic contact angle less than 90°) of the fiber-reinforced filler are used to form a three-dimensional support network in the PTFE dispersion, thereby improving the structural stability of the aerogel. Moreover, the network structure of the high aspect ratio fiber-reinforced filler can effectively disperse stress, thereby improving the mechanical strength of the aerogel material and reducing the volume shrinkage of the aerogel to a certain extent during the heat treatment of the aerogel, thereby maintaining a high porosity and finely regulating the pore size.

[0035] 5. After heat treatment, the PTFE particles in the composite dispersion overlap and entangle with the skeleton reinforcement phase and fiber-reinforced filler. The in-situ curing and crosslinking of the skeleton reinforcement phase, as well as the melting and interfacial diffusion of the PTFE particles, occur simultaneously, maintaining a good interfacial interaction between the skeleton reinforcement phase and PTFE. The strong polarity of the skeleton reinforcement phase also enables a good interfacial interaction with the fiber-reinforced filler. The excellent hydrophilicity of the fiber-reinforced filler (hydrophilic contact angle less than 90°) enables it to disperse evenly in the aqueous system and also has good interfacial wettability with the PTFE dispersion. This interfacial reinforcement of the three-phase system further improves the mechanical properties of the aerogel, resulting in a low thermal conductivity polytetrafluoroethylene composite aerogel material with excellent overall performance.

[0036] 6. The present invention uses freeze-drying and heat treatment processes to melt PTFE particles at high temperatures, embedding the reinforcing phase into the PTFE matrix, further enhancing the overall strength and stability of the material, preparing high-strength PTFE aerogel, and also improving the environmental tolerance of the aerogel.

[0037] 7. The pore structure of the PTFE aerogel prepared by the present invention is adjustable, and the aerogel has strong adaptability in application. The pore structure can be adjusted according to different needs, and it has wide application value in the fields of aerospace, chemical industry, construction or environmental protection materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a flow chart for preparing the polytetrafluoroethylene composite aerogel of the present invention;

[0039] Figure 2 The microstructure diagram and three-dimensional structure schematic diagram of the aerogel prepared in Example 1. DETAILED DESCRIPTION

[0040] In the present invention, unless otherwise specified, the required raw materials or reagents are commercially available products well known to those skilled in the art.

[0041] like Figure 1 As shown, the present invention provides a method for preparing a polytetrafluoroethylene composite aerogel material, comprising the following steps:

[0042] Mixing a dianhydride monomer and a diamine monomer with an organic solvent to carry out a polymerization reaction, and mixing the obtained polyimide precursor with an amino-containing organic base hydrophilic modifier to carry out modification to obtain a polyimide precursor salt as a skeleton reinforcement phase;

[0043] Mixing the skeleton reinforcement phase with a fiber-reinforced filler to obtain a functional reinforcement filler;

[0044] Mixing PTFE dispersion, functional strengthening filler and water to obtain a composite dispersion;

[0045] The composite dispersion is placed in a mold and placed on a substrate. Liquid nitrogen is added to a metal container under the substrate to cool the substrate. The composite dispersion is directionally frozen vertically upward from the bottom in contact with the substrate by cooling, and then freeze-dried to obtain an aerogel.

[0046] After the aerogel is demoulded, it is heat-treated by programmed temperature increase to obtain a polytetrafluoroethylene composite aerogel material.

[0047] The present invention mixes dianhydride monomers, diamine monomers and organic solvents to carry out polymerization reaction, and mixes the obtained polyimide precursor with an amino-containing organic base hydrophilic modifier to carry out modification to obtain a polyimide precursor salt as a skeleton reinforcement phase.

[0048] In the present invention, the dianhydride monomer preferably includes one or more of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride; the diamine monomer preferably includes one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminobenzanilide, 4,4'-diaminobenzophenone, and 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl. When the dianhydride monomer or diamine monomer is two or more of the above, the present invention does not specifically limit the ratio of the different types of reagents, and any ratio can be used.

[0049] In the present invention, the organic solvent is preferably N,N-dimethylacetamide, N-methylpyrrolidone or N,N-dimethylformamide. The amount of the organic solvent used is not particularly limited and can be adjusted according to actual needs to ensure smooth reaction.

[0050] In the present invention, the polymerization reaction time is preferably 4 to 16 hours, more preferably 8 to 12 hours, further preferably 10 hours, and the temperature is preferably room temperature.

[0051] After the polymerization reaction is completed, the present invention preferably adds an amino-containing organic base hydrophilic modifier to the resulting product system, performs modification (salification) under stirring conditions, and precipitates the resulting water-soluble polyimide precursor salt solution in acetone to obtain a water-soluble polyimide precursor salt solid (as a skeleton reinforcement phase, its advantages are reflected in toughness, good dispersibility and system compatibility).

[0052] In the present invention, the amino-containing organic base hydrophilic modifier preferably includes one or more of triethylamine, triethylenediamine, N,N-dimethylethanolamine, and pyridine. When two or more of the above-mentioned amino-containing organic base hydrophilic modifiers are used, the present invention does not specifically limit the ratio of the different types of agents; any ratio is acceptable. The present invention utilizes the hydrophilic modifier to convert the synthesized polyimide precursor into a water-soluble polyimide precursor salt.

[0053] In the present invention, the molar ratio of the dianhydride monomer, the diamine monomer and the amino-containing organic base hydrophilic modifier is preferably 100:98 to 100:200, more preferably 100:100:200.

[0054] In the present invention, the modification temperature is preferably 25° C., and the modification time is preferably 2 to 8 hours, more preferably 2 to 4 hours, and even more preferably 2 to 3 hours.

[0055] The present invention has no particular limitation on the precipitation, and the precipitation may be carried out according to a process well known in the art.

[0056] The present invention mixes the skeleton reinforcement phase with the fiber reinforcement filler to obtain the function-reinforced filler.

[0057] In the present invention, the fiber-reinforced filler is preferably a fiber filler with a hydrophilic contact angle of less than 90°, and more preferably includes one or more of glass fiber, carbon fiber, cellulose and polyimide fiber; the cellulose preferably includes nanofibrillated cellulose (NFC) or cellulose nanocrystals; when the fiber-reinforced filler is two or more of the above, the present invention has no special limitation on the ratio of different types of fiber-reinforced fillers, and any ratio is acceptable.

[0058] In the present invention, the mass of the skeleton reinforcement phase preferably accounts for 40 to 60% of the total mass of the functional reinforcement filler; the mass of the fiber reinforcement filler preferably accounts for 40 to 60% of the total mass of the functional reinforcement filler, the average length of the fiber reinforcement filler is preferably 20 to 300 μm, and the fiber diameter is preferably 0.2 to 1 μm.

[0059] After obtaining the function-enhancing filler, the present invention mixes the PTFE dispersion, the function-enhancing filler and water to obtain a composite dispersion.

[0060] In the present invention, the solid content of polytetrafluoroethylene in the PTFE dispersion is 40 to 80 wt%, more preferably 60 wt%, and the particle size is 0.1 to 0.5 μm.

[0061] In the present invention, the mass ratio of the PTFE dispersion, the functional reinforcing filler and water is preferably 2-10:1-10:80-95, more preferably 4-8:1-2:90-95.

[0062] In the present invention, the composite dispersion preferably also includes a thickener. The mass ratio of the PTFE dispersion to the thickener is preferably 4:0-2, more preferably 4:0-1. The thickener preferably includes one or more of polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, sodium polyacrylate, and water-based polyurethane. The water-based polyurethane is not particularly limited in the present invention; any commercially available product known in the art may be used. The thickener is used in the present invention to improve the dispersibility of the filler and to buffer internal stress during the freezing process to prevent cracking.

[0063] After obtaining the composite dispersion, the present invention places the composite dispersion in a mold and places it on a substrate. Liquid nitrogen is added to a metal container under the substrate to cool the substrate. The composite dispersion is directionally frozen vertically upward from the bottom in contact with the substrate by cooling, and then freeze-dried to obtain an aerogel.

[0064] In the present invention, the mold is preferably a square container measuring 30 mm by 30 mm by 50 mm or a circular container with a diameter of 25 mm. The mold is used to control the dimensions of the PTFE aerogel. The mold is preferably made of one or more of nitrile rubber, silicone rubber, polystyrene plastic, and polycarbonate plastic. The present invention does not specifically limit the specific preparation and source of the mold; it can be obtained using methods well known in the art.

[0065] The present invention has no special limitation on the metal container, and any metal container that can hold liquid nitrogen known in the art can be used; in the embodiment, it is specifically a 304 stainless steel container.

[0066] In the present invention, the substrate is preferably a copper plate; the thickness of the substrate is preferably 0.5 mm to 5 mm, and the length and width of the substrate are adjusted according to the external dimensions of the prepared PTFE. The present invention utilizes the substrate to achieve unidirectional heat conduction of the composite dispersion, thereby achieving directional freezing of the composite dispersion; the directional freezing cools the substrate by liquid nitrogen, and achieves unidirectional heat conduction to the sample through the cooling process of the substrate; during the directional freezing, the liquid nitrogen temperature is preferably -196°C.

[0067] After freezing, the obtained product is placed in a freeze dryer for freeze drying; the freeze drying time is preferably 48 to 96 hours, more preferably 72 hours.

[0068] After obtaining the aerogel, the present invention demoulds the aerogel and then performs a programmed temperature sintering process to achieve in-situ curing and cross-linking of the skeleton reinforcement phase and melting, interface diffusion and molecular chain entanglement of PTFE particles in the PTFE dispersion to obtain a polytetrafluoroethylene composite aerogel material.

[0069] In the present invention, the programmed temperature rise preferably includes: the first section temperature is 70-120°C, and the sintering time is 40-80min; the second section temperature is 190-240°C, and the sintering time is 40-80min; the third section temperature is 280-330°C, and the sintering time is 40-80min; the fourth section temperature is 370-390°C, and the sintering time is 20-40min; the first section temperature is preferably 100-110°C, and the sintering time is 60min; the second section temperature is 200-220°C, and the sintering time is 50-60min; the third section temperature is 300-320°C, and the sintering time is 60-70min; the fourth section temperature is 380-390°C, and the sintering time is 30min.

[0070] The present invention provides a polytetrafluoroethylene composite aerogel material prepared by the preparation method described in the above technical solution.

[0071] The present invention provides the application of the polytetrafluoroethylene composite aerogel material described in the above technical solution in the fields of aerospace, chemical industry, construction or environmental protection. The present invention does not specifically limit the method of application, and the application can be carried out according to methods well known in the art.

[0072] The technical solutions provided by the present invention are described in detail below with reference to the examples, but they should not be construed as limiting the scope of protection of the present invention. The raw materials and equipment used in the specific embodiments of the present invention are all known products and were obtained by purchasing commercial products.

[0073] In the following examples, the solid content of polytetrafluoroethylene in the polytetrafluoroethylene dispersion is 60 wt % and the particle size is 0.1 to 0.5 μm.

[0074] Example 1

[0075] Dosage of each component:

[0076]

[0077]

[0078] Among them, the average length of nanofibrillated cellulose is 100-300 μm, and the fiber diameter is 0.2-0.5 μm;

[0079] The molar ratio of each component when synthesizing the polyimide precursor salt:

[0080] Dianhydride (pyromellitic dianhydride, PMDA) 100

[0081] Diamine (4,4'-diaminodiphenyl ether, ODA) 100

[0082] Hydrophilic modifier (triethylamine) 200

[0083] Step 1: 10.012 g (0.05 mol) of diamine and 10.906 g (0.05 mol) of dianhydride were added to 90 g of N, N-dimethylacetamide in sequence. After being evenly dispersed, the system was placed at room temperature for reaction for 8 hours to generate polyamic acid. 10.119 g (0.1 mol) of triethylamine was then added dropwise to the reaction system and stirred at 25°C for 2 hours to form a salt to obtain a soluble polyimide precursor salt solution. The reaction system was then precipitated in acetone to obtain a soluble polyimide precursor salt solid, i.e., a skeleton reinforcement phase.

[0084] The skeleton reinforcement phase is mixed with nanofibrillated cellulose in the above ratio to obtain a functional reinforcement filler;

[0085] Step 2: Mix the PTFE dispersion, deionized water, functional strengthening filler and thickener in the above proportions and stir evenly to obtain a composite dispersion;

[0086] Step 3: The composite dispersion is placed in a 30*30*50 square mold, and then placed on a 2mm thick copper plate. Liquid nitrogen is added to a 304 stainless steel container under the substrate to cool the substrate. The liquid nitrogen temperature is -196°C. The composite dispersion is directionally frozen from the bottom in contact with the substrate upwards by cooling.

[0087] Step 4: After freezing is completed, the directionally frozen sample is placed in a freeze dryer and dried for 48 hours to obtain aerogel;

[0088] Step 5: After demolding the prepared aerogel, place the aerogel in a muffle furnace and calcine at 100°C, 200°C, and 300°C for 1 hour, respectively, and finally calcine at 380°C for 30 minutes to obtain a polytetrafluoroethylene composite aerogel material.

[0089] Figure 2 The microstructure diagram and three-dimensional structure schematic diagram of the aerogel prepared in Example 1.

[0090] The performance of the composite aerogel material prepared in this example was tested:

[0091] Thermal conductivity was measured using a hot disk according to the international standard ISO 22007-2.2. The aerogel porosity was calculated to be 96.4%, based on the ratio of the aerogel density to the density of polytetrafluoroethylene. Due to the high porosity and oriented pore structure of the prepared aerogel, the PTFE aerogel exhibits significant anisotropic thermal conductivity. The measured axial thermal conductivity was 0.041 W / (m·K), and the radial thermal conductivity was 0.020 W / (m·K).

[0092] 2) Aerogel compression testing, conducted according to ASTM D1621, yielded axial and radial compressive strengths of 200 kPa and 160 kPa, respectively, at 50% strain at a test rate of 0.5 mm / min. After 10 cycles of compression, the axial and radial strain recoveries were 86% and 82%, respectively. This demonstrates the effectiveness of the fiber-supported layered structure design.

[0093] 3) The water contact angle measured by an optical contact angle meter is 151°, showing a significant superhydrophobic effect.

[0094] 4) At room temperature, the sample can maintain 95% of its mechanical properties after being immersed in 1M sodium hydroxide, hydrochloric acid, or sodium chloride for one month. It also maintains flexibility at -196°C.

[0095] 5) The sound insulation performance of aerogel was tested in accordance with GB / T19889.3-2005. The test results showed that at a frequency of 1kHz, the sound insulation coefficient reached 28dB.

[0096] 6) Testing the flame retardancy of the aerogel: In a vertical combustion test, the sample self-extinguished in 12 seconds and did not drip, demonstrating its excellent flame retardancy.

[0097] Therefore, while having excellent thermal insulation properties, the aerogel prepared in this embodiment has excellent mechanical properties and strong environmental stability, and is suitable for long-term applications in complex environments in aerospace, chemical industry, construction, environmental protection and other fields.

[0098] Example 2

[0099] Dosage of each component:

[0100]

[0101] Among them, the average length of cellulose nanocrystals is 50-200 μm, and the whisker diameter is 0.2-0.5 μm;

[0102] The molar ratio of each component when synthesizing the polyimide precursor salt:

[0103] Dianhydride (pyromellitic dianhydride, PMDA) 100

[0104] Diamine (4,4'-diaminodiphenyl ether, ODA) 100

[0105] Hydrophilic modifier (triethylamine) 200

[0106] Step 1: 10.012 g of diamine and 10.906 g of dianhydride were added to 90 g of N,N-dimethylacetamide in sequence. After uniform dispersion, the system was placed at room temperature for reaction for 8 hours to generate polyamic acid. 10.119 g of triethylamine was then added dropwise to the reaction system and stirred at 25°C for 2 hours to form a salt to obtain a soluble polyimide precursor salt solution. The reaction system was then precipitated in acetone to obtain a soluble polyimide precursor salt solid, i.e., the skeleton reinforcement phase.

[0107] The skeleton reinforcement phase is mixed with cellulose nanocrystals in the above ratio to obtain a functional reinforcement filler;

[0108] Step 2: Mix the PTFE dispersion, deionized water, functional strengthening filler and thickener in the above proportions and stir evenly to obtain a composite dispersion;

[0109] Step 3: Place the prepared composite dispersion in a 30*30*50 square mold, and then place it on a 3mm thick copper plate. Add liquid nitrogen to a 304 stainless steel container under the substrate to cool the substrate. The liquid nitrogen temperature is -196°C. The composite dispersion is directionally frozen from the bottom in contact with the substrate upwards by cooling.

[0110] Step 4: After freezing is completed, the directionally frozen sample is placed in a freeze dryer and dried for 48 hours to obtain aerogel;

[0111] Step 5: After demolding the prepared aerogel, the aerogel was placed in a muffle furnace and calcined at 110°C, 210°C, and 310°C for 70 minutes, respectively, and finally calcined at 380°C for 20 minutes to obtain a polytetrafluoroethylene composite aerogel material.

[0112] The aerogel prepared in this example was subjected to performance testing using the same testing methods and standards as in Example 1. The results showed an axial thermal conductivity of 0.083 W / (m·K), a radial thermal conductivity of 0.031 W / (m·K), a porosity of 94.2%, axial and radial compressive strengths of 180 kPa and 200 kPa, respectively, at 50% strain, axial and radial strain recovery rates of 85% and 84%, respectively, after 10 cycles, and a water contact angle of 158°. Therefore, while exhibiting excellent thermal insulation properties, the aerogel prepared in this example also exhibits high mechanical strength and excellent recovery properties, making it suitable for applications in lightweight flexible devices and equipment.

[0113] Example 3

[0114] Dosage of each component:

[0115]

[0116]

[0117] Among them, the average length of carbon fiber is 100-300 μm, and the fiber diameter is 0.2-1 μm;

[0118] The molar ratio of each component when synthesizing the polyimide precursor salt:

[0119] Dianhydride (pyromellitic dianhydride, PMDA) 100

[0120] Diamine (4,4'-diaminodiphenyl ether, ODA) 98

[0121] Hydrophilic modifier (triethylamine) 200

[0122] Step 1: Add diamine and dianhydride to N,N-dimethylacetamide in the above ratios, and after uniform dispersion, place the system at room temperature for reaction for 12 hours to generate polyamic acid. Then, add triethylamine dropwise to the reaction system according to the above ratio, stir at 25°C for 2.5 hours to form a salt, and obtain a soluble polyimide precursor salt solution. Then, precipitate the reaction system in acetone to obtain a soluble polyimide precursor salt solid, i.e., the skeleton reinforcement phase.

[0123] The skeleton reinforcement phase is mixed with carbon fibers in the above ratio to obtain a functional reinforcement filler;

[0124] Step 2: Mix the PTFE dispersion, deionized water, functional strengthening filler and thickener in the above proportions and stir evenly to obtain a composite dispersion;

[0125] Step 3: Place the prepared composite dispersion in a 30*30*50 square mold, and then place it on a 5mm thick copper plate. Add liquid nitrogen to a 304 stainless steel container under the substrate to cool the substrate. The liquid nitrogen temperature is -196°C. The composite dispersion is directionally frozen from the bottom in contact with the substrate upwards by cooling.

[0126] Step 4: After the sample is frozen, place the directionally frozen sample in a freeze dryer and dry it for 72 hours to obtain aerogel;

[0127] Step 5: After demolding the prepared aerogel, the aerogel was placed in a muffle furnace and calcined at 120°C, 220°C, and 320°C for 50 minutes, respectively, and finally calcined at 390°C for 20 minutes to obtain a polytetrafluoroethylene composite aerogel material.

[0128] The aerogel prepared in this example was subjected to performance testing using the same testing methods and standards as in Example 1. The results showed an axial thermal conductivity of 0.134 W / (m·K), a radial thermal conductivity of 0.038 W / (m·K), a porosity of 92.6%, axial and radial compressive strengths of 620 kPa and 400 kPa, respectively, at 50% strain, and a water contact angle of 150.8°. The aerogel prepared in this example exhibits enhanced mechanical properties and excellent thermal insulation properties, and is widely applicable in fields such as construction, industrial equipment, and the automotive industry.

[0129] Example 4

[0130] Dosage of each component:

[0131]

[0132] Among them, the average length of nanofibrillated cellulose is 50 to 200 μm, and the fiber diameter is 0.2 to 0.5 μm;

[0133] The molar ratio of each component when synthesizing the polyimide precursor salt:

[0134] Dianhydride (3,3',4,4'-biphenyltetracarboxylic dianhydride, BPDA) 100

[0135] Diamine (4,4'-diaminodiphenyl ether, ODA) 98

[0136] Hydrophilic modifier (triethylenediamine, DABCO) 200

[0137] Step 1: Add diamine and dianhydride to N,N-dimethylacetamide in the above ratios, and after uniform dispersion, place the system at room temperature for reaction for 10 hours to generate polyamic acid. Then, add triethylenediamine solid according to the above ratio and stir at 25°C for 4 hours to form a salt to obtain a soluble polyimide precursor salt solution. Then, precipitate the reaction system in acetone to obtain a soluble polyimide precursor salt solid, i.e., the skeleton reinforcement phase.

[0138] The skeleton reinforcement phase is mixed with nanofibrillated cellulose in the above ratio to obtain a functional reinforcement filler;

[0139] Step 2: Mix the PTFE dispersion, deionized water, functional strengthening filler and thickener in the above proportions and stir evenly to obtain a composite dispersion;

[0140] Step 3: The prepared composite dispersion is placed in a circular mold with a diameter of 25 mm, and then placed on a copper plate with a thickness of 3 mm. Liquid nitrogen is added to a 304 stainless steel container under the substrate to cool the substrate. The liquid nitrogen temperature is -196°C. The composite dispersion is directionally frozen from the bottom in contact with the substrate upwards by cooling.

[0141] Step 4: After the sample is frozen, place the directionally frozen sample in a freeze dryer and dry it for 96 hours to obtain aerogel;

[0142] Step 5: After demolding the prepared aerogel, the aerogel was placed in a muffle furnace and calcined at 120°C, 220°C, and 320°C for 1 hour, respectively, and finally calcined at 380°C for 30 minutes to obtain a polytetrafluoroethylene composite aerogel material.

[0143] The aerogel prepared in this example was subjected to performance testing using the same testing methods and standards as in Example 1. The results showed an axial thermal conductivity of 0.091 W / (m·K), a radial thermal conductivity of 0.0026 W / (m·K), a porosity of 94.1%, and axial and radial strain recovery rates of 94% and 90%, respectively, after 10 compression cycles. The operating temperature was ≥240°C. Therefore, in addition to excellent thermal insulation properties, the aerogel prepared in this example also exhibited excellent mechanical recovery, making it suitable for applications in flexible electronic devices and sensors.

[0144] Comparative Example 1

[0145] The only difference from Example 1 is that the amount of nanofibrillated cellulose added is adjusted to 0, the amount of polyimide precursor salt added is adjusted to 1 (keeping the filler ratio unchanged), and the remaining raw material amounts and steps are the same as in Example 1.

[0146] The aerogel prepared in comparative example 1 was subjected to performance testing, and the test methods and test standards were the same as those in Example 1. The test conditions for tensile properties, creep properties, and dielectric properties were the same as those in Example 1. The axial thermal conductivity was measured to be 0.11 W / (m·K), and the radial thermal conductivity was 0.052 W / (m·K). The axial thermal conductivity and radial thermal conductivity increased to 275% and 260% of the original values, respectively, compared with Example 1, and the porosity decreased from 96.4% to 79.2%. This indicates that the comparative aerogel has undergone severe shrinkage, which means that cellulose is critical to suppressing the shrinkage of the aerogel. It was further measured that the axial and radial compressive strengths under 50% strain increased slightly, rising to 220 KPa and 170 KPa, respectively. This is attributed to the increase in the density of the material, which makes the material denser and more difficult to deform when compressed. The axial and radial strain recovery rates after 10 cycles decreased to 51% and 40%, respectively, which shows that the elastic network structure of cellulose is crucial to the realization of high recovery performance. The water contact angle measured by an optical contact angle meter was 138°. The slight decrease may be due to the disappearance of cellulose, which reduces the roughness of the aerogel surface and weakens the lotus effect on the surface. The performance maintenance ability is basically the same as that of Example 1. The sound insulation performance was tested and it was found that at a frequency of 1kHz, the sound insulation coefficient dropped to 18dB. This may be because the increase in aerogel density, the increase in pores, and the decrease in specific surface area led to a decrease in viscous dissipation capacity and a significant deterioration in sound insulation performance. The flame retardant performance is basically the same as that of Example 1. In the vertical combustion test, the self-extinguishing time of the sample was 13 seconds and there was no dripping, which shows its good flame retardant properties.

[0147] Comparative Example 2

[0148] The only difference from Example 1 is that the amount of polyimide precursor salt added is adjusted to 0, the amount of nanofibrillated cellulose added is adjusted to 1 (keeping the filler ratio unchanged), and the amounts of other cellulose raw materials and the steps are the same as in Example 1.

[0149] The aerogel prepared in comparative example 2 was subjected to performance testing, and the test methods and test standards were the same as those in Example 1. The test conditions for tensile properties, creep properties, and dielectric properties were the same as those in Example 1. The axial thermal conductivity was measured to be 0.048 / (m·K), and the radial thermal conductivity was measured to be 0.039W / (m·K). The axial thermal conductivity and radial thermal conductivity increased to 117% and 195% of the original values, respectively, compared with Example 1, and the porosity decreased from 96.4% to 94.2%. This indicates that the comparative aerogel has shrunk slightly. At the same time, the differential decrease in thermal conductivity in the two directions can be attributed to the destruction of the layered orientation structure caused by the failure to add polyimide precursor salt. The axial and radial compressive strengths were tested under 50% strain, and it was found that under the same compression test conditions, the aerogel would undergo obvious axial damage, and the strength would drop significantly to 20Kpa, while the radial compressive strength would also decrease to 54Kpa. This was attributed to the disappearance of the skeleton reinforcement phase, which led to a decrease in the interfacial effect, thereby reducing the strength of the aerogel. After 10 cycles, the axial and radial strain recovery rates dropped to 42% and 59%, respectively, with a significant decrease in axial recovery performance. This indicates that the loss of the polyimide microstructure significantly degrades the aerogel's mechanical properties. Testing of its sound insulation performance revealed a drop to 15dB at 1kHz, likely due to a decrease in interfacial strength, which reduced the system's viscous dissipation capacity and significantly degraded sound insulation performance. The flame retardant properties were essentially consistent with those of Example 1. In a vertical combustion test, the sample self-extinguished in 12 seconds and exhibited no dripping, demonstrating its excellent flame retardancy.

[0150] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a polytetrafluoroethylene composite aerogel material, characterized in that: The following steps are involved: Mixing a dianhydride monomer and a diamine monomer with an organic solvent to carry out a polymerization reaction, and mixing the obtained polyimide precursor with an amino-containing organic base hydrophilic modifier to carry out modification to obtain a polyimide precursor salt as a skeleton reinforcement phase; Mixing the skeleton reinforcement phase with a fiber-reinforced filler to obtain a functional reinforcement filler; Mixing PTFE dispersion, functional strengthening filler and water to obtain a composite dispersion; The composite dispersion is placed in a mold and placed on a substrate. Liquid nitrogen is added to a metal container below the substrate to cool the substrate. The composite dispersion is directionally frozen vertically upward from the bottom in contact with the substrate by cooling, and then freeze-dried to obtain an aerogel. After demoulding the aerogel, heat treatment is performed by programmed temperature increase to obtain a polytetrafluoroethylene composite aerogel material; The fiber-reinforced filler includes one or more of cellulose and polyimide fibers; The average length of the fiber-reinforced filler is 20-300 μm, and the fiber diameter is 0.2-1 μm.

2. The preparation method according to claim 1, characterized in that The dianhydride monomer includes one or more of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride and 3,3'4,4'-benzophenone tetracarboxylic dianhydride; The diamine monomer includes one or more of 4,4'-diaminodiphenyl ether, p-phenylenediamine, 4,4'-diaminodiphenyl sulfone, 4,4'-diaminobenzanilide, 4,4'-diaminobenzophenone and 4,4'-diamino-2,2'-dimethyl-1,1'-biphenyl; The amino-containing organic base hydrophilic modifier includes one or more of triethylamine, triethylenediamine, N,N-dimethylethanolamine and pyridine; The molar ratio of the dianhydride monomer, the diamine monomer and the amino-containing organic base hydrophilic modifier is 100:98 to 100:200; The polymerization reaction time is 4 to 16 hours; the modification time is 2 to 8 hours.

3. The preparation method according to claim 1, characterized in that The fiber-reinforced filler is a fiber filler with a hydrophilic contact angle less than 90°, and the cellulose includes nanofibrillated cellulose or cellulose nanocrystals.

4. The preparation method according to claim 1 or 3, characterized in that The mass of the skeleton reinforcement phase accounts for 40-60% of the total mass of the functional reinforcement filler; the mass of the fiber reinforcement filler accounts for 40-60% of the total mass of the functional reinforcement filler.

5. The preparation method according to claim 1, characterized in that The solid content of polytetrafluoroethylene in the PTFE dispersion is 40-80 wt %, and the particle size is 0.1-0.5 μm; the mass ratio of the PTFE dispersion, the functional strengthening filler and water is 2-10:1-10:80-95.

6. The preparation method according to claim 1 or 5, characterized in that The composite dispersion also includes a thickener, and the mass ratio of the PTFE dispersion to the thickener is 4:0-2; the thickener includes one or more of polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, sodium polyacrylate and water-based polyurethane.

7. The preparation method according to claim 1, characterized in that The thickness of the substrate is 0.5 mm to 5 mm. During the directional freezing, the temperature of liquid nitrogen is -196° C.; and the freeze-drying time is 48 to 96 hours.

8. The preparation method according to claim 1, characterized in that The programmed temperature rise includes: the first stage temperature is 70-120°C, and the sintering time is 40-80 minutes; the second stage temperature is 190-240°C, and the sintering time is 40-80 minutes; the third stage temperature is 280-330°C, and the sintering time is 40-80 minutes; the fourth stage temperature is 370-390°C, and the sintering time is 20-40 minutes.

9. The polytetrafluoroethylene composite aerogel material prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the polytetrafluoroethylene composite aerogel material according to claim 9 in the fields of aerospace, chemical industry, construction or environmental protection.

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