Polytetrafluoroethylene composite aerogel material as well as preparation method and application thereof

Abstract

The invention provides a polytetrafluoroethylene composite aerogel material as well as a preparation method and application thereof, and belongs to the technical field of aerogel materials. According to the invention, the polytetrafluoroethylene dispersion liquid is used as a raw material, framework reinforced phases with different types, forms and functions and fiber reinforced fillers are compounded and mixed to prepare functional reinforced fillers, and the polytetrafluoroethylene dispersion liquid and the functional reinforced fillers are compounded; the polytetrafluoroethylene composite aerogel material is prepared through directional freezing, freeze drying and heat treatment. The prepared polytetrafluoroethylene composite aerogel material has the functions of low thermal conductivity, excellent mechanical properties, good chemical stability, wide temperature-resistant range, super-hydrophobicity, strong weather resistance and the like.

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 (0.01-0.3 g / cm 3 ), high specific surface area (100~2000m 2 / g) and low thermal conductivity (less than 0.02W / mK) have great potential as thermal insulation materials. 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 more, and recent studies have achieved thermal conductivity as low as 14mW / m·K. However, in actual applications, these aerogels have the problem of dusting due to brittleness. Therefore, organic aerogels have gradually been studied. Compared with brittle inorganic aerogels, organic aerogels often have better comprehensive mechanical properties. At the same time, due to its flexible molecular designability, it has application potential in many fields. So far, the reported organic aerogel materials mainly include cellulose, polyacrylonitrile, polyvinyl alcohol, polyimide and polyurethane. However, while these aerogels have excellent processing properties and rich design, it also means that their molecular segments are relatively fragile, and their high and low temperature resistance and chemical resistance are significantly lower than those of inorganic aerogels, which greatly limits their application in complex environments such as aerospace, chemical industry, and cold climates. In order to ensure the service life of aerogels in complex environments such as aerospace, chemical industry, construction and environmental protection, it is crucial to achieve long-term stability of aerogel performance.

[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 foam 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 Baska kov 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°. Isopropanol, 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, although the above studies have successfully prepared PTFE aerogels and demonstrated their excellent performance in the field of oil-water separation, the related studies have only stayed on the construction of aerogel superhydrophobic surfaces to achieve better hydrophobic and oleophilic effects. There are still many unresolved issues, such as insufficient research on the relationship between the microstructure of aerogels and their macroscopic properties. First, for any material used in industrial applications, strength and sustainability are key factors, and related studies have not paid attention to the mechanical properties and recovery properties of PTFE aerogel composites. As we all know, the reason why PTFE aerogels are difficult to prepare is that their intermolecular forces are weak, which makes them difficult to shape and their mechanical strength is very poor. At the same time, PTFE molecular chains are prone to creep under stress. Even if aerogels are made, they are easy to deform under stress and the deformation cannot be recovered. However, these problems have been avoided in existing studies. Second, although related studies have successfully obtained PTFE aerogels using reinforcing fillers, most of them choose to use high-proportion and large-scale carbon fillers, lacking consideration of the aerogel density and filler interface problems that lead to the degradation of aerogel performance. Finally, related studies lack research on structural regulation. The current research on PTFE aerogels is only focused on the field of oil-water separation, which makes it difficult to give full play to the advantages of PTFE. It is also crucial to regulate the structure and performance through fillers, thereby broadening the application of PTFE aerogels. In order to obtain PTFE aerogels with better performance and realize the wide application of PTFE aerogels, these issues cannot be ignored.

[0005] The main reason for the construction problem of PTFE aerogel 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 for PTFE to construct 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 is 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 puts higher requirements on the treatment 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 it is urgent 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 a preparation method and application thereof, so as to 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] The dianhydride monomer and the diamine monomer are mixed with an organic solvent to carry out a polymerization reaction, and the obtained polyimide precursor is mixed 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 the fiber-reinforced filler to obtain a functionally reinforced filler;

[0011] Mixing the PTFE dispersion, the 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, and liquid nitrogen is added to a metal container below the substrate to cool the substrate, and 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'-benzophenonetetracarboxylic 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 40-80wt%, 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.

[0022] Preferably, 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.

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

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

[0027] The present invention uses polytetrafluoroethylene dispersion as raw material, and mixes a water-soluble skeleton reinforcement phase that can be in-situ cured and cross-linked during the subsequent programmed temperature rise sintering process and a fiber-reinforced filler of different types, forms and functions with a high aspect ratio and a hydrophilic contact angle of less than 90° to obtain a functional reinforced filler, and then the polytetrafluoroethylene dispersion and the functional reinforced filler are compounded to obtain a polytetrafluoroethylene composite aerogel material through directional freezing, freeze drying and heat treatment. The polytetrafluoroethylene composite aerogel material prepared by the present invention has the functions of low thermal conductivity, excellent mechanical properties, good chemical stability, wide temperature resistance range, super hydrophobicity, strong weather resistance and the like.

[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 forms a supporting structure by freeze drying, and then forms a skeleton reinforcing phase, i.e., a polyimide microphase, by in-situ curing and crosslinking in the subsequent program temperature rise sintering process. Since the skeleton reinforcing phase is formed by in-situ curing and crosslinking 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; At the same time, the second reinforcing phase (fiber reinforced filler) of the high aspect ratio fiber is introduced, and 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 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 during the heat treatment of the aerogel, and 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 the fiber-reinforced filler. The in-situ curing and cross-linking of the skeleton reinforcement phase and the melting and interfacial diffusion of the PTFE particles occur simultaneously, so that the skeleton reinforcement phase and PTFE maintain a good interface effect. At the same time, the strong polarity of the skeleton reinforcement phase also makes it have a good interface effect with the fiber-reinforced filler; and the good hydrophilicity of the fiber-reinforced filler (hydrophilic contact angle is less than 90°) makes it dispersed evenly in the aqueous system, and it also has good interfacial wettability with the PTFE dispersion. This interfacial enhancement of the three-phase system will further improve the mechanical properties of the aerogel, and then obtain 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. The present invention overcomes the cracking and shrinkage phenomenon in the processing of PTFE aerogel by adding a skeleton reinforcement phase and a fiber-reinforced filler to the PTFE dispersion, and solves the problem of aerogel molding by freeze drying and gradually heating the process. The addition of the skeleton reinforcement phase improves the high temperature resistance and toughness of the aerogel and prevents the generation of microcracks; while the fiber-reinforced filler provides a network support structure, disperses the internal stress, and significantly improves the mechanical behavior and pore structure of the aerogel.

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

[0032] 2. The present invention adopts directional freezing technology and uses the different behaviors of the skeleton reinforcement phase and the fiber-reinforced filler during directional ice crystal growth to construct a network-supported layered channel structure. This design optimizes the heat conduction path, so that the final aerogel has low thermal conductivity in the radial direction perpendicular to the freezing direction, ensuring the excellent thermal insulation performance of the material. At the same time, due to the larger size of the fiber-reinforced filler, the orientation effect is smaller and it is distributed in a network, so that the mechanical properties of the aerogel in two directions are balanced.

[0033] 3. The present invention adopts PTFE dispersion as raw material, and the raw material contains a surfactant that assists the dispersion of PTFE particles. Charged surfactant molecules adsorbed on the surface of PTFE particles are used to generate charge interactions with polar molecules or ionic salts (corresponding to the 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 reinforcing material. This improvement significantly improves the uniformity of the material and the overall performance of the 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, which will be embedded in the PTFE matrix during the subsequent molding and heating sintering process, 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 fiber is introduced, and 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 to improve 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 during the heat treatment of the aerogel, achieving the maintenance of high porosity and the refinement and regulation of pore size.

[0035] 5. After heat treatment, the PTFE particles in the composite dispersion overlap and entangle with the skeleton reinforcement phase and the fiber-reinforced filler. The in-situ curing and cross-linking of the skeleton reinforcement phase and the melting and interfacial diffusion of the PTFE particles occur simultaneously, so that the skeleton reinforcement phase and PTFE have a good interface effect. At the same time, the strong polarity of the skeleton reinforcement phase also makes it have a good interface effect with the fiber-reinforced filler; and the good hydrophilicity of the fiber-reinforced filler (hydrophilic contact angle is less than 90°) makes it dispersed evenly in the aqueous system, and it also has good interfacial wettability with the PTFE dispersion. This interfacial enhancement of the three-phase system will further improve the mechanical properties of the aerogel, and thus obtain a low thermal conductivity polytetrafluoroethylene composite aerogel material with excellent comprehensive 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 The present invention is a flow chart of the preparation of polytetrafluoroethylene composite aerogel;

[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] The dianhydride monomer and the diamine monomer are mixed with an organic solvent to carry out a polymerization reaction, and the obtained polyimide precursor is mixed 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 the fiber-reinforced filler to obtain a functionally reinforced filler;

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

[0045] The composite dispersion liquid is placed in a mold and placed on a substrate, and liquid nitrogen is added to a metal container under the substrate to cool the substrate, and 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;

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

[0047] The invention mixes dianhydride monomers, diamine monomers and organic solvents for polymerization reaction, mixes the obtained polyimide precursor with an amino-containing organic base hydrophilic modifier for modification, and obtains 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, biphenyl tetracarboxylic 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 has no special limitation on the ratio of different types of reagents, and any ratio is acceptable.

[0049] In the present invention, the organic solvent is preferably N,N-dimethylacetamide, N-methylpyrrolidone or N,N-dimethylformamide. The present invention has no particular limitation on the amount of the organic solvent, which 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 completing the polymerization reaction, 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 the amino-containing organic base hydrophilic modifier is two or more of the above, the present invention has no special restrictions on the ratio of different types of reagents, and any ratio is acceptable. The present invention utilizes a 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 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-enhanced 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 40-60% of the total mass of the functional reinforcement filler; the mass of the fiber reinforced filler preferably accounts for 40-60% of the total mass of the functional reinforcement filler, the average length of the fiber reinforced filler is preferably 20-300 μm, and the fiber diameter is preferably 0.2-1 μm.

[0059] After obtaining the function-enhanced filler, the present invention mixes the PTFE dispersion, the function-enhanced 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, and more preferably 4-8:1-2:90-95.

[0062] In the present invention, the composite dispersion preferably further includes a thickener, and 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 waterborne polyurethane. The present invention has no special limitation on the waterborne polyurethane, and commercially available products well known in the art can be used. The present invention utilizes a thickener to improve the dispersibility of the filler and buffer the internal stress of the freezing process to avoid cracking.

[0063] After obtaining the composite dispersion, the present invention loads the composite dispersion into a mold, places the mold on a substrate, adds liquid nitrogen into a metal container under the substrate to cool the substrate, freezes the composite dispersion vertically upward from the bottom in contact with the substrate by cooling, and then freezes and dries it to obtain an aerogel.

[0064] In the present invention, the mold is preferably a square container of 30mm*30mm*50mm or a round container of 25mm in diameter. The mold is used to control the external dimensions of the PTFE aerogel. The material of the mold is preferably one or more of nitrile rubber, silicone rubber, polystyrene plastic and polycarbonate plastic. The present invention has no special restrictions on the specific preparation and source of the mold, which can be obtained in a manner well known in the art.

[0065] The present invention has no special limitation on the metal container, and any metal container known in the art that can hold liquid nitrogen 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 outer dimensions of the prepared PTFE. The present invention utilizes the substrate to achieve unidirectional heat conduction to 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 the aerogel is obtained, the invention demolds 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 of the above technical solution in the fields of aerospace, chemical industry, construction or environmental protection. The present invention has no special limitation on the application method, and the application can be carried out according to the method known in the art.

[0072] The technical solution provided by the present invention is described in detail below in conjunction with the embodiments, but they should not be understood as limiting the scope of protection of the present invention. The raw materials and equipment used in the specific implementation of the present invention are all known products, which are 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 are successively added to 90 g of N, N-dimethylacetamide, and after being evenly dispersed, the system is placed at room temperature for reaction for 8 hours to generate polyamic acid, and then 10.119 g (0.1 mol) of triethylamine is 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, and then the reaction system is 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 functionally enhanced filler;

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

[0086] Step 3: placing the composite dispersion in a 30*30*50 square mold, and then placing it on a copper plate with a thickness of 2 mm, adding liquid nitrogen in a 304 stainless steel container under the substrate to cool the substrate, the liquid nitrogen temperature is -196°C, and the composite dispersion is directionally frozen vertically upward from the bottom in contact with the substrate 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] 1) Thermal conductivity was tested using Hot Disk according to international standard ISO22007-2.2. The porosity of the aerogel was calculated to be 96.4% by the ratio of the aerogel density to the density of polytetrafluoroethylene. Due to the high porosity of the prepared aerogel and the oriented pore structure, the PTFE aerogel has obvious anisotropic thermal conductivity. The axial thermal conductivity was measured to be 0.041 W / (m·K) and the radial thermal conductivity was 0.020 W / (m·K).

[0092] 2) Aerogel compression test follows ASTM D1621 standard, and the axial and radial compressive strengths under 50% strain are 200KPa and 160KPa respectively at an experimental rate of 0.5mm / min, and the axial and radial strain recovery rates after 10 cycles of compression are 86% and 82% respectively. This also illustrates the effectiveness of the fiber-supported layered structure design.

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

[0094] 4) Under room temperature, the mechanical properties of the samples can be maintained at 95% of the samples before immersion after being immersed in 1M sodium hydroxide, hydrochloric acid, and sodium chloride for one month. At the same time, the samples can still maintain 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 retardant properties of the aerogel. In the vertical combustion experiment, the self-extinguishing time of the sample was 12 seconds and there was no dripping, which showed its good flame retardant properties.

[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 the fields of aerospace, chemical industry, construction, and environmental protection.

[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 are added to 90 g of N, N-dimethylacetamide successively, and after being evenly dispersed, the system is placed at room temperature for reaction for 8 hours to generate polyamic acid, and then 10.119 g of triethylamine is 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, and then the reaction system is precipitated in acetone to obtain a soluble polyimide precursor salt solid, i.e., a skeleton reinforcement phase;

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

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

[0109] Step 3: placing the prepared composite dispersion in a 30*30*50 square mold, and then placing it on a copper plate with a thickness of 3 mm, adding liquid nitrogen in a 304 stainless steel container under the substrate to cool the substrate, the liquid nitrogen temperature is -196°C, and the composite dispersion is directionally frozen vertically upward from the bottom in contact with the substrate 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, place the aerogel in a muffle furnace and calcine at 110° C., 210° C., and 310° C. for 70 min, respectively, and finally calcine at 380° C. for 20 min to obtain a polytetrafluoroethylene composite aerogel material.

[0112] The performance test of the aerogel prepared in this embodiment was carried out. The test method and test standard were the same as those in Example 1. The axial thermal conductivity was measured to be 0.083W / (m·K), the radial thermal conductivity was 0.031W / (m·K), the aerogel porosity was 94.2%, the axial and radial compressive strengths at 50% strain were 180KPa and 200KPa respectively, the axial and radial strain recovery rates measured after 10 cycles were 85% and 84% respectively, and the water contact angle was 158°. Therefore, while having excellent thermal insulation performance, the aerogel prepared in this embodiment has high mechanical strength and excellent recovery performance, and is suitable for applications in lightweight flexible devices and flexible 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 ratio, and after being evenly dispersed, place the system at room temperature for 12 hours to react to generate polyamic acid, and then add triethylamine to the reaction system according to the above ratio, stir at 25°C for 2.5 hours to form a salt to obtain a soluble polyimide precursor salt solution, and then precipitate the reaction system in acetone to obtain a soluble polyimide precursor salt solid, i.e., a 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 according to the above proportions, stir evenly, and obtain a composite dispersion;

[0125] Step 3: placing the prepared composite dispersion in a 30*30*50 square mold, and then placing it on a copper plate with a thickness of 5 mm, adding liquid nitrogen in a 304 stainless steel container under the substrate to cool the substrate, the liquid nitrogen temperature is -196°C, and the composite dispersion is directionally frozen vertically upward from the bottom in contact with the substrate 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, place the aerogel in a muffle furnace and calcine at 120° C., 220° C., and 320° C. for 50 min, respectively, and finally calcine at 390° C. for 20 min to obtain a polytetrafluoroethylene composite aerogel material.

[0128] The performance test of the aerogel prepared in this embodiment was carried out, and the test method and test standard were the same as those in Example 1. The axial thermal conductivity was measured to be 0.134 W / (m·K), the radial thermal conductivity was 0.038 W / (m·K), the aerogel porosity was 92.6%, the axial and radial compressive strengths at 50% strain were 620 KPa and 400 KPa respectively, and the water contact angle was 150.8°. The mechanical properties of the aerogel prepared in this embodiment are enhanced, and the thermal insulation performance is relatively excellent, and it can be widely used in the fields of 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 50-200 μm, and the fiber diameter is 0.2-0.5 μm;

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

[0134] Dianhydride (3,3',4,4'-biphenyltetracarboxylic acid 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 ratio, and after being evenly dispersed, place the system at room temperature for reaction for 10 hours to generate polyamic acid, and then add triethylenediamine solid according to the above ratio, stir at 25°C for 4 hours to form a salt, and obtain a soluble polyimide precursor salt solution, and then precipitate the reaction system in acetone to obtain a soluble polyimide precursor salt solid, that is, a skeleton reinforcement phase;

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

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

[0140] Step 3: placing the prepared composite dispersion in a circular mold with a diameter of 25 mm, and then placing it on a copper plate with a thickness of 3 mm, adding liquid nitrogen in a 304 stainless steel container under the substrate to cool the substrate, the liquid nitrogen temperature is -196°C, and the composite dispersion is directionally frozen vertically upward from the bottom in contact with the substrate 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, place the aerogel in a muffle furnace and calcine at 120° C., 220° C., and 320° C. for 1 h, respectively, and finally calcine at 380° C. for 30 min to obtain a polytetrafluoroethylene composite aerogel material.

[0143] The performance test of the aerogel prepared in this embodiment was carried out, and the test method and test standard were the same as those in Example 1. The axial thermal conductivity was measured to be 0.091 W / (m·K), the radial thermal conductivity was 0.0026 W / (m·K), the aerogel porosity was 94.1%, the axial and radial strain recovery rates after 10 cycles of compression were 94% and 90%, respectively, and the use temperature was ≥240°C. Therefore, while having excellent thermal insulation performance, the aerogel prepared in this embodiment has excellent mechanical recovery performance and is suitable for applications in flexible electronic devices, sensors, etc.

[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 performance test of the aerogel prepared in comparative example 1 was carried out, and the test method and test standard 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 0.11W / (m·K), and the radial thermal conductivity was 0.052W / (m·K). The axial thermal conductivity and radial thermal conductivity increased to 275% and 260% of the original, respectively, compared with Example 1, and the porosity decreased from 96.4% to 79.2%. This shows that the aerogel in the comparative example has undergone severe shrinkage, which means that cellulose is critical to the suppression of aerogel shrinkage. It was further measured that the axial and radial compressive strengths under 50% strain increased slightly, rising to 220KPa and 170Kpa, respectively, which was attributed to the increase in the density of the material, making the material denser and more difficult to deform when compressed. The axial and radial strain recovery rates of 10 cycles dropped 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 was measured to be 138° using an optical contact angle meter. 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 in 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 density of the aerogel increased, the pores increased, and the specific surface area decreased, resulting in a decrease in the viscous dissipation capacity and a significant deterioration in the sound insulation performance. The flame retardant performance is basically the same as that in Example 1. In the vertical combustion experiment, 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 performance test of the aerogel prepared in comparative example 2 was carried out, and the test method and test standard 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 0.039W / (m·K). The axial thermal conductivity and radial thermal conductivity increased to 117% and 195% of the original, respectively, compared with Example 1, and the porosity decreased from 96.4% to 94.2%. This shows that the aerogel in this comparative example has shrunk slightly. At the same time, the differential decrease in thermal conductivity in both 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 be significantly damaged in the axial direction, and the strength dropped significantly to 20Kpa, while the radial compressive strength also decreased to 54Kpa, which was attributed to the disappearance of the skeleton reinforcement phase. The decrease in interfacial effect caused the decrease in the strength of the aerogel. The axial and radial strain recovery rates after 10 cycles dropped to 42% and 59%, respectively, and the axial recovery performance dropped significantly, indicating that the disappearance of the polyimide microphase structure significantly deteriorated the mechanical properties of the aerogel. The sound insulation performance was tested and it was found that at a frequency of 1kHz, the sound insulation coefficient dropped to 15dB. This may be because the interface strength decreased, which reduced the viscous dissipation capacity of the system and significantly deteriorated the sound insulation performance. The flame retardant performance is basically the same as that of Example 1. In the vertical combustion experiment, the self-extinguishing time of the sample was 12 seconds, and there was no dripping, which showed its good flame retardant properties.

[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 principle of the present invention. These improvements and modifications should also be regarded as 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: The dianhydride monomer and the diamine monomer are mixed with an organic solvent to carry out a polymerization reaction, and the obtained polyimide precursor is mixed 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 the fiber-reinforced filler to obtain a functionally reinforced filler; Mixing the PTFE dispersion, the functional strengthening filler and water to obtain a composite dispersion; The composite dispersion liquid is placed in a mold and placed on a substrate, and liquid nitrogen is added to a metal container below the substrate to cool the substrate, and 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; After the aerogel is demoulded, it is heat treated by programmed temperature increase to obtain a polytetrafluoroethylene composite aerogel material.

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 of less than 90°, and the fiber-reinforced filler includes one or more of glass fiber, carbon fiber, cellulose and polyimide fiber; the cellulose includes nanofibrillated cellulose or cellulose nanocrystal.

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, the average length of the fiber reinforcement filler is 20-300 μm, and the fiber diameter is 0.2-1 μm.

5. The preparation method according to claim 1, characterized in that: The solid content of polytetrafluoroethylene in the PTFE dispersion is 40-80wt%, and the particle size is 0.1-0.5μm; 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 40-80min; the second stage temperature is 190-240°C and the sintering time is 40-80min; the third stage temperature is 280-330°C and the sintering time is 40-80min; the fourth stage temperature is 370-390°C and the sintering time is 20-40min.

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

10. Application 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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