Phthalonitrile modified phenolic aerogel as well as preparation method and application thereof

Abstract

The invention discloses a phthalonitrile modified phenolic aerogel and a preparation method and application thereof. Linear phenolic aldehyde, 4-nitrophthalonitrile and a basic catalyst are dissolved in an organic solvent, a water bath reaction is conducted, powder is prepared, and phthalonitrile etherified phenolic resin is obtained; preparing a mixed solvent, and dissolving linear phenolic aldehyde and phthalonitrile etherified phenolic resin in the mixed solvent to obtain a mixed solution; adding a cross-linking agent into the mixed solution, carrying out sol-gel reaction to obtain wet gel, and aging, segmentally curing and drying the wet gel to obtain the phthalonitrile modified phenolic aerogel. The pyrolytic property of the phenolic aerogel is improved, the phthalonitrile modified phenolic aerogel with high carbon residue and high thermal stability is obtained under the condition of low-temperature heating, and the method is suitable for being applied to the field of thermal insulation ablation of a thermal protection system of aerospace equipment.

Description

Technical Field

[0001] The invention belongs to the technical field of preparation of ablative heat protection materials, and in particular relates to a phthalonitrile-modified phenolic aerogel and a preparation method and application thereof. Background Art

[0002] As human exploration of space gradually deepens, spacecraft will face more extreme and complex external environment tests. When spacecraft moves at high speed in the atmosphere, it will have strong friction with the air, forming a serious aerodynamic heating effect, generating a large amount of heat energy, causing the surface of the spacecraft to reach a very high temperature. Therefore, in order to protect the normal operation of internal systems and equipment, the thermal protection system is one of the indispensable systems in aerospace equipment. Returnable vehicles and space return capsules need to brake and slow down when returning, which is a compromise made for the insufficient performance of their thermal protection systems. In thermal protection systems, ablative thermal protection systems are mostly used in returnable vehicles. Phenolic aerogels are widely used in this field due to their high residual carbon, low density, low thermal conductivity, and excellent thermal insulation and ablation capabilities. The heat-resistant outsole of the US "Dragon" spacecraft uses a carbon fiber reinforced phenolic aerogel composite material with extremely low thermal conductivity and good ablation and thermal insulation properties.

[0003] At present, phenolic aerogel is mainly obtained by sol-gel reaction, solvent replacement and supercritical drying or freeze drying with resorcinol and formaldehyde as raw materials. Under high-temperature ablation, the methylene bonds of phenolic aerogel cleave too quickly, the residual carbon is relatively low, and the thermal stability is insufficient, which is not conducive to obtaining better thermal insulation ablation performance. Therefore, it is very important to modify the phenolic aerogel to obtain a modified phenolic aerogel with higher residual carbon, good thermal stability and better thermal insulation ablation performance. Summary of the invention

[0004] The invention aims to introduce phthalonitrile into a phenolic aerogel skeleton, provide a phthalonitrile-modified phenolic aerogel and a preparation method and application thereof, and prepare the phthalonitrile-modified phenolic aerogel by blending phthalonitrile-etherified phenolic with thermoplastic phenolic under appropriate conditions. The phthalonitrile-modified phenolic aerogel has excellent properties of high residual carbon and high thermal stability, and the method has low cost, mild conditions, and the preparation method is simple and easy to operate.

[0005] The present invention is achieved through the following technical solutions.

[0006] One aspect of the present invention provides a method for preparing phthalonitrile-modified phenolic aerogel, comprising the following steps:

[0007] a. dissolving linear phenolic acid aldehyde: 4-nitrophthalonitrile: alkaline catalyst in an organic solvent in a mass ratio of (5-8): (2-5): (2-5), stirring evenly, reacting in a water bath, cooling, adjusting pH, and powdering to obtain phthalonitrile etherified phenolic resin;

[0008] b. dissolving the purchased linear phenolic resin and phthalonitrile etherified phenolic resin in a mixed solvent in a mass ratio of (5-8): (2-5), stirring evenly to obtain a mixed solution;

[0009] c. Add the crosslinking agent to the mixed solution at a mass ratio of 100:(2-4) and stir evenly to obtain a precursor solution;

[0010] d. The precursor solution undergoes a sol-gel reaction to obtain a wet gel, which is then aged, segmentally cured, and dried to obtain a phthalonitrile-modified phenolic aerogel.

[0011] Preferably, the organic solvent is any one of pyrrolidone, dimethyl sulfoxide, dimethylformamide or dimethylacetamide.

[0012] The mixed solvent is prepared by mixing a main solvent and a secondary solvent in a mass ratio of (5-8):(3-6), wherein the main solvent is ethanol and the secondary solvent is ethyl acetate.

[0013] Preferably, the alkaline catalyst is any one of NaOH, KOH, K2CO3, Na2CO3 or Ba(OH)2.

[0014] Preferably, the cross-linking agent is hexamethylenetetramine.

[0015] Preferably, in step a, the reaction is carried out in a water bath at 80-90° C. for 5-10 hours, then the temperature is cooled to room temperature and the pH is adjusted to neutral.

[0016] Preferably, in step d, the sol-gel reaction is carried out at a temperature of 90-150° C. for 24-72 hours and aged at room temperature for 1-3 days.

[0017] Preferably, the staged curing process is: 90°C / 0.5h, 110°C / 1h, 140°C / 2h, 150°C / 3h, 180°C / 2h, 200°C / 2h, 220°C / 4h, and drying at normal pressure.

[0018] Another aspect of the present invention provides a phthalonitrile-modified phenolic aerogel prepared by the method.

[0019] The present invention adopts the above technical solution, which has the following beneficial effects:

[0020] 1. The present invention introduces phthalonitrile into the phenolic aerogel skeleton. Since phthalonitrile itself has good thermal stability, its introduction improves the pyrolysis performance of the phenolic aerogel, significantly increases the initial pyrolysis temperature, improves the thermal stability of the phenolic aerogel, and increases the application scope of the phenolic aerogel in the field of thermal insulation and ablation, so that it can be applied in a more severe environment. The prepared phthalonitrile-modified phenolic aerogel can be used in a thermal protection system of aerospace equipment.

[0021] 2. The present invention uses phthalonitrile etherified phenolic resin in the preparation of aerogel, phthalonitrile is connected to phenolic resin to obtain BPN and then blended with phenolic, gel-sol to obtain aerogel, phthalonitrile is introduced into the phenolic aerogel skeleton at a relatively low temperature, and two resins with different solubility can be blended to prepare aerogel by using a mixed solvent. The invention successfully expands the application of phthalonitrile to aerogel. The carbon residue rate of the aerogel prepared by the invention can reach up to 68.14%, and the initial decomposition temperature and the maximum rate thermal decomposition temperature are significantly improved relative to phenolic aerogel. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of the present application, and do not constitute an improper limitation of the present invention. In the drawings:

[0023] Figure 1 is a preparation flow chart of the present invention;

[0024] Figure 2 This is the TG curve of the phthalonitrile-modified phenolic aerogel prepared in the present invention. DETAILED DESCRIPTION

[0025] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The exemplary embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0026] The present invention provides a method for preparing phthalonitrile-modified phenolic aerogel, such as Figure 1 As shown, the following steps are included:

[0027] a. Dissolve linear phenolic acid: 4-nitrophthalonitrile: alkaline catalyst in an organic solvent in a mass ratio of (5-8): (3-5): (4-6), stir evenly, react in a water bath at 80-90° C. for 5-10 hours, cool, adjust pH to neutral, and powder to obtain phthalonitrile etherified phenolic resin;

[0028] Wherein, the alkaline catalyst is any one of NaOH, KOH, K2CO3, Na2CO3 or Ba(OH)2.

[0029] The organic solvent is any one of N-dimethylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) or dimethylacetamide (DMAC).

[0030] The phthalonitrile used is ortho-nitro-substituted phthalonitrile or meta-nitro-substituted phthalonitrile, and the amount used is 0-100% of the molar number of phenolic hydroxyl group of phenolic aldehyde, but not 0, such as 50%-70%.

[0031] b. Preparing a main solvent (ethanol) and a secondary solvent (ethyl acetate) into a mixed solvent in a mass ratio of (4-7):(3-6); dissolving the purchased linear phenolic resin and phthalonitrile etherified phenolic resin in the mixed solvent in a mass ratio of (5-8):(2-5), stirring evenly, and obtaining a mixed solution;

[0032] c. Add the crosslinking agent to the mixed solution at a mass ratio of 100:(2-4) and stir evenly to obtain a precursor solution;

[0033] The cross-linking agent is any one of ammonium persulfate, potassium persulfate or hexamethylenetetramine.

[0034] d. The precursor solution is subjected to a sol-gel reaction at a temperature of 90-150°C for 24-72 hours to obtain a wet gel, which is aged at room temperature for 1-3 days, and is cured in stages at 90°C / 0.5h, 110°C / 1h, 140°C / 2h, 150°C / 3h, 180°C / 2h, 200°C / 2h, and 220°C / 4h, and dried at normal pressure to obtain phthalonitrile-modified phenolic aerogel.

[0035] Since the reaction temperature of phthalonitrile resin is relatively high, it usually starts to solidify at 270°C, which is too high for most conditions of preparing aerogels. Therefore, phthalonitrile is connected to the phenolic hydroxyl group of phenolic resin, thereby introducing phthalonitrile into the phenolic skeleton, and taking advantage of the low temperature curing condition of phenolic resin to introduce phthalonitrile into the phenolic aerogel skeleton.

[0036] Because PF and BPN have different solubility, the solubility of phenolic resin modified by phthalonitrile changes with that before modification, and phthalonitrile etherified phenolic resin is soluble in organic solvents such as acetone, ethyl acetate, tetrahydrofuran, dimethylformamide, dimethylacetamide, dimethyl sulfoxide, pyrrolidone, etc., while linear phenolic resin is soluble in most organic solvents of ethanol. The solvent for preparing aerogel at phenolic normal pressure is ethanol, but phthalonitrile etherified phenolic resin is insoluble in ethanol, so this patent adopts a mixed solvent of ethanol and ethyl acetate, successfully dissolving phthalonitrile etherified phenolic and linear phenolic, obtaining the required solution, making it possible to introduce phthalonitrile into phenolic aerogel skeleton. And the aerogel prepared by mixed solvent dissolving resin can be prepared by normal pressure drying, which simplifies the drying process and shortens the preparation time.

[0037] The method of the present invention is further described in detail below through different embodiments.

[0038] Example 1

[0039] Mass ratio: W(phenolic PF):W(phthalonitrile etherified phenolic BPN)=8:2

[0040] Dissolve linear phenolic acid: 4-nitrophthalonitrile: K2CO3 in N-dimethylpyrrolidone (NMP) in a mass ratio of 5:3:4, stir evenly, react in a water bath at 90°C for 7 hours, cool, adjust the pH to neutral, and powder to obtain phthalonitrile etherified phenolic acid resin;

[0041] 25 g of ethanol and ethyl acetate solvents were respectively mixed in a mass ratio of 5:5 to obtain 50 g of a mixed solvent, and linear phenolic acid and phthalonitrile etherified phenolic resin were added to the mixed solvent in a mass ratio of 8:2, and stirred at room temperature until completely dissolved to obtain a mixed solution;

[0042] According to the mixed solution: cross-linking agent mass ratio of 100:2, hexamethylenetetramine was added to the mixed solution and stirred evenly. After obtaining a uniform solution, it was placed in a reactor and placed in a constant temperature environment of 90°C for sol-gel reaction for 72 hours. After the reaction was complete, the reactor was taken out and cooled to room temperature. The wet gel was taken out and aged at room temperature for 2 days, and dried at 80°C under normal pressure for 12 hours until there was no change in weight, and then segmented curing was performed to obtain phthalonitrile-modified phenolic aerogel.

[0043] The prepared phthalonitrile modified phenolic aerogel was tested to have a 10% compression strength of 2.13 MPa, a room temperature thermal conductivity of 0.0390 W / m / K, an initial pyrolysis temperature of 435.15°C, a maximum pyrolysis rate temperature of 584.73°C, and a residual carbon content of 63.21%.

[0044] Example 2

[0045] Mass ratio W(phenolic PF):W(phthalonitrile etherified phenolic BPN)=7:3

[0046] Dissolve linear phenolic acid: 4-nitrophthalonitrile: Na2CO3 in dimethyl sulfoxide (DMSO) in a mass ratio of 6:4:5, stir evenly, react in a water bath at 90°C for 5 hours, cool, adjust the pH to neutral, and powder to obtain phthalonitrile etherified phenolic acid resin;

[0047] Ethanol and ethyl acetate solvents were respectively mixed in a mass ratio of 6:4 to obtain 50 g of a mixed solvent, and linear phenolic acid and phthalonitrile etherified phenolic resin were added to the mixed solvent in a mass ratio of 7:3, and stirred at room temperature until completely dissolved to obtain a mixed solution;

[0048] Potassium persulfate was added into the mixed solution at a mass ratio of mixed solution to cross-linking agent of 100:2.5 and stirred evenly. After obtaining a uniform solution, the solution was placed in a reactor and placed in a constant temperature environment of 100°C for sol-gel reaction for 48 hours. After the reaction was complete, the reactor was taken out and cooled to room temperature. The wet gel was taken out and aged at room temperature for 3 days, dried at 80°C under normal pressure for 12 hours until the weight did not change, and then cured in sections to obtain phthalonitrile-modified phenolic aerogel.

[0049] The prepared phthalonitrile modified phenolic aerogel was tested to have a 10% compression strength of 2.34 MPa, a room temperature thermal conductivity of 0.0391 W / m / K, an initial pyrolysis temperature of 464.46°C, a maximum pyrolysis rate temperature of 610.36°C, and a residual carbon content of 65.05%.

[0050] Example 3

[0051] Mass ratio: W(phenolic PF):W(phthalonitrile etherified phenolic BPN)=6:4

[0052] Dissolve linear phenolic acid: 4-nitrophthalonitrile: KOH in N,N-dimethylformamide (DMF) at a mass ratio of 7:4:6, stir evenly, react in a water bath at 85°C for 8h, cool, adjust the pH to neutral, and grind to obtain phthalonitrile etherified phenolic acid resin;

[0053] Ethanol and acetone solvents were respectively mixed in a mass ratio of 7:3 to obtain 50 g of a mixed solvent, phenolic resin and phthalonitrile etherified phenolic resin were added to the mixed solvent in a mass ratio of 6:4, and stirred at room temperature until completely dissolved to obtain a mixed solution;

[0054] According to the mixed solution: cross-linking agent mass ratio of 100:3, ammonium persulfate was added to the mixed solution and stirred evenly. After obtaining a uniform solution, it was placed in a reactor and placed in a constant temperature environment of 150°C for sol-gel reaction for 24 hours. After the reaction was complete, the reactor was taken out and cooled to room temperature. The wet gel was taken out and aged at room temperature for 2 days, and dried at 80°C under normal pressure for 12 hours until there was no change in weight, and then segmented curing was performed to obtain phthalonitrile-modified phenolic aerogel.

[0055] The prepared phthalonitrile modified phenolic aerogel was tested to have a 10% compression strength of 2.92 MPa, a room temperature thermal conductivity of 0.0394 W / m / K, an initial pyrolysis temperature of 495.08°C, a maximum pyrolysis rate temperature of 615.97°C, and a residual carbon content of 68.14%.

[0056] Example 4

[0057] Mass ratio: W(phenolic PF):W(phthalonitrile etherified phenolic BPN)=5:5

[0058] Dissolve linear phenolic acid: 4-nitrophthalonitrile: Ba(OH)2 in dimethylacetamide (DMAC) in a mass ratio of 8:5:6, stir evenly, react in a water bath at 80°C for 10 hours, cool, adjust the pH to neutral, and grind to obtain phthalonitrile etherified phenolic acid resin;

[0059] Ethanol and ethyl acetate were mixed in a mass ratio of 4:6 to prepare 50 g of a mixed solvent, phenolic resin and phthalonitrile etherified phenolic resin were added to the mixed solvent in a mass ratio of 5:5, and stirred at room temperature until completely dissolved to obtain a mixed solution;

[0060] According to the mixed solution: cross-linking agent mass ratio of 100:4, hexamethylenetetramine was added to the mixed solution and stirred evenly. After obtaining a uniform solution, it was placed in a reactor and placed in a constant temperature environment of 120°C for sol-gel reaction for 36 hours. After the reaction was complete, the reactor was taken out and cooled to room temperature. The wet gel was taken out and aged at room temperature for 1 day, and dried at 80°C under normal pressure for 12 hours until there was no change in weight. It was then cured in sections to obtain phthalonitrile-modified phenolic aerogel.

[0061] The prepared phthalonitrile modified phenolic aerogel was tested to have a 10% compression strength of 3.45 MPa, a room temperature thermal conductivity of 0.0412 W / M / K, an initial pyrolysis temperature of 513.12°C, a maximum pyrolysis rate temperature of 632.57°C, and a residual carbon of 67.96%.

[0062] Comparative Example 1

[0063] Mass ratio: W(phenolic PF):W(phthalonitrile etherified phenolic BPN) = 10:0

[0064] 25 g of ethanol and ethyl acetate solvent were respectively mixed in a mass ratio of 1:1 to obtain 50 g of a mixed solvent, 12.5 g of phenolic resin was added to the mixed solvent, and stirred evenly at room temperature until it was completely dissolved, 1.875 g of hexamethylenetetramine was added to the solution and stirred evenly to obtain a uniform solution, which was placed in a reactor and placed in a constant temperature environment for sol-gel reaction for 3 days. After the reaction was complete, the reactor was taken out and cooled to room temperature, the wet gel was taken out and aged at room temperature for 2 days, and dried at 80° C. under normal pressure for 12 hours until the weight did not change, and then cured in sections to obtain phthalonitrile-modified phenolic aerogel.

[0065] The prepared phthalonitrile modified phenolic aerogel was tested to have a 10% compression strength of 1.56 MPa, a room temperature thermal conductivity of 0.0369 W / m / K, an initial pyrolysis temperature of 364.21°C, a maximum pyrolysis rate temperature of 540.06°C, and a residual carbon of 55.39%.

[0066] Density, compression, simultaneous thermal analysis, and performance tests were performed on Examples 1-4 and compared with Comparative Example 1.

[0067] The test results are shown in Table 1

[0068]

[0069] From Table 1 and Figure 2 The TG curve of the phthalonitrile-modified phenolic aerogel prepared by the present invention shows that the phthalonitrile-modified phenolic aerogel prepared by the method of the present invention has a significantly improved thermal stability as the phthalonitrile content increases, while its density, compression strength and thermal conductivity are improved, that is, the initial pyrolysis temperature is increased by 148.91°C, the maximum pyrolysis rate temperature is increased by 92.51°C, and the residual carbon rate is increased by 12.57%. Generally speaking, when the BPN resin content is below 50%, the heat resistance stability of the aerogel is gradually improved as the phthalonitrile content increases. The introduction of phthalonitrile into phenolic aerogel achieves the goal of improving the thermal stability of phenolic aerogel and increasing the residual carbon rate without significantly increasing its density and thermal conductivity.

[0070] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solution disclosed in the present invention, technicians in this field can make some substitutions and deformations to some technical features therein according to the disclosed technical content without creative labor, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A method for preparing phthalonitrile-modified phenolic aerogel, characterized in that: The following steps are involved: a. dissolving linear phenolic acid aldehyde, 4-nitrophthalonitrile and alkaline catalyst in an organic solvent in a mass ratio of (5-8): (2-5): (2-5), stirring uniformly, reacting in a water bath, cooling, adjusting the pH and powdering to obtain phthalonitrile etherified phenolic resin; b. dissolving the purchased linear phenolic resin and phthalonitrile etherified phenolic resin in a mixed solvent in a mass ratio of (5-8): (2-5), stirring uniformly to obtain a mixed solution; c. Add the crosslinking agent to the mixed solution at a mass ratio of 100:(2-4) and stir evenly to obtain a precursor solution; d. The precursor solution undergoes a sol-gel reaction to obtain a wet gel, which is then aged, segmentally cured, and dried to obtain a phthalonitrile-modified phenolic aerogel.

2. The method for preparing phthalonitrile-modified phenolic aerogel according to claim 1, wherein The organic solvent is any one of pyrrolidone, dimethyl sulfoxide, dimethylformamide or dimethylacetamide.

3. The method for preparing phthalonitrile-modified phenolic aerogel according to claim 1, wherein The mixed solvent is prepared by configuring a main solvent and a secondary solvent in a mass ratio of (5-8):(2-5), the main solvent is ethanol, and the secondary solvent is ethyl acetate or tetrahydrofuran.

4. The method for preparing phthalonitrile-modified phenolic aerogel according to claim 1, wherein The alkaline catalyst is any one of NaOH, KOH, K2CO3, Na2CO3 or Ba(OH)2.

5. The method for preparing phthalonitrile-modified phenolic aerogel according to claim 1, wherein The cross-linking agent is any one of ammonium persulfate, potassium persulfate or hexamethylenetetramine.

6. The method for preparing phthalonitrile-modified phenolic aerogel according to claim 1, wherein In step a, react in a water bath at 80-90° C. for 5-10 hours, cool to room temperature, and adjust the pH to neutral.

7. The method for preparing phthalonitrile-modified phenolic aerogel according to claim 1, wherein: In step d, a sol-gel reaction is carried out at a temperature of 90 to 150° C. for 24 to 72 hours, and then aged at room temperature for 1 to 3 days.

8. The method for preparing phthalonitrile-modified phenolic aerogel according to claim 1, wherein The segmented curing process is: 90℃ / 0.5h, 110℃ / 1h, 140℃ / 2h, 150℃ / 3h, 180℃ / 2h, 200℃ / 2h, 220℃ / 4h, and drying at normal pressure.

9. The phthalonitrile-modified phenolic aerogel prepared by the method according to any one of claims 1 to 8.

10. Use of the phthalonitrile-modified phenolic aerogel according to claim 9 in a thermal protection system of aerospace equipment.

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