A high-temperature resistant vermiculite / polyimide composite foam material and its preparation method

Abstract

A high-temperature resistant vermiculite / polyimide composite foam material and its preparation method are disclosed. The method comprises the following steps: Step 1, organic modification and exfoliation of vermiculite; Step 2, preparation of a foaming polyamic acid (PAA) resin solution; Step 3, formulation and homogenization of the foaming slurry to obtain a composite foaming slurry; Step 4, casting and pre-curing; and Step 5, a step-by-step foaming-imidization-vermiculite expansion synergistic reaction. The prepared material is particularly suitable for applications with extreme temperature resistance requirements, such as: secondary insulation layers in hypersonic vehicle thermal protection systems, external insulation for cryogenic tanks in reusable launch vehicles, high-temperature zone insulation for aero-engine nacelles, insulation of high-temperature steam pipelines in deep-well extra-heavy oil extraction, insulation of high-temperature reactors in chemical plants, and fire barriers in special buildings.

Description

Technical Field

[0001] This invention relates to the field of high-performance polymer-based composite materials technology, and in particular to a high-temperature resistant vermiculite / polyimide composite foam material and its preparation method. Background Technology

[0002] Polyimide (PI) foam, as one of the highest-performance polymer foam materials, has become a core material in high-end fields such as thermal and sound insulation for aerospace vehicles (e.g., passenger planes, helicopters), fire-resistant compartments for ships, and fireproof insulation for high-speed trains, thanks to its excellent heat resistance (long-term operating temperature 250-300℃), inherent flame retardancy (LOI>36%), low smoke and low toxicity, excellent chemical stability, and good mechanical properties. For example, in the aviation field, it is used for thermal and sound insulation of engine compartments and APU compartments; in deep space exploration, it is used in the thermal protection systems of spacecraft.

[0003] However, with the development of aerospace technology towards high Mach number vehicles (such as hypersonic vehicles) and reusable launch vehicles, as well as the urgent need for energy conservation in ultra-high temperature pipelines and kilns in the energy and chemical industries, existing polyimide foam materials face severe challenges. The surface skin of hypersonic vehicles can reach aerodynamic heating temperatures of hundreds to thousands of degrees Celsius during flight. Although active cooling and ceramic matrix composites serve as the main load-bearing layer of the thermal protection system (TPS), an efficient secondary insulation layer is still needed internally to prevent heat transfer into the cabin. This insulation layer must maintain structural integrity and low thermal conductivity for extended periods at temperatures of 400-600°C or even higher. Traditional polyimide foams undergo severe thermal decomposition, significant shrinkage, carbonization, and pulverization at these temperatures, leading to insulation failure. Simply modifying the PI molecular structure (such as introducing alkynyl groups, benzoxazoles, or phosphorus-based structures) to improve heat resistance often results in a sharp increase in precursor resin viscosity, deterioration of processability, difficulty in foaming, and increased brittleness, with limited improvement and extremely high costs.

[0004] Inorganic nano / micro filler composites are an effective strategy for improving the heat resistance, barrier properties, and mechanical properties of polymers. Vermiculite, a natural, abundant, and inexpensive layered silicate mineral, possesses a unique layered structure and high-temperature expansion characteristics. Its interlayers contain hydrated cations; at high temperatures (typically 800-1000℃), the interlayer water rapidly vaporizes, causing the layers to expand tens of times along the c-axis, forming low-density, high-porosity expanded vermiculite. It has an extremely low thermal conductivity (0.04 W / m•K to 0.07 W / m•K) and an extremely high melting point (>1300℃), making it an ideal inert thermal insulation filler.

[0005] However, directly combining expanded vermiculite with polyimide precursors to prepare high-performance foams faces fundamental challenges: 1. Dispersion problem: Expanded vermiculite is bulky, has low density and poor mechanical strength, and is very easy to agglomerate, float or settle in resin slurry, making it difficult to achieve uniform dispersion at the nano / micro scale.

[0006] 2. Structural damage: During the high-shear mixing process of the foaming process, the fragile porous structure of expanded vermiculite is easily damaged, losing its intrinsic lamellar structure and reinforcing potential.

[0007] 3. Weak interface: The surface of expanded vermiculite is inert and has weak interfacial bonding with the PI matrix, which becomes a stress concentration point and a rapid channel for heat and mass transfer, which may impair the overall performance.

[0008] 4. Process interference: A large amount of inorganic filler severely hinders bubble nucleation, growth and stability, resulting in uneven foam pore size, low closed-cell rate, uncontrolled density, and even failure to foam.

[0009] Therefore, existing technologies typically use expanded vermiculite as a simple physical filler, mixing it with binders to prepare rigid insulation boards, or combining it with elastomers to prepare low-frequency vibration damping materials. This fails to fully leverage its synergistic enhancement effect with high-performance polymers at the molecular / nanoscale. Overcoming these bottlenecks and perfectly combining the ultra-high temperature stability of vermiculite with the excellent comprehensive properties of polyimide in foam structures to achieve a synergistic effect of "1+1>2," and developing novel composite foams with significantly improved temperature resistance, remains a long-standing technical challenge in this field. Summary of the Invention

[0010] This invention provides a high-temperature resistant vermiculite / polyimide composite foam material and its preparation method to overcome the shortcomings of the prior art. Its temperature resistance, high-temperature dimensional stability and thermal insulation are significantly improved compared with pure polyimide foam.

[0011] In order to achieve the objectives of this invention, the following technologies are proposed: One method for preparing high-temperature resistant vermiculite / polyimide composite foam material is provided, including the following steps: Step 1: Organic modification and exfoliation of vermiculite; Step 2, Preparation of foamed polyamic acid (PAA) resin solution; Step 3: Preparation and homogenization of the foaming slurry to obtain a composite foaming slurry; Step 4, casting and pre-curing; Step 5: Step-by-step foaming-iminolation-vermiculite expansion synergistic reaction.

[0012] Further, in step 1, during the organic modification and exfoliation of vermiculite, the following method is used: Raw material pretreatment: Natural vermiculite ore with a complete layered structure is selected, crushed, ball-milled, and sieved to obtain vermiculite powder with a particle size distribution of 5 to 50 micrometers. The powder is treated with a 1 mol / L to 3 mol / L hydrochloric acid solution at 60 to 80°C for 4 to 8 hours to remove impurities such as carbonates and displace some of the interlayer exchangeable cations, increasing the interlayer spacing and reactivity. After acid washing, the powder is filtered, washed with deionized water until neutral, and dried at 80 to 100°C for 12 hours to obtain acidified vermiculite (H-VMT).

[0013] Organic Intercalation and Exfoliation: The acidified vermiculite described above is dispersed in deionized water or an ethanol / water mixture to prepare a suspension of 3 wt% to 8 wt%. An organic intercalating agent is added under stirring and heating (50°C to 70°C). The organic intercalating agent is selected from: long-chain alkyl quaternary ammonium salts (such as hexadecyltrimethylammonium bromide CTAB, octadecyltrimethylammonium chloride STAC); amino or hydroxyl-containing organic compounds (such as 12-aminododecanoic acid, p-aminobenzoic acid); and silane coupling agents (such as γ-aminopropyltriethoxysilane KH550, γ-methacryloyloxypropyltrimethoxysilane KH570). The amount of intercalating agent used is 1.0 to 2.5 times the cation exchange capacity (CEC) of the vermiculite. After reacting for 4 to 10 hours, the resulting mixture is subjected to high-energy treatment to achieve exfoliation. The high-energy treatment includes: high-speed homogenous shearing (>10000 rpm, 20 min to 40 min) or high-power ultrasonic treatment (400 W to 800 W, 1 h to 2 h).

[0014] Post-processing: The product after high-energy treatment was collected by high-speed centrifugation and washed multiple times with deionized water and ethanol to remove free intercalating agent. Finally, it was freeze-dried or spray-dried to obtain loose, free-flowing organic modified vermiculite powder, denoted as O-VMT. X-ray diffraction (XRD) characterization showed that its (002) characteristic diffraction peak shifted to a smaller angle, indicating that the interlayer spacing increased from about 0.76 nm in the original ore to about 1.4 nm. From the provided XRD pattern and combined with the Bragg equation (2dsinθ=nλ), under the condition of fixed X-ray wavelength, the shift of the diffraction peak to a smaller angle directly corresponds to the increase of the interplanar spacing (d). This proves that the modification treatment allows interlayer material to enter the vermiculite interlayer, thereby increasing the interlayer spacing.

[0015] Furthermore, the preparation of the foamed polyamic acid (PAA) resin solution in step 2 is carried out as follows: Under the protection of a dry inert gas (such as nitrogen or argon), aromatic diamine monomers in an equimolar ratio are dissolved in a polar aprotic solvent in a reaction vessel equipped with a stirrer, thermometer, and drying tube. Aromatic dianhydride monomers in an equimolar ratio to the diamine are added in batches under ice-water bath cooling (0°C to 5°C) and vigorous stirring, controlling the addition rate to keep the reaction system temperature below 30°C. After the addition is complete, the reaction continues at a low temperature (<20°C) for 4 to 8 hours to obtain a high-viscosity polyamic acid (PAA) solution.

[0016] The aromatic diamine may be selected from: 4,4'-diaminodiphenyl ether (ODA), p-phenylenediamine (p-PDA), 4,4'-diaminodiphenylmethane (MDA), 2,2'-bis[4-(4-aminophenoxy)phenyl]propane (BAPP), etc.

[0017] Aromatic dianhydrides can be selected from: pyromellitic dianhydride (PMDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), 4,4'-oxobisphthalic anhydride (ODPA), 2,2'-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA), etc.

[0018] The solvents used are N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), or a mixture thereof.

[0019] To obtain a suitable foaming viscosity, the solid content should be controlled between 10% and 20%.

[0020] To the prepared PAA solution, the following additives based on the PAA solid mass meter were added, and the mixture was stirred evenly at low temperature to obtain a foamable resin solution: Chemical foaming agent: The content is 2wt%, preferably a foaming agent with a decomposition temperature in the range of 180℃ to 250℃ that decomposes to produce inert gases such as N2 and CO2, such as azodicarbonamide (ADC, decomposition temperature 150℃ to 210℃), 4,4'-oxobisbenzenesulfonyl hydrazine (OBSH, decomposition temperature 140℃ to 160℃), 5-phenyltetrazole (PT), etc.

[0021] Surfactant (foam stabilizer): content is 0.2 wt%, used to reduce surface tension and stabilize bubbles, preferably polysiloxane-polyether copolymer, fluorocarbon surfactant, etc.

[0022] Iminolation accelerator: 8 wt% of a tertiary amine (such as pyridine, triethylamine, or isoquinoline); Imidizing dehydrating agent: an acid anhydride (such as acetic anhydride or phthalic anhydride) at a content of 6 wt%, which is combined with an imidization accelerator to promote the cyclization and dehydration of amide acid to generate imide and absorb the water generated in the reaction.

[0023] Furthermore, the preparation and homogenization of the foaming slurry in step 3 are carried out as follows: The organic vermiculite powder (O-VMT) obtained in step 1 is added to the same solvent as in step 2 according to the designed ratio, and ultrasonically dispersed (power 200W to 400W, time 0.5 hours to 1 hour) to prepare a uniform O-VMT suspension. This suspension is then slowly added to the foamable PAA resin solution prepared in step 2 under stirring. The amount of O-VMT added is a percentage of the total mass of the final composite foam solids (i.e., PAA solids + O-VMT), ranging from 0.5 wt% to 25 wt%, preferably 3 wt% to 12 wt%. If the amount added is too low, the reinforcing effect is not obvious, while if it is too high, the slurry viscosity will be too high, making it difficult for bubbles to grow and deteriorating the foam structure. The mixture is first mechanically stirred (500 rpm to 1000 rpm) at room temperature for 1 to 2 hours, and then treated by an ultrasonic cell disruptor or a high-speed homogenizer (power 300W to 600W, on / off pulse mode, total time 10 to 30 minutes) to ensure that O-VMT is uniformly dispersed at the nano / micro scale in the PAA solution, forming a stable, homogeneous composite foaming slurry without visible particles.

[0024] Furthermore, the casting and pre-curing in step 4 are carried out in the following manner: The composite foaming slurry is poured onto a horizontally placed clean glass plate, steel belt, or peelable polyimide film substrate, and the wet film thickness is controlled between 1.0 mm and 4.0 mm using an adjustable scraper. The substrate is then placed in a programmable temperature oven or tunnel drying chamber for step-by-step pre-curing under air or low-ventilation conditions. A typical procedure is: 40°C to 60°C for 1 to 3 hours, followed by 80°C to 100°C for 0.5 to 2 hours. This process aims to slowly remove most of the solvent, allowing the PAA molecular chains to form a physical cross-linked network (gelation) through hydrogen bonding and other interactions. Simultaneously, the foaming agent begins to slightly decompose, generating initial bubble nuclei, and the slurry transforms from a viscous liquid into a self-supporting "wet gel" film with a certain strength and elasticity. Precise control of the temperature and solvent evaporation rate at this stage is crucial for obtaining a uniform and fine cell structure.

[0025] Furthermore, the stepwise foaming-iminolation-vermiculite expansion synergistic reaction in step 5 proceeds as follows: Carefully remove the pre-cured gel film from the substrate (or transfer it along with the substrate) to a programmable temperature-controlled high-temperature oven, autoclave (at atmospheric or slightly negative pressure), or muffle furnace. Perform a precisely designed stepped heat treatment process in an air atmosphere (or an inert atmosphere such as nitrogen to obtain a higher char yield): Stage 1: Foaming Initiation and Initial Imidification (180℃ to 220℃). The temperature is increased to 200±10℃ at a rate of 1-2℃ / min and held at this temperature for 30 to 90 minutes. During this stage, the chemical foaming agent decomposes rapidly and in large quantities, producing gases (such as N2 and CO) that expand within the pre-gelled PAA network, forming the main cell structure. Simultaneously, PAA begins to undergo intramolecular dehydration cyclization (iminolation reaction), resulting in a sharp increase in viscosity. A pre-cured PI network begins to form, providing structural support for the cells and "locking in" the initial foam morphology. At this point, the system temperature is insufficient to trigger a dramatic expansion of vermiculite; the organically converted vermiculite is uniformly dispersed within the polymer matrix of the cell walls.

[0026] The second stage: deep imidization and confined expansion of vermiculite (280℃ to 350℃). The temperature is further increased to 300℃ to 320℃ at a rate of 1℃ / min to 1.5℃ / min, and held at this temperature for 60 to 150 minutes. Within this temperature range, the imidization reaction of PI tends to be complete, the rigidity of the molecular chains increases significantly, and the strength of the foam matrix is ​​significantly improved. Simultaneously, this temperature triggers the intense vaporization of the interlayer bound water in vermiculite (including small molecules generated by the decomposition of the intercalating agent). Since the vermiculite sheets are now firmly embedded in the partially solidified but still somewhat flexible PI matrix, their thermal expansion is strongly constrained by the three-dimensional PI network. This "confined expansion" results in: firstly, the fine vermiculite sheets cannot expand disorderly and loosely by tens of times as in their free state, but rather expand in a direction conducive to their expansion, i.e., mainly along a direction parallel to the cell walls, resulting in limited and ordered interlayer expansion (the expansion factor is usually controlled between 5 and 15 times). Secondly, some fully intercalated vermiculite can be further exfoliated into thinner nanosheets. Finally, the "expansion force" generated by the expansion makes the vermiculite sheets bond more tightly to the PI matrix. At the same time, the functional groups at the ends of the intercalating agent (such as -NH2, -COOH, -CH=CH2) may react chemically with the ends or segments of the PI molecular chain to form chemical bonds, greatly enhancing the interfacial bonding. This process forms a unique "brick (vermiculite sheets) - mud (PI matrix)" reinforced structure.

[0027] The third stage: post-curing and structural relaxation (350℃ to 400℃). The temperature is increased to 370℃ to 390℃ at a rate of 0.5℃ / min to 1℃ / min, and held at this temperature for 30 to 60 minutes. This stage aims to complete the imidization reaction, eliminate internal stress, improve the crystallinity of PI, and further stabilize the micro and macroscopic structure of the composite foam. Finally, the temperature is programmed to decrease at a rate not exceeding 2℃ / min or cooled in the furnace to room temperature to obtain the final high-temperature resistant vermiculite / polyimide composite foam material.

[0028] On the other hand, a high-temperature resistant vermiculite / polyimide composite foam material is also provided, which has the following characteristics and properties: Macroscopic appearance: The color is usually light gold to brownish-red, the texture is soft and elastic, the closed-cell rate is high, and the pores are uniform and fine.

[0029] Microstructure: Scanning electron microscopy (SEM) revealed that the foam has a three-dimensional interconnected or partially closed porous structure with an average pore size ranging from 50 μm to 300 μm. Under high-magnification SEM, the pore walls are not smooth but are embedded with a large number of lamellar materials (vermiculite). The lamellar planes are mostly arranged parallel to the pore wall direction and have no visible gaps with the PI matrix, showing a strong bond.

[0030] Basic physical properties: density can reach 30 kg / m³ 3 Up to 120 kg / m 3 Within the specified range, through formulation and process adjustments, 50 kg / m³ is preferred. 3 Up to 80 kg / m 3 Within this density range, the room temperature thermal conductivity can be as low as 0.026 W / (m·K) to 0.035 W / (m·K).

[0031] Heat resistance and thermophysical properties: Long-term heat resistance: After 1000 hours of heat aging in a static air atmosphere at 350℃, the mass loss rate is ≤5% and the compressive strength (10% deformation) retention rate is ≥80%.

[0032] High-temperature dimensional stability: After exposure in a muffle furnace at 600℃ for 30 minutes, the linear shrinkage rate (volume direction) is ≤4%, the mass retention rate is ≥92%, and the foam basically maintains its original shape without melting, flowing or collapsing.

[0033] Thermal stability: Under nitrogen atmosphere, the 5% thermal weight loss temperature (Td5%) is more than 20°C higher than that of pure PI foam with the same formula, and the char rate at 800°C is increased by 5 to 15 percentage points.

[0034] Flame retardancy: Limiting oxygen index (LOI) ≥48%, vertical burning reaches UL94 V-0 rating, and smoke density rating (SDR) is significantly reduced.

[0035] Mechanical properties: At similar densities, the compressive strength, flexural strength, and modulus are increased by 20% to 50% compared to pure PI foam.

[0036] The advantages of the above technical solution are: This invention abandons the simplistic approach of directly adding pre-expanded vermiculite, and innovatively adopts an "in-situ restricted expansion" strategy. Through precise control of the heat treatment process, the expansion of vermiculite (200℃ to 300℃) and the curing and shaping of polyimide (200℃ to 350℃) are precisely coordinated in time and space. The vermiculite expands under the constraint of the PI network, and the PI network becomes more heat-resistant due to the reinforcement and barrier effect of the vermiculite. The two mutually promote each other, achieving a synergistic reinforcement effect of "1+1>2".

[0037] This invention, through organic modification, not only increases the compatibility of vermiculite with solvents and resins, but also allows its terminal reactive functional groups to chemically interact with PI chains during heat treatment, thereby constructing a strong interfacial bond between inorganic vermiculite and organic PI. This effectively transfers stress, hinders crack propagation, and prevents the diffusion of thermal degradation products. High-energy processing and slurry homogenization processes ensure uniform dispersion of vermiculite at the nano / micro scale, preventing agglomeration.

[0038] The composite foam prepared by this invention has a long-term service temperature that is increased from about 300°C to 350°C to 400°C for pure PI foam, and a short-term tolerance temperature that is increased from about 500°C to 600°C to 650°C. The dimensional stability and mass retention rate at high temperatures are greatly improved, solving the bottleneck problem of PI foam being prone to failure at extreme high temperatures.

[0039] This invention significantly improves heat resistance while simultaneously optimizing the material's thermal insulation, flame retardant properties, mechanical strength, and modulus. The vermiculite flakes act as a physical barrier within the cell walls, extending the heat conduction path and reducing the thermal conductivity. Its non-flammability and char-promoting effect further enhance the flame retardant rating.

[0040] The raw material used in this invention, vermiculite, is widely available and inexpensive, and the synthesis and foaming processes for PI precursors are mature. The method of this invention is based on existing PI foam production equipment, requiring no major modifications; it can be achieved by optimizing process parameters and has the potential for large-scale production.

[0041] The materials prepared by this invention are particularly suitable for fields with extreme requirements for temperature resistance, such as: secondary insulation layers of hypersonic vehicle thermal protection systems, external insulation of cryogenic storage tanks of reusable launch vehicles, high-temperature zone insulation of aero-engine nacelles, insulation of high-temperature steam pipelines for deep-well extra-heavy oil extraction, insulation of high-temperature reactors in chemical plants, and fire barriers for special buildings. Attached Figure Description

[0042] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will now be described in further detail with reference to the accompanying drawings.

[0043] Figure 1 The XRD patterns of the organically modified vermiculite of Example 1 after exfoliation and natural vermiculite are shown.

[0044] Figure 2 The microstructure of the foam in Comparative Example 1 (pure PI foam) is shown.

[0045] Figure 3 The microstructure of the foam cells in Example 1 (5% O-VMT / PI composite foam) is shown.

[0046] Figure 4 A diagram of the composite foam prepared in Example 1 is shown.

[0047] Figure 5 A diagram of the composite foam prepared in Comparative Example 1 is shown. Detailed Implementation

[0048] Main raw materials used in the examples and comparative examples: Vermiculite: Natural phlogopite-type vermiculite, produced in Lingshou, Hebei Province, with a particle size D50=30μm and CEC≈100 meq / 100g.

[0049] Aromatic diamine: 4,4'-diaminodiphenyl ether (ODA), chemically pure.

[0050] Aromatic dianhydride: Pyromellitic dianhydride (PMDA), chemically pure.

[0051] Solvent: N,N-dimethylacetamide (DMAc), reagent grade.

[0052] Organic intercalating agents: hexadecyltrimethylammonium bromide (CTAB), analytical grade; γ-aminopropyltriethoxysilane (KH550), industrial grade.

[0053] Foaming agent: Azodicarbonamide (ADC), industrial grade.

[0054] Surfactant: Organosilicon surfactant (DC-193).

[0055] Iminolation accelerator: pyridine, analytical grade.

[0056] Imidizing dehydrating agent: acetic anhydride, analytical grade.

[0057] Testing and characterization methods: Apparent density: determined according to GB / T 6343-2009.

[0058] Cell structure: Observed using a scanning electron microscope (SEM, Hitachi SU-8010).

[0059] Thermal conductivity: Tested at 25°C and 200°C using a Hot Disk TPS 2500S thermal constant analyzer.

[0060] Thermogravimetric analysis (TGA): Using a Netzsch STA 449 F3 thermometer under nitrogen atmosphere at a heating rate of 10 °C / min, Td5% and char residue at 800 °C were recorded.

[0061] Limiting oxygen index (LOI): determined according to GB / T 2406.2-2009.

[0062] Vertical burning (UL94): Determined according to ANSI / UL 94-2013.

[0063] Compression performance: According to GB / T 8813-2008, use a universal testing machine, compression rate 2 mm / min, and take the stress value at 10% deformation.

[0064] High-temperature thermal aging: Place the sample (50×50×20 mm³) in an air-circulating oven at a set temperature, periodically remove it, weigh it, and test its compressive strength.

[0065] High-temperature thermal shock: The sample (50×50×20 mm³) was placed in a muffle furnace preheated to 600℃ and kept for 30 minutes. After cooling, the size and mass changes were measured, and the linear shrinkage rate and mass retention rate were calculated.

[0066] Example 1 Step 1, Organic modification of vermiculite: 100g of acidified vermiculite (H-VMT) was dispersed in 2L of deionized water, and 40g of CTAB (approximately 1.8 times the amount of CEC) was added. The mixture was stirred at 70℃ for 6 hours. The reaction solution was treated with a high-speed disperser (13000 rpm) for 30 minutes, followed by ultrasonic treatment (600W) for 1.5 hours. After centrifugation, washing with water, and washing with alcohol three times, the mixture was freeze-dried to obtain organified vermiculite O-VMT-1. XRD showed that its d(002) interlayer spacing increased from approximately 0.76 nm in the original ore to approximately 1.4 nm.

[0067] Step 2, Preparation of foamable PAA solution: Under nitrogen protection, 19.82 g of ODA was dissolved in 300 g of DMAc. Under ice-water bath cooling, 21.81 g of PMDA was added in portions, with the temperature controlled below 25 °C, and the reaction was carried out for 6 hours to obtain a PAA solution with a solid content of approximately 12.5%. Then, 0.52 g of ADC (2 wt%), 0.05 g of DC-193 (0.2 wt%), 2.08 g of pyridine (8 wt%), and 1.56 g of acetic anhydride (6 wt%) were added, and the mixture was stirred until homogeneous.

[0068] Step 3, preparation of composite slurry: Weigh 1.30 g of O-VMT-1 (5 wt% of the final solid mass), add 20 g of DMAc, and ultrasonically disperse for 40 minutes. Add the above PAA solution while stirring, stir at room temperature for 1 hour, and then ultrasonically treat (400 W, pulse mode) for 20 minutes to obtain a homogeneous slurry.

[0069] Step 4, Casting and Pre-curing: The slurry was coated onto a glass plate to a wet film thickness of 2.0 mm. The plate was then placed in an oven at 50°C for 2 hours, followed by 90°C for 1 hour to obtain a self-supporting gel film.

[0070] Step 5, Stepped foaming - imidization: The gel membrane was transferred to a high-temperature oven and the temperature was programmed as follows: from 2℃ / min to 200℃, held for 60 min; from 1℃ / min to 320℃, held for 120 min; from 1℃ / min to 380℃, held for 40 min; then allowed to cool naturally. A light gold composite foam was obtained, denoted as PI / VMT-5%.

[0071] Example 2 Step 1, Organic modification of vermiculite: 100g of acidified vermiculite (H-VMT) was dispersed in 2L of deionized water, and 40g of CTAB (approximately 1.8 times the amount of CEC) was added. The mixture was stirred at 70℃ for 6 hours. The reaction solution was treated with a high-speed disperser (13000 rpm) for 30 minutes, followed by ultrasonic treatment (600W) for 1.5 hours. After centrifugation, washing with water, and washing with alcohol three times, the mixture was freeze-dried to obtain organo-modified vermiculite O-VMT-1.

[0072] Step 2, Preparation of foamable PAA solution: Under nitrogen protection, 19.82 g of ODA was dissolved in 300 g of DMAc. Under ice-water bath cooling, 21.81 g of PMDA was added in portions, with the temperature controlled below 25 °C, and the reaction was carried out for 6 hours to obtain a PAA solution with a solid content of approximately 12.5%. Then, 0.52 g of ADC (2 wt%), 0.05 g of DC-193 (0.2 wt%), 2.08 g of pyridine (8 wt%), and 1.56 g of acetic anhydride (6 wt%) were added, and the mixture was stirred until homogeneous.

[0073] Step 3, preparation of composite slurry: Weigh 0.65 g of O-VMT-1 (2.5 wt% of the final solid mass), add 20 g of DMAc, and ultrasonically disperse for 40 minutes. Add the above PAA solution while stirring, stir at room temperature for 1 hour, and then ultrasonically treat (400 W, pulse mode) for 20 minutes to obtain a homogeneous slurry.

[0074] Step 4, Casting and Pre-curing: The slurry was coated onto a glass plate to a wet film thickness of 2.0 mm. The plate was then placed in an oven at 50°C for 2 hours, followed by 90°C for 1 hour to obtain a self-supporting gel film.

[0075] Step 5, Stepped foaming - imidization: The gel membrane was transferred to a high-temperature oven and the temperature was programmed as follows: from 2℃ / min to 200℃, held for 60 min; from 1℃ / min to 320℃, held for 120 min; from 1℃ / min to 380℃, held for 40 min; then allowed to cool naturally. A light golden composite foam was obtained, denoted as PI / VMT-2.5%.

[0076] Example 3 Step 1, Organic modification of vermiculite: 100g of acidified vermiculite (H-VMT) was dispersed in 2L of deionized water, and 40g of CTAB (approximately 1.8 times the amount of CEC) was added. The mixture was stirred at 70℃ for 6 hours. The reaction solution was treated with a high-speed disperser (13000 rpm) for 30 minutes, followed by ultrasonic treatment (600W) for 1.5 hours. After centrifugation, washing with water, and washing with alcohol three times, the mixture was freeze-dried to obtain organo-modified vermiculite O-VMT-1.

[0077] Step 2, Preparation of foamable PAA solution: Under nitrogen protection, 19.82 g of ODA was dissolved in 300 g of DMAc. Under ice-water bath cooling, 21.81 g of PMDA was added in portions, with the temperature controlled below 25 °C, and the reaction was carried out for 6 hours to obtain a PAA solution with a solid content of approximately 12.5%. Then, 0.52 g of ADC (2 wt%), 0.05 g of DC-193 (0.2 wt%), 2.08 g of pyridine (8 wt%), and 1.56 g of acetic anhydride (6 wt%) were added, and the mixture was stirred until homogeneous.

[0078] Step 3, preparation of composite slurry: Weigh 1.95g of O-VMT-1 (7.5 wt% of the final solid mass), add 20g of DMAc, and ultrasonically disperse for 40 minutes. Add the above PAA solution while stirring, stir at room temperature for 1 hour, and then ultrasonically treat (400W, pulse mode) for 20 minutes to obtain a homogeneous slurry.

[0079] Step 4, Casting and Pre-curing: The slurry was coated onto a glass plate to a wet film thickness of 2.0 mm. The plate was then placed in an oven at 50°C for 2 hours, followed by 90°C for 1 hour to obtain a self-supporting gel film.

[0080] Step 5, Stepped foaming - imidization: The gel membrane was transferred to a high-temperature oven and the temperature was programmed to rise: from 2℃ / min to 200℃ and hold for 60 min; from 1℃ / min to 320℃ and hold for 120 min; from 1℃ / min to 380℃ and hold for 40 min; then allowed to cool naturally. A light golden composite foam was obtained, denoted as PI / VMT-7.5%.

[0081] Example 4 Step 1, Organic modification of vermiculite: 100g of acidified vermiculite (H-VMT) was dispersed in 2L of deionized water, and 40g of CTAB (approximately 1.8 times the amount of CEC) was added. The mixture was stirred at 70℃ for 6 hours. The reaction solution was treated with a high-speed disperser (13000 rpm) for 30 minutes, followed by ultrasonic treatment (600W) for 1.5 hours. After centrifugation, washing with water, and washing with alcohol three times, the mixture was freeze-dried to obtain organo-modified vermiculite O-VMT-1.

[0082] Step 2, Preparation of foamable PAA solution: Under nitrogen protection, 19.82 g of ODA was dissolved in 300 g of DMAc. Under ice-water bath cooling, 21.81 g of PMDA was added in portions, with the temperature controlled below 25 °C, and the reaction was carried out for 6 hours to obtain a PAA solution with a solid content of approximately 12.5%. Then, 0.52 g of ADC (2 wt%), 0.05 g of DC-193 (0.2 wt%), 2.08 g of pyridine (8 wt%), and 1.56 g of acetic anhydride (6 wt%) were added, and the mixture was stirred until homogeneous.

[0083] Step 3, preparation of composite slurry: Weigh 2.60 g of O-VMT-1 (10 wt% of the final solid mass), add 20 g of DMAc, and ultrasonically disperse for 40 minutes. Add the above PAA solution while stirring, stir at room temperature for 1 hour, and then ultrasonically treat (400 W, pulse mode) for 20 minutes to obtain a homogeneous slurry.

[0084] Step 4, Casting and Pre-curing: The slurry was coated onto a glass plate to a wet film thickness of 2.0 mm. The plate was then placed in an oven at 50°C for 2 hours, followed by 90°C for 1 hour to obtain a self-supporting gel film.

[0085] Step 5, Stepped foaming - imidization: The gel membrane was transferred to a high-temperature oven and the temperature was programmed as follows: from 2℃ / min to 200℃, held for 60 min; from 1℃ / min to 320℃, held for 120 min; from 1℃ / min to 380℃, held for 40 min; then allowed to cool naturally. A light golden composite foam was obtained, denoted as PI / VMT-10%.

[0086] Example 5 Step 1, Organic modification of vermiculite: 100 g of H-VMT was dispersed in an ethanol / water (4:1 v / v) mixture, 20 g of KH550 was added, and the pH was adjusted to 5 with acetic acid. The mixture was refluxed at 70 °C for 8 hours. The reaction solution was treated with a high-speed disperser (13000 rpm) for 30 minutes, followed by ultrasonic treatment (600 W) for 1.5 hours. After centrifugation, washing with water, and washing with alcohol three times, the solution was freeze-dried to obtain organo-modified vermiculite O-VMT-2.

[0087] Step 2, Preparation of foamable PAA solution: Under nitrogen protection, 19.82 g of ODA was dissolved in 300 g of DMAc. Under ice-water bath cooling, 21.81 g of PMDA was added in portions, with the temperature controlled below 25 °C, and the reaction was carried out for 6 hours to obtain a PAA solution with a solid content of approximately 12.5%. Then, 0.52 g of ADC (2 wt%), 0.05 g of DC-193 (0.2 wt%), 2.08 g of pyridine (8 wt%), and 1.56 g of acetic anhydride (6 wt%) were added, and the mixture was stirred until homogeneous.

[0088] Step 3, preparation of composite slurry: Weigh 1.30g of O-VMT-2 (5 wt% of the final solid mass), add 20g of DMAc, and ultrasonically disperse for 40 minutes. Add the above PAA solution while stirring, stir at room temperature for 1 hour, and then ultrasonically treat (400W, pulse mode) for 20 minutes to obtain a homogeneous slurry.

[0089] Step 4, Casting and Pre-curing: The slurry was coated onto a glass plate to a wet film thickness of 2.0 mm. The plate was then placed in an oven at 50°C for 2 hours, followed by 90°C for 1 hour to obtain a self-supporting gel film.

[0090] Step 5, Stepped foaming - imidization: The gel membrane was transferred to a high-temperature oven and the temperature was programmed to rise: from 2℃ / min to 200℃ and hold for 60 min; from 1℃ / min to 320℃ and hold for 120 min; from 1℃ / min to 380℃ and hold for 40 min; then allowed to cool naturally. A light gold composite foam was obtained, denoted as PI / KH550-VMT-5%.

[0091] Example 6 Step 1, Organic modification of vermiculite: 100g of acidified vermiculite (H-VMT) was dispersed in 2L of deionized water, and 40g of CTAB (approximately 1.8 times the amount of CEC) was added. The mixture was stirred at 70℃ for 6 hours. The reaction solution was treated with a high-speed disperser (13000 rpm) for 30 minutes, followed by ultrasonic treatment (600W) for 1.5 hours. After centrifugation, washing with water, and washing with alcohol three times, the mixture was freeze-dried to obtain organo-modified vermiculite O-VMT-1. XRD showed that its d(002) increased from 1.42 nm to 2.65 nm.

[0092] Step 2, Preparation of foamable PAA solution: Under nitrogen protection, 19.82 g BPDA was dissolved in 300 g DMAc. Under ice-water bath cooling, 21.81 g p-PDA was added in portions, with the temperature controlled below 25 °C, and the reaction was carried out for 6 hours to obtain a PAA solution with a solid content of approximately 15%. Then, 0.52 g ADC (2 wt%), 0.05 g DC-193 (0.2 wt%), 2.08 g pyridine (8 wt%), and 1.56 g acetic anhydride (6 wt%) were added and stirred until homogeneous.

[0093] Step 3, preparation of composite slurry: Weigh 1.30 g of O-VMT-1 (5 wt% of the final solid mass), add 20 g of DMAc, and ultrasonically disperse for 40 minutes. Add the above PAA solution while stirring, stir at room temperature for 1 hour, and then ultrasonically treat (400 W, pulse mode) for 20 minutes to obtain a homogeneous slurry.

[0094] Step 4, Casting and Pre-curing: The slurry was coated onto a glass plate to a wet film thickness of 2.0 mm. The plate was then placed in an oven at 50°C for 2 hours, followed by 90°C for 1 hour to obtain a self-supporting gel film.

[0095] Step 5, Stepped foaming - imidization: The gel membrane was transferred to a high-temperature oven and the temperature was programmed as follows: from 2℃ / min to 200℃, held for 60 min; from 1℃ / min to 320℃, held for 120 min; from 1℃ / min to 380℃, held for 40 min; then allowed to cool naturally. A light gold composite foam was obtained, denoted as BPDA-PI / VMT-5%.

[0096] Comparative Example 1 (Pure PI Foam) Step 1, Preparation of foamable PAA solution: Under nitrogen protection, 19.82 g of ODA was dissolved in 300 g of DMAc. Under ice-water bath cooling, 21.81 g of PMDA was added in portions, with the temperature controlled below 25 °C, and the reaction was carried out for 6 hours to obtain a PAA solution with a solid content of approximately 12.5%. Then, 0.52 g of ADC (2 wt%), 0.05 g of DC-193 (0.2 wt%), 2.08 g of pyridine (8 wt%), and 1.56 g of acetic anhydride (6 wt%) were added, and the mixture was stirred until homogeneous.

[0097] Step 2, Casting and Pre-curing: The slurry was coated onto a glass plate to a wet film thickness of 2.0 mm. The plate was then placed in an oven at 50°C for 2 hours, followed by 90°C for 1 hour to obtain a self-supporting gel film.

[0098] Step 3, Stepped foaming - imidization: The gel membrane was transferred to a high-temperature oven and the temperature was programmed to rise at 2℃ / min to 200℃ and hold for 60 min; at 1℃ / min to 320℃ and hold for 120 min; at 1℃ / min to 380℃ and hold for 40 min; then allowed to cool naturally. A light golden composite foam was obtained, denoted as Neat PI Foam.

[0099] Comparative Example 2 (Physically Mixed Expanded Vermiculite) Instead of using organic vermiculite, we directly purchase commercially available 20-40 mesh expanded vermiculite, and simply grind and sieve it.

[0100] In step S3 of Example 1, an equal mass (5 wt% of solids) of expanded vermiculite powder was added to the PAA solution and mechanically stirred for 30 minutes (uneven dispersion could not be achieved, and a large amount of agglomeration occurred). Subsequent steps were the same as in Example 1. The resulting sample was designated PI / ExVMT-blend. Its foam structure was coarse and uneven, with vermiculite particles visible to the naked eye.

[0101] Performance data table of examples and comparative examples Based on the table above and as shown below Figures 1 to 3 As shown, we know that: Structure and basic properties: As shown in the table, the composite foams prepared by the method of this invention (Examples 1-6) have a smaller average pore size than pure PI foam (Comparative Example 1) at similar densities, indicating that uniformly dispersed O-VMT can serve as effective heterogeneous nucleation sites, refining the pores. The thermal conductivity is generally lower than that of the comparative example, especially at high temperatures (200℃), where the thermal conductivity decreases significantly, thanks to the thermal barrier effect of the vermiculite sheets.

[0102] Thermal stability: The Td5% and char residue at 800°C of all examples were significantly higher than those of Comparative Example 1, and increased with increasing vermiculite content (Examples 1, 3, 4). This indicates that the vermiculite sheets effectively hindered heat and mass transfer, delayed the decomposition of the PI matrix, and promoted the formation of higher char residue. Comparative Example 2 showed limited improvement due to dispersion and interface issues.

[0103] Flame retardancy: The LOI value increases significantly with the increase of vermiculite content, indicating that vermiculite plays a synergistic role in flame retardancy in both the gas phase and condensed phase.

[0104] Mechanical properties: The compressive strength of the composite foam was generally improved by more than 20%, proving that the well-dispersed and interfacially bonded vermiculite layers played an effective reinforcing role. Comparative Example 2, due to poor interfacial structure and uneven dispersion, showed no improvement in strength and even a slight decrease.

[0105] High temperature performance: Thermal shock at 600℃: The composite foam of this invention exhibits overwhelming advantages. Comparative Example 1 showed a linear shrinkage of 15.2% and a mass loss of nearly 25%, with severe foam deformation and shrinkage. While Comparative Example 2 showed slight improvement, its structure was uneven and the edges were powdery. The linear shrinkage rates of Examples 1-4 were all below 6%, and the mass retention rates were above 88%. Among them, Example 4 (10% content) had a shrinkage rate of only 2.5% and a mass retention rate of 94.2%, with its appearance basically remaining unchanged. This directly proves that the robust "brick-and-mortar" structure formed by "restricted expansion" can effectively support the foam skeleton and inhibit shrinkage and decomposition under extreme high temperatures.

[0106] Long-term thermal aging at 350℃: After 1000 hours of aging, the pure PI foam lost more than half of its strength, while the composite foam in the example generally retained more than 80% of its strength, and the mass loss was also much smaller. This indicates that the composite foam has superior structural stability and lifespan under long-term medium and high temperatures.

[0107] Effects of different modifiers on the PI system: Example 5 (KH550 modified) and Example 1 (CTAB modified) have comparable performance, indicating that silane coupling agents can also achieve effective interfacial modification. Example 6 (BPDA / p-PDA system) has higher heat resistance and more outstanding performance after composite, indicating that the method of the present invention has good universality with different PI systems.

[0108] In summary, this invention, through an ingenious design of "organic intercalation – slurry composite – restricted expansion and simultaneous curing," successfully prepared vermiculite / polyimide composite foam with significantly improved temperature resistance, high-temperature dimensional stability, and overall performance. Its core performance, particularly its dimensional stability under 600℃ high-temperature thermal shock, far surpasses that of existing pure PI foam, solving a key bottleneck in its application in extreme environments.

[0109] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Obviously, those skilled in the art can make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention also intends to include these modifications and variations.

Claims

1. A high-temperature resistant vermiculite / polyimide composite foam material, characterized in that: Its density is 30 kg / m³ 3 Up to 120 kg / m 3 ; Its room temperature thermal conductivity ranges from 0.026 W / (m•K) to 0.035 W / (m•K); After 1000 hours of heat aging at 350℃, the mass loss rate is ≤5%, and the compressive strength retention rate is ≥80% under 10% deformation; After exposure at 600℃ for 30 minutes, the linear shrinkage in the volume direction is ≤4%, and the mass retention rate is ≥92%. Under a nitrogen atmosphere, the carbon residue rate at 800℃ increases by 5 to 15 percentage points; Limiting oxygen index ≥48%, vertical combustion reaches UL94 V-0 rating.

2. A method for preparing the high-temperature resistant vermiculite / polyimide composite foam material as described in claim 1, characterized in that, Including the following steps: Step 1, organic modification and exfoliation of vermiculite; Step 2, Preparation of foamed polyamic acid (PAA) resin solution; Step 3: Preparation and homogenization of the foaming slurry to obtain a composite foaming slurry; Step 4, casting and pre-curing; Step 5: Step-by-step foaming-iminolation-vermiculite expansion synergistic reaction.

3. The method for preparing high-temperature resistant vermiculite / polyimide composite foam material according to claim 2, characterized in that, The organic modification and exfoliation of vermiculite in step 1 includes the following steps: Step 100: The natural vermiculite ore is crushed, ball-milled, and sieved to obtain vermiculite powder with a particle size distribution of 5 to 50 micrometers; Step 101: The obtained vermiculite powder is treated with a hydrochloric acid solution with a concentration of 1 mol / L to 3 mol / L at 60°C to 80°C for 4 to 8 hours. Step 102: The acid-washed vermiculite powder is filtered, washed with deionized water until neutral, and dried at 80°C to 100°C for 12 hours to obtain acidified vermiculite H-VMT. Step 103: Disperse the obtained acidified vermiculite H-VMT in deionized water or an ethanol / water mixed solvent to prepare a suspension of 3 wt% to 8 wt%. Step 104: Add an organic intercalating agent to the suspension and react for 4-10 hours under stirring and at 50°C to 70°C. Step 105: The mixture obtained in step 104 is homogenized and sheared at >10000 rpm for 20 to 40 minutes and / or sonicated at 400 W to 800 W for 1 to 2 hours to achieve exfoliation; Step 106: Collect the product after step 105 by centrifugation, and wash the collected product multiple times with deionized water and ethanol. Step 107: The product obtained in step 106 is freeze-dried or spray-dried to obtain organically modified vermiculite powder with an interlayer spacing of 1.8 nm to 3.5 nm.

4. The method for producing high-temperature resistant vermiculite / polyimide composite foam material according to claim 3, characterized in that, In step 104, the organic intercalating agent is one or more of the following: long-chain alkyl quaternary ammonium salt, organic compounds containing amino or hydroxyl groups, and silane coupling agents; The amount of organic intercalating agent used is 1 to 2.5 times the cation exchange capacity of vermiculite.

5. The method for producing high-temperature resistant vermiculite / polyimide composite foam material according to claim 2, characterized in that, The preparation of the foamed polyamic acid (PAA) resin solution in step 2 is carried out through the following steps: Step 200: Under an inert gas atmosphere, dissolve equimolar amounts of aromatic diamine monomers in a polar aprotic solvent. Step 201: At 0°C to 5°C, add aromatic dianhydride monomer in batches to the product obtained in step 200 in an equimolar ratio with the aromatic diamine monomer, and control the reaction temperature to be below 30°C during addition. Step 202: React at a temperature below 20°C for 4 to 8 hours to obtain a polyamic acid solution with a solid content of 10 wt% to 20 wt%. Step 203: Add 2 wt% foaming agent, 0.2 wt% foam stabilizer, 8 wt% imidization accelerator, and 6 wt% imidization dehydrating agent to the polyamic acid solution obtained in step 202.

6. The method for producing high-temperature resistant vermiculite / polyimide composite foam material according to claim 2, characterized in that, Step 3, the preparation and homogenization of the foaming slurry, includes the following steps: Step 300: Add the organo-modified vermiculite powder obtained in Step 1 to the solvent used in Step 2 to prepare the foamed polyamic acid PAA resin solution, and ultrasonically disperse it at 200W to 400W for 0.5 hours to 1 hour to form an organo-modified vermiculite powder suspension. Step 301: The organic vermiculite powder suspension obtained in step 300 is added to the foamed polyamic acid (PAA) resin solution obtained in step 2. When adding, the amount of organic vermiculite powder suspension added accounts for 0.5 wt% to 25 wt% of the total mass of the high-temperature resistant vermiculite / polyimide composite foam material. Step 302: At room temperature, the product obtained in step 301 is mechanically stirred at 500 rpm to 1000 rpm for 1 to 2 hours, and then treated with an ultrasonic cell disruptor or a high-speed homogenizer at a power of 300W to 600W for 10 to 30 minutes to obtain a composite foaming slurry.

7. The method for producing high-temperature resistant vermiculite / polyimide composite foam material according to claim 2, characterized in that, In step 4, during the casting and pre-curing process, the following steps are performed: Step 400: Place the composite foaming slurry obtained in step 3 onto the foaming substrate; Step 401: Heat the foaming slurry to pre-cur it. During heating, keep it at 40°C to 60°C for 1 to 3 hours, and then keep it at 80°C to 100°C for 0.5 to 2 hours, so that the composite foaming slurry forms a wet film with a thickness of 1.0 mm to 4.0 mm.

8. The method for producing high-temperature resistant vermiculite / polyimide composite foam material according to claim 2, characterized in that, The synergistic reaction of step-foaming-iminolation-vermiculite expansion in step 5 proceeds through the following steps: Step 500: The obtained wet film is heated to 180°C to 220°C at a rate of 1°C / min to 2°C / min and kept at that temperature for 30 minutes to 90 minutes. Step 501: Continue to raise the temperature to 280°C to 350°C at a rate of 1°C / min to 1.5°C / min, and hold at that temperature for 60 minutes to 150 minutes; Step 502: Continue heating to 350°C to 400°C at a rate of 0.5°C / min to 1°C / min, and hold for 30 minutes to 60 minutes to obtain a high-temperature resistant vermiculite / polyimide composite foam material.

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