Method for synthesizing polyimide aerogel by taking waste polyimide fiber as raw material

Abstract

The invention discloses a method for synthesizing polyimide aerogel by taking waste polyimide fiber as a raw material, which comprises the following steps: mixing the waste polyimide fiber with a first solvent, and stirring until the waste polyimide fiber is hydrolyzed to obtain hydrolysate; adjusting the pH value of the hydrolysate to 1-3, stirring until precipitate is separated out, filtering to obtain a polyamide acid filter cake, dispersing the polyamide acid filter cake in a second solvent under an ice bath condition, dropwise adding triethylamine, stirring until the solution becomes clear, freeze-drying to obtain water-soluble polyamide acid, and stirring in a third solvent under the ice bath condition until the water-soluble polyamide acid is completely dissolved to obtain the water-soluble polyamide acid. The preparation method comprises the following steps: preparing a polyamide acid precursor solution, freezing and then freeze-drying to obtain a polyamide acid aerogel green body, and carrying out thermal imidization treatment on the polyamide acid aerogel green body under the protection of nitrogen to obtain the polyimide aerogel, and the polyimide aerogel has low heat conductivity coefficient and flame-retardant and smoke-suppression performance.

Description

Technical Field

[0001] This invention belongs to the field of polymer material recycling and functional material preparation technology, specifically relating to a method for synthesizing polyimide aerogel using waste polyimide fibers as raw materials. Background Technology

[0002] Polyimide aerogel, as an advanced porous material, combines the excellent mechanical properties and thermal stability of polyimide resin with the ultra-low density and high thermal insulation properties unique to aerogel nanomaterials, making it a promising candidate for applications in aerospace, precision electronic devices, and high-efficiency energy storage and conversion.

[0003] Traditional polyimide aerogels are typically prepared by polymerizing diamine monomers (such as 4,4'-diaminodiphenyl ether and 4,4'-diaminodiphenylmethane) with dianhydride monomers (such as pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, and 3,3',4,4'-benzophenone tetracarboxylic dianhydride) in a polar aprotic solvent (such as N,N-dimethylformamide) to form a polyamic acid solution. This solution is then subjected to a complex multi-step process involving chemical crosslinking, gelation, solvent displacement, and supercritical drying. The traditional polyimide aerogel preparation process is complex and costly, requiring the use of large amounts of toxic organic solvents, which is inconsistent with green processes and makes it difficult to recycle waste polyimide materials.

[0004] To address the recycling of polyimide materials, existing technologies have attempted to depolymerize waste polyimide through alkaline aqueous solutions to obtain hydrolysate products containing polyamic acid fragments, or to directly prepare water-based polyamic acid systems. However, directly using the hydrolysate to construct stable, high-performance aerogel structures still faces significant challenges: the polyamic acid recovered through hydrolysis exhibits poor solubility and gelling properties in aqueous systems. Therefore, developing an integrated method that can directly use waste polyimide fibers as raw materials to efficiently prepare high-performance aerogels in an environmentally friendly process has significant technical and economic value. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for synthesizing polyimide aerogel using waste polyimide fibers as raw materials. This method is mild and environmentally friendly, and can achieve efficient and targeted recycling of waste polyimide fibers.

[0006] Another object of the present invention is to provide a polyimide aerogel obtained by the above method.

[0007] The objective of this invention is achieved through the following technical solution.

[0008] A method for synthesizing polyimide aerogel using waste polyimide fibers as raw materials includes the following steps:

[0009] Step 1) Mix waste polyimide fibers with the first solvent and stir at 50-80 °C until the waste polyimide fibers are completely hydrolyzed to obtain a hydrolysate; then adjust the pH of the hydrolysate to 1-3 and stir until a precipitate is formed to obtain a mixture containing polyamic acid. Filter the mixture containing polyamic acid to obtain a polyamic acid filter cake. The mass fraction of the waste polyimide fibers to the volume fraction of the first solvent is (1-2):(30-60), where mass fraction is in g and volume fraction is in mL. The first solvent includes alkali and water, and the mass fraction of alkali to water in the first solvent is (5-10):(95-90).

[0010] In step 1), the waste polyimide fiber is stirred at 50-80 °C for 1-2 h until it is completely hydrolyzed.

[0011] In step 1), stir for 1 to 3 hours until a precipitate forms.

[0012] In step 1), the alkali includes sodium hydroxide and / or potassium hydroxide.

[0013] Step 2): Under ice bath conditions, the polyamic acid filter cake is dispersed in a second solvent, and then triethylamine (TEA) is added dropwise under stirring. After the addition is complete, the mixture is stirred until the solution becomes clear, and then freeze-dried to obtain water-soluble polyamic acid (PAA-TEA). The ratio of the mass fraction of the polyamic acid filter cake, the volume fraction of the second solvent, and the volume fraction of the triethylamine is (3~15):30:4. The mass fraction is expressed in g, the volume fraction is expressed in mL, and the second solvent is water.

[0014] In step 2), stir for 0.5 to 1 h until the solution becomes clear.

[0015] Step 3), dissolve the water-soluble polyamic acid in the third solvent, and then stir under ice bath conditions until the water-soluble polyamic acid is completely dissolved to obtain a precursor solution of polyamic acid, wherein, by mass parts, the ratio of the water-soluble polyamic acid to the third solvent is (2~15):(85~98), and the third solvent includes water;

[0016] In step 3), the third solvent further includes triethylamine, and the ratio of water to triethylamine in the third solvent by volume is (90~99.9):(0.1~10).

[0017] Step 4), freeze the precursor solution at -196~-20 ℃ for 20~60 min, and then freeze-dry it to obtain the polyamic acid aerogel preform;

[0018] In step 4), after freezing, the freeze is frozen at -60 to -40 °C for at least 24 hours before freeze-drying.

[0019] In step 4), the freeze-drying time is 12-48 h.

[0020] Step 5) Under nitrogen protection, the polyamic acid aerogel preform is subjected to thermal imidization treatment to obtain polyimide aerogel (PI-TEA). The thermal imidization treatment includes holding the preform at 80~120 ℃, 130~170 ℃, 180~220 ℃ and 230~270 ℃ for 1~2 h in sequence.

[0021] In the above technical solution, the temperature of the ice bath is 0~5℃.

[0022] In the above technical solution, the heating rate during the thermal imidization process is 1~3 ℃ / min.

[0023] The polyimide aerogel obtained by the above method.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The method of the present invention does not involve solvent replacement of traditional polyimide aerogels, and does not require the use of large amounts of organic solvents (such as N,N-dimethylformamide, N-methylpyrrolidone, etc.), thus avoiding the consumption of large amounts of toxic organic solvents and greatly reducing environmental hazards and cost pressures.

[0026] 2. The polyimide aerogel of the present invention has low thermal conductivity and flame-retardant and smoke-suppressing properties. Its heat release rate curve maintains a stable plateau period of 200-1000 s, mainly attributed to the formation of a dense protective carbon layer on the surface of the polyimide aerogel, effectively blocking the transfer of heat and oxygen to the interior.

[0027] 3. The method of the present invention uses waste polyimide fibers as raw materials to prepare polyimide aerogels. From an environmental protection perspective, it reuses defective polyimide fibers, effectively realizing the recycling and reuse of waste. Attached Figure Description

[0028] Figure 1 Infrared spectra of waste polyimide fibers and recycled polyamic acid;

[0029] Figure 2 The ¹H NMR spectrum of the recovered polyamic acid (PAA);

[0030] Figure 3 The XRD pattern of the recovered polyamic acid (PAA) is shown.

[0031] Figure 4 XPS of the recovered polyamic acid (PAA), where (a) is the full spectrum, (b) is the C1s spectrum, (c) is the N1s spectrum, and (d) is the O1s spectrum;

[0032] Figure 5 Infrared spectra of the recovered polyamic acid, polyimide (PI), triethylamine (TEA), water-soluble polyamic acid (PAA-TEA) from Example 2, and polyimide aerogel (PI-TEA) obtained from Example 2;

[0033] Figure 6 Electron micrograph of the recovered polyamic acid;

[0034] Figure 7 Electron micrographs of the polyimide aerogels (PI-TEA) obtained in Examples 1-2 and Examples 5-6;

[0035] Figure 8 The TG and DTG spectra of waste polyimide fibers, polyimide, and the polyimide aerogel obtained in Example 2 are shown.

[0036] Figure 9 The thermal conductivity diagrams are for the polyimide aerogels obtained in Examples 1, 3, 5, and 7.

[0037] Figure 10 The thermal conductivity diagrams are for the polyimide aerogels obtained in Examples 2, 4, 6, and 8.

[0038] Figure 11 This is a flowchart of the synthesis of polyimide aerogel using waste polyimide fibers as raw materials;

[0039] Figure 12 The following are characteristic curves of the polyimide aerogel obtained in Example 2, where (a) is the heat release rate and total heat release, (b) is the smoke generation rate and total smoke generation, (c) is the heat release rate and mass loss rate, and (d) is the carbon monoxide generation rate and carbon dioxide generation rate. Detailed Implementation

[0040] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0041] The raw materials used in the following embodiments are from the following sources:

[0042] Waste polyimide fibers were purchased from defective polyimide fibers from Jiangsu Aoshen New Material Co., Ltd.

[0043] Sodium hydroxide (analytical grade) and hydrochloric acid (HCl concentration in hydrochloric acid was 36~38 wt%) were purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd.

[0044] Triethylamine (analytical grade) was purchased from Tianjin Kemeo Chemical Reagent Co., Ltd.

[0045] The device information used in the following embodiments is detailed in Table 1:

[0046] Table 1

[0047]

[0048] Examples 1-8

[0049] A method for synthesizing polyimide aerogel using waste polyimide fibers as raw materials includes the following steps:

[0050] Step 1) The waste polyimide fiber is mixed with the first solvent and stirred at 60 °C for 1.5 h until the waste polyimide fiber is completely hydrolyzed to obtain a hydrolysate; then hydrochloric acid is added to the hydrolysate to adjust the pH to 2, and stirred for 1 h until a precipitate is formed to obtain a mixture containing polyamic acid; the mixture containing polyamic acid is filtered to obtain a polyamic acid filter cake, wherein the mass fraction of waste polyimide fiber to the volume fraction of the first solvent is 2:40, the mass fraction is in g, and the volume fraction is in mL; the first solvent is a mixture of alkali and deionized water, the pH of the first solvent is 13±1, and the mass fraction of alkali to deionized water in the first solvent is 6:94, and the alkali is sodium hydroxide;

[0051] Step 2): Under ice bath conditions (0~5 ℃), the polyamic acid filter cake is completely dispersed in the second solvent (deionized water, 30 mL). Triethylamine (TEA) is then slowly added dropwise under stirring. After the addition is complete, the mixture is stirred for 1 h until the solution becomes clear (at this point, the solution is a homogeneous solution, and the polyamic acid in the polyamic acid filter cake is neutralized and converted into a water-soluble salt). The mixture is then freeze-dried for 24 h to obtain water-soluble polyamic acid (PAA-TEA, powder). The ratio of the mass fraction of the polyamic acid filter cake, the volume fraction of the second solvent, and the volume fraction of triethylamine is 8:30:4. The mass fraction is expressed in g, and the volume fraction is expressed in mL.

[0052] Step 3): Dissolve the water-soluble polyamic acid in the third solvent, and then stir under ice bath conditions (0~5 ℃) until the water-soluble polyamic acid is completely dissolved to obtain the precursor solution of polyamic acid. The ratio of water-soluble polyamic acid to the third solvent by mass is X; the third solvent is a mixture of deionized water and triethylamine, and the ratio of deionized water to triethylamine in the third solvent by volume is 99:1.

[0053] Step 4): Pour 2.5 mL of the precursor solution into a mold (the length and width of the mold are both 2 cm), freeze at Y ℃ for 60 min to set it, freeze and store at -40 ℃ for 24 h, and then freeze-dry in a vacuum environment for 24 h to obtain the polyamic acid aerogel preform.

[0054] Step 5): Under nitrogen protection, the polyamic acid aerogel preform was placed in a tube furnace for thermal imidization treatment to obtain polyimide aerogel (PI-TEA). The thermal imidization treatment included sequentially holding the aerogel at 100 °C, 150 °C, 200 °C, and 250 °C for 1 h each, with a heating rate of 2.5 °C / min. The dimensions of the polyimide aerogels obtained in Examples 1-8 were: length 1.5 ± 0.1 cm, width 1.5 ± 0.1 mm, and thickness 4 ± 0.2 mm.

[0055] The concentrations of polyamic acid in X, Y, and the precursor solutions in different embodiments are shown in Table 2.

[0056] Table 2

[0057]

[0058] The polyamic acid mixture from Example 2 was freeze-dried for 24 h to obtain recovered polyamic acid (powder, PAA).

[0059] Infrared spectroscopy was performed on waste polyimide fiber (PI fibre) and recycled polyamic acid (PAA), and the results are as follows: Figure 1 As shown. By Figure 1 It can be seen that the recycled polyamic acid ( Figure 1 The “Recycled PAA” in the text is at 1780 cm. -1 (C=O asymmetric stretching vibration peak), 1380 cm⁻¹ -1 (CN stretching vibration peak) and 725 cm -1 The characteristic peaks at the position of (C=O bending vibration peak) (these three characteristic peaks are characteristic peaks of the imide ring in waste polyimide fibers) are compared to those in waste polyimide fibers. Figure 1 The "PI fibre" in the spectrum is significantly weakened. This indicates that during the process of obtaining recycled polyamic acid from waste polyimide fibers, the imide rings of the waste polyimide fibers undergo a hydrolytic ring-opening reaction, transforming back into amide bonds and carboxyl groups. Therefore, in the infrared spectrum of the recycled polyamic acid, the three characteristic peaks belonging to the imide rings weaken or even disappear completely. Furthermore, a peak at 1660 cm⁻¹ is observed. -1 (Absorption peak of stretching vibration of amide carbonyl group) and 1540 cm⁻¹ -1The characteristic peaks of ammonium acid were reproduced at positions such as (coupling of NH bending vibration and CN stretching vibration), proving that waste polyimide fibers have been successfully recycled and converted into polyammonium acid.

[0060] The recovered polyamic acid (PAA) was subjected to NMR spectroscopy, and the results are as follows: Figure 2 As shown. The top image shows the structural formula of polyamic acid, and the bottom image shows the NMR spectrum of the recovered polyamic acid. Figure 2 It can be seen that a broad and weak characteristic peak belonging to the carboxyl proton (-COOH) was observed at 12.79 ppm (peak e) in the NMR 1H spectrum, and a sharp single peak of the amide proton (-NH-) appeared at 10.54 ppm (peak d), directly proving that the recovered polyamic acid has the characteristic functional groups of polyamic acid. At the same time, the characteristic peak observed at 7.04 ppm belongs to the aromatic hydrogen (peak b) that shifts to a higher field due to the electron donor characteristics of the ether bond (RO-R'), and the characteristic peak at 7.70 ppm is the aromatic hydrogen (peak c) affected by the electron-withdrawing effect of the adjacent amide carbonyl group (C=O). In addition, due to the microenvironmental differences and magnetic inequivalence caused by the rotational isomerism of the amide bond, a set of multiplets appeared at 8.31, 8.18, 7.96 and 7.80 ppm (peak a). Most importantly, integrating the peak areas of peaks a, b, c, and d using peak a as a baseline yielded a ratio of approximately 2:4:4:2, which perfectly matches the theoretical value, confirming the successful recovery of polyamic acid. In summary, this demonstrates that the mixture obtained after hydrolyzing waste polyimide fibers contains polyamic acid.

[0061] X-ray diffraction analysis was performed on the recovered polyamic acid (PAA), and its XRD data are as follows: Figure 3 As shown. By Figure 3 It can be seen that polyamic acid exhibits a broad dispersion peak in the range of 15~35°, which is a typical characteristic of the amorphous structure of polyamic acid. This indicates that the present invention successfully reverse-converts highly crystalline or highly oriented waste polyimide fibers into amorphous polyamic acid.

[0062] Figure 4 The XPS spectrum of the recovered polyamic acid is shown below. Figure 4 (a) represents the full spectrum. Figure 4 (b) to (d) are the fine spectra of C1s, N1s, and O1s, respectively. Figure 4 Quantitative analysis of the full spectrum in (a) revealed that the atomic percentages of C, O and N in the recovered polyamic acid were 69.71%, 21.65% and 6.75%, respectively (with trace amounts of Na, Cl and Si, totaling <2%), which are highly consistent with the theoretical calculations of polyamic acid. Figure 4The fine spectrum of C1s in (b) clearly shows the various chemical environments of carbon in the recovered polyamic acid, corresponding to Figure 2 The structural formula of polyamic acid is shown. The fine C1s spectrum includes: the C=C bond (284.8 eV) serving as the aromatic backbone of the polymer; the CN bond connecting the amide to the benzene ring and a possible CO bond (286.05 eV); and the carbonyl carbon derived from the amide bond (C=O, 288.46 eV). Furthermore, a characteristic π-π satellite peak (291.47 eV) appears at a higher binding energy, further confirming the successful preservation of the aromatic ring structure. This demonstrates that the recovered polyamic acid possesses the standard carbon backbone and functional groups of polyamic acid. Figure 4 As shown in (c), the fine spectrum of N1s exhibits a single, symmetrical, and standard-positioned peak, with its binding energy (399.93 eV) precisely corresponding to the nitrogen atom in the amide bond. The symmetry of this peak and the absence of other forms of nitrogen signals (such as imine nitrogen or amino nitrogen) indicate that the nitrogen atoms in the recovered polyamic acid exist almost entirely in the form of amide bonds, resulting in a highly homogeneous chemical environment. This confirms that the core reaction for successfully recovering and regenerating polyamic acid from waste polyimide fibers has been efficiently completed. Figure 4 The fine O1s spectrum in (d) further confirms the characteristic oxygen environment of polyamic acid. The peak with lower binding energy (531.88 eV) in the O1s spectrum belongs to the carbonyl oxygen (O=C) in the amide bond, which corroborates the C=O signal in the C1s spectrum; while the peak with higher binding energy (533.29 eV) originates from the hydroxyl oxygen (OH) in the carboxyl group (-COOH) or the ether oxygen bond (COC) between aromatic rings. This is completely consistent with the simultaneous presence of carbonyl and carboxyl / ether oxygen oxygen in the polyamic acid molecule, providing strong evidence for the overall structure identification. This indicates that the recycling process has successfully converted waste polyimide fibers into polyamic acid with correct structure and good purity.

[0063] The method for obtaining polyimide (PI) includes: placing the recovered polyamic acid in a tube furnace for heat treatment under nitrogen protection to obtain polyimide (PI), wherein the heat treatment conditions are: holding at 100 ℃, 200 ℃ and 300 ℃ for 1 h each in sequence, and the heating rate of the heat treatment is 5 ℃ / min.

[0064] The recovered polyamic acid (PAA), polyimide (PI), triethylamine (TEA), the water-soluble polyamic acid (PAA-TEA) from Example 2, and the polyimide aerogel (PI-TEA) obtained in Example 2 were characterized by infrared spectroscopy, and the results are as follows: Figure 5 As shown. By Figure 5 It is known that polyamic acid (PAA) has a viscosity of 2500~3500 cm⁻¹. -1The infrared spectrum of polyimide (PI) exhibits a broad and strong characteristic OH peak of carboxylic acid. After the addition of triethylamine (TEA), the OH peak of the water-soluble polyamic acid (PAA-TEA) becomes significantly narrower or shifts, indicating that triethylamine interacts with the carboxyl groups of polyamic acid to form a water-soluble triethylamine salt of polyamic acid. Further comparison of the infrared spectra of polyimide (PI) and polyimide aerogel (PI-TEA) reveals that their spectral shapes are essentially identical. This indicates that during the thermal imidization process, triethylamine first escapes, then undergoes cyclization, dehydration, and ring closure regeneration of polyimide to form polyimide aerogel, without damaging the molecular structure of polyimide. The main chain of the polyimide aerogel in Example 2 remains predominantly composed of imide rings, demonstrating structural stability.

[0065] Based on the above analysis, the process for synthesizing polyimide aerogel using waste polyimide fibers as raw materials in this invention is as follows: Figure 11 As shown, waste polyimide fibers are hydrolyzed in an alkaline aqueous solution, causing the imide ring to open and form sodium polyamic acid. Hydrochloric acid is added to the hydrolysate for acidification, causing the sodium carboxylate to protonate and form polyamic acid. The polyamic acid precipitates due to a sharp drop in solubility, while sodium chloride remains in the aqueous phase, resulting in a mixture containing polyamic acid. Under ice bath conditions, triethylamine is added to the polyamic acid (polyamic acid and triethylamine form a salt), converting it into a water-soluble triethylamine polyamic acid salt (a water-soluble salt) and further promoting its solubility to obtain a precursor solution. The precursor solution is then subjected to freeze-setting, freeze-drying, and thermal imidization treatments. During this process, triethylamine escapes, polyamic acid undergoes cyclization and dehydration, and ring-closure regeneration occurs to form a polyimide aerogel.

[0066] Scanning electron microscopy (SEM) was performed on the recovered polyamic acid (PAA), the polyimide aerogels obtained in Examples 1-2 and 5-6, and the SEM results are as follows: Figure 6 and Figure 7 As shown. Among them, Figure 6 This is recycled polyamic acid (PAA). Figure 7 The images show the morphology of the polyimide aerogels obtained in Examples 1-2 and Examples 5-6. Figure 6 It can be seen that the morphology of the recovered polyamic acid is that of sheet-like aggregates of varying sizes. From Figure 7It can be seen that the polyimide aerogels all exhibit rich porosity, continuous structure, and uniform pore distribution. Specifically, the polyimide aerogels obtained in Examples 1 and 5 have a layered structure (anisotropic), while those obtained in Examples 2 and 6 exhibit a three-dimensional network structure (isotropic). When the concentration of polyamic acid in the precursor solution is the same, comparing the morphologies of the polyimide aerogels obtained in Examples 1 and 2, and Examples 5 and 6, reveals that the lower the freezing temperature, the more pronounced the layered structure and the more orderly the arrangement of the resulting polyimide aerogels. This is because at lower freezing temperatures (-196 °C), the pore walls extend along the ice crystal growth direction, transforming the three-dimensional network structure of the polyimide aerogel from random to ordered. Further comparison of the morphologies of the polyimide aerogels obtained in Examples 1 and 5 shows that, at the same freezing temperature, the pore size of the resulting polyimide aerogels significantly increases with increasing concentration of polyamic acid in the precursor solution. This indicates that by controlling the concentration of polyamic acid in the precursor solution and the freezing temperature, the three-dimensional network and pore structure of polyimide aerogel can be effectively adjusted. The microstructure of polyimide aerogel endows it with properties such as extremely light weight, ultra-high specific surface area, and excellent thermal insulation performance.

[0067] Figure 8 (a) TG and (b) DTG spectra of waste polyimide fibers (PI Fibers), polyimide (PI), and the polyimide aerogel (PI-TEA) of Example 2. Figure 8 (a) and Figure 8 As shown in (b), polyimide aerogel loses 10% of its weight at 548 °C, and its maximum decomposition rate occurs at 556 °C. Waste polyimide fibers and polyimide lose 10% of their weight at 584 °C and 556 °C, respectively, and their maximum decomposition rates occur at 586 °C and 570 °C, respectively. Comparatively, the thermal stability of polyimide aerogel is slightly lower than that of waste polyimide fibers and polyimide, but it still exhibits thermal stability greater than 500 °C. Figure 8 As shown in (a), the curves in the DTG spectrum all exhibit a single, symmetrical, sharp peak shape, and Figure 8 In (b), the TG plot shows that the char residue of waste polyimide fibers, polyimide, and polyimide aerogel at 700 °C is greater than 63%. This demonstrates that during the synthesis of polyimide aerogel, neither the introduction of triethylamine into polyamic acid (steps 2 and 3) nor the pyrolysis and volatilization of the polyamic acid aerogel preform under thermal imidization treatment significantly damages the chemical structure of the polyimide aerogel.

[0068] Example 9

[0069] The thermal conductivity of the polyimide aerogels obtained in Examples 1-8 was tested under the following conditions: At room temperature, the thermal conductivity of the polyimide aerogel (dimensions: length 1.5 ± 0.1 cm, width 1.5 ± 0.1 mm, thickness 4 ± 0.2 mm) was tested using a TPS 2500S thermal constant analyzer in an atmosphere of normal pressure air (transient measurement, not involving heating rate). The polyimide aerogel used was one of the polyimide aerogels obtained in Examples 1-8, and the probe model of the TPS2500S thermal constant analyzer used was 7531*. The thermal conductivity of the polyimide aerogels obtained in Examples 1-8 is as follows: Figure 9 and Figure 10 As shown. By Figure 9 and Figure 10 It can be seen that, regardless of whether the polyimide aerogels are anisotropic (obtained in Examples 1, 3, 5, and 7) or isotropic (obtained in Examples 2, 4, 6, and 8), the thermal conductivity of the polyimide aerogels exhibits the following trend: as the concentration of polyamic acid in the precursor solution increases, the thermal conductivity of the resulting polyimide aerogels gradually increases. This trend is mainly attributed to the decisive role of the concentration of polyamic acid in the nanoporous structure of the polyimide aerogels. Figure 10 The polyimide aerogel obtained in Example 2 has a thermal conductivity as low as 0.03209 W / (m•K), indicating its excellent thermal insulation performance. This suggests that as the concentration of polyamic acid in the precursor solution increases, the polymer molecular chain density in the precursor solution increases, making it easier to form a more robust and dense solid-state framework network, thereby affecting the thermal insulation performance.

[0070] The thermal conductivity may be affected in two ways: firstly, the dense solid-state framework network provides a more efficient heat conduction path for phonon transport, significantly enhancing solid-phase thermal conductivity; secondly, although the polyimide aerogel still maintains extremely high porosity, the thickened pore walls and changes in micropore size weaken the inhibition of air convection within the pores, thus increasing the contribution of gas-phase thermal conductivity and improving the thermal conductivity. Therefore, a relatively low concentration of polyamic acid (3-5 wt% in the precursor solutions of Examples 1-4) can just right form a sufficiently robust solid framework that maintains a three-dimensional nanoporous structure while being extremely sparse and fine. The solid framework of the polyimide aerogel obtained in Example 2 minimizes solid-phase thermal conductivity, fully utilizing the confinement effect of the nanopores inside the polyimide aerogel on air molecules, allowing gas-phase thermal conductivity to be maintained at an extremely low level. This study demonstrates that precisely controlling the concentration of polyamic acid in the precursor solution can optimize the microstructure of polyimide aerogel, thereby providing a key performance control method for its thermal insulation applications in aerospace, high-end buildings, and electronic equipment.

[0071] Example 10

[0072] Flame retardant performance test: The flame retardant performance of the polyimide aerogel obtained in Example 2 was tested using a cone calorimeter (model: VOUCH 6810, Suzhou Yangyi Wolchi Testing Technology Co., Ltd.) according to GB / T 16172-2007 standard. Specific conditions were as follows: The sample (dimensions: length 10 cm, width 10 cm, thickness 3 mm) wrapped in aluminum foil was placed in the sample holder on the weighing sensor, ensuring it was horizontal and centered. The cone heater was started, and the heat radiation flux was allowed to stabilize at 50 kW / m². 2 Then, the sample was pushed into the test position, and data recording began simultaneously. The high-voltage spark generator was moved above the sample, ignition was initiated, and the ignition time (TTI) of the sample was recorded. The sample was prepared by grinding and pressing 17.94 g of the polyimide aerogel obtained in Example 2. The polyimide aerogel obtained in Example 2 was equilibrated at 25 °C and 50% RH for 48 h before testing. The results are as follows... Figure 12 As shown, (a) represents the heat release rate (HRR) and total heat release, (b) represents the smoke generation rate (SPR) and total smoke generation, (c) represents the heat release rate and mass loss rate (MLR), and (d) represents the CO generation rate and CO2 generation rate (toxic gas release curves).

[0073] Depend on Figure 12 From (a), we know that at 50 kW / m 2 Under high-intensity thermal radiation, the ignition time (TTI) of the polyimide aerogel obtained in Example 2 can be extended to 40 s, effectively suppressing the heat release rate (HRR) in the initial stage of a fire, and the peak value of the heat release rate is significantly suppressed to 79.57 kW / m². 2 (This value is far below 100 kW / m) 2 The "highly efficient flame retardant" threshold is at an extremely low level, and the heat release rate drops sharply after reaching its peak, remaining at 30-35 kW / m² for 200-1000 s (the plateau period corresponds to 800 s). 2 The plateau phase indicates that the polyimide aerogel obtained in Example 2 is inherently flame-retardant and possesses an "internal self-inhibition" characteristic, which is direct evidence of char layer formation and its continued effectiveness. The total heat release (THR) slowly increases to 34.65 MJ / m² within 1200 s. 2 The curve maintains a low slope increase within 1200 s without any sudden jumps, demonstrating excellent thermal stability and low fire load characteristics. This indicates that the polyimide aerogel obtained in Example 2 has excellent flame retardant and smoke-suppressing potential.

[0074] Depend on Figure 12As shown in (b), the smoke generation rate (SPR) decreases by more than 75% after the formation of the char layer, with a peak value of 0.0124 m. 2 / s, plateauing period remained at 0.002~0.004 m 2 The level was extremely low, with a maximum total smoke production (TSP) of 3.78 m³ / s. 2 This proves that the carbon layer has excellent smoke suppression function.

[0075] Depend on Figure 12 As shown in (c), the peak value of the mass loss rate (MLR) curve is 0.0658 g / s, and the plateau period can be maintained at a low level of 0.01~0.03 g / s. This slow and controlled mass loss process shown in the MLR curve indicates that a dense protective char layer is formed on the surface of the polyimide aerogel obtained in Example 2 at high temperature, which effectively blocks the transfer of heat and oxygen to its interior, achieving highly efficient flame retardancy at the kinetic level.

[0076] Depend on Figure 12 As can be seen from (d), the carbon dioxide generation rate and the carbon monoxide generation rate both remain at a low level. Although the CO generation rate increases slightly due to the incomplete combustion of the gas phase caused by the charcoal layer, this is precisely the inevitable cost and direct evidence that the charcoal layer plays a barrier role: by sacrificing part of the complete combustion, the ultimate control of the fire is achieved.

[0077] In summary, the combustion behavior of the polyimide aerogel obtained in Example 2 constitutes a complete chain of evidence: the sharp decrease in HRR and the long plateau period, the simultaneous control of MLR, the significant inhibition of SPR, and the increase in carbon monoxide formation rate indicate that the polyimide aerogel obtained in Example 2 rapidly forms a dense char layer after ignition. This char layer can continuously suppress heat release, mass loss, and smoke release, i.e., self-consistent combustion behavior. This self-consistent combustion behavior is a typical characteristic of condensed phase flame retardant mechanisms. Its practical significance in fire safety is: an extremely low heat release rate (79.6 kW / m³). 2 This means that it is difficult to ignite surrounding items (the fire will not spread), and the nearly 14-minute plateau period provides ample window for personnel to escape. The 20-minute slow burning buys valuable time for fire rescue. Therefore, the polyimide aerogel obtained in Example 2 is a high-performance flame retardant material with both high thermal stability and efficient charring ability, showing extremely high application potential in terms of fire safety performance.

[0078] The present invention has been described above by way of example. It should be noted that any simple modifications, alterations or other equivalent substitutions that can be made by those skilled in the art without creative effort without departing from the core of the present invention fall within the protection scope of the present invention.

Claims

1. A method for synthesizing polyimide aerogel using waste polyimide fibers as raw materials, characterized in that, Includes the following steps: Step 1) Mix waste polyimide fibers with the first solvent and stir at 50-80 °C until the waste polyimide fibers are completely hydrolyzed to obtain a hydrolysate; then adjust the pH of the hydrolysate to 1-3 and stir until a precipitate is formed to obtain a mixture containing polyamic acid. Filter the mixture containing polyamic acid to obtain a polyamic acid filter cake. The mass fraction of the waste polyimide fibers to the volume fraction of the first solvent is (1-2):(30-60), where mass fraction is in g and volume fraction is in mL. The first solvent includes alkali and water, and the mass fraction of alkali to water in the first solvent is (5-10):(95-90). Step 2): Under ice bath conditions, the polyamic acid filter cake is dispersed in a second solvent, and then triethylamine is added dropwise under stirring. After the addition is complete, the mixture is stirred until the solution becomes clear, and then freeze-dried to obtain water-soluble polyamic acid. The ratio of the mass fraction of the polyamic acid filter cake, the volume fraction of the second solvent, and the volume fraction of the triethylamine is (3~15):30:

4. The mass fraction is expressed in g, the volume fraction is expressed in mL, and the second solvent is water. Step 3), dissolve the water-soluble polyamic acid in the third solvent, and then stir under ice bath conditions until the water-soluble polyamic acid is completely dissolved to obtain a precursor solution of polyamic acid. The ratio of the water-soluble polyamic acid to the third solvent is (2~15):(85~98) by mass. The third solvent includes water. Step 4), freeze the precursor solution at -196~-20 ℃ for 20~60 min, and then freeze-dry it to obtain the polyamic acid aerogel preform; Step 5) Under nitrogen protection, the polyamic acid aerogel preform is subjected to thermal imidization treatment to obtain polyimide aerogel. The thermal imidization treatment includes holding the preform at 80~120 ℃, 130~170 ℃, 180~220 ℃ and 230~270 ℃ for 1~2 h in sequence.

2. The method according to claim 1, characterized in that, In step 1), the waste polyimide fiber is stirred at 50-80 °C for 1-2 h until it is completely hydrolyzed.

3. The method according to claim 1, characterized in that, In step 1), stir for 1 to 3 hours until a precipitate forms.

4. The method according to claim 1, characterized in that, In step 1), the alkali includes sodium hydroxide and / or potassium hydroxide.

5. The method according to claim 1, characterized in that, In step 2), stir for 0.5 to 1 h until the solution becomes clear.

6. The method according to claim 1, characterized in that, In step 3), the third solvent further includes triethylamine, and the ratio of water to triethylamine in the third solvent by volume is (90~99.9):(0.1~10).

7. The method according to claim 1, characterized in that, In step 4), the freeze-drying time is 12-48 hours.

8. The method according to claim 1, characterized in that, The temperature of the ice bath is 0~5℃.

9. The method according to claim 1, characterized in that, The heating rate during the thermal imidization process is 1~3℃ / min.

10. The polyimide aerogel obtained by the method according to any one of claims 1 to 9.

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