Polyacrylonitrile-based hollow carbon fibers for thermal protection materials and their preparation method

Abstract

This invention belongs to the field of ablation-resistant thermal insulation composite material preparation technology, specifically relating to a polyacrylonitrile-based hollow carbon fiber for thermal protection materials and its preparation method. The invention first pre-oxidizes porous polyacrylonitrile precursor fibers to obtain porous pre-oxidized fibers. Then, phenolic resin is coated onto the surface of the pre-oxidized fibers through solution polymerization. After impregnation in an organosilicon source solution, rapid Joule heating at high temperature deposits both phenolic carbon microspheres with excellent ablation resistance and a silica coating on the fiber surface, forming a unique hollow carbon fiber structure with excellent thermal insulation properties. This solves the problem that polyacrylonitrile-based carbon fibers cannot simultaneously achieve both thermal insulation and ablation resistance. The thermal conductivity of the polyacrylonitrile-based hollow carbon fiber prepared by this method is about 17% lower than that of porous polyacrylonitrile carbon fibers prepared by traditional carbonization methods, making it applicable to the field of integrated ablation-resistant thermal protection materials.

Description

Technical Field

[0001] This invention belongs to the field of ablation thermal insulation composite material technology, specifically relating to a polyacrylonitrile-based hollow carbon fiber for thermal protection materials and its preparation method. Background Technology

[0002] Solid rocket motors are the power subsystems of launch vehicles and other aircraft, providing continuous power to meet design requirements. Ablation-resistant and heat-insulating integrated composite materials are among the most widely used thermal protection materials in solid rocket motors, offering excellent ablation resistance and thermal insulation properties that ensure the safe operation of these motors. Polyacrylonitrile-based carbon fibers possess excellent mechanical properties, good thermal stability, and high ablation residue, making them the preferred reinforcing material for ablation-resistant and heat-insulating composite materials. However, this fiber has a significantly higher thermal conductivity compared to other fibers, resulting in poorer thermal insulation performance, and the thermal insulation properties of carbon fiber insulation materials prepared from it are not ideal. Therefore, to meet the application requirements of polyacrylonitrile-based carbon fibers in the field of thermal protection materials, the preparation of ablation-resistant and heat-insulating integrated carbon fibers should be achieved through carbon fiber structural design and microstructure control.

[0003] Porous carbon fibers possess extremely low thermal conductivity due to their abundant pore structure. However, compared to traditional carbon fibers, the weaker skeletal structure leads to a decrease in the ablation resistance of porous carbon fibers. Therefore, developing a polyacrylonitrile-based carbon fiber that can balance the thermal insulation and ablation resistance properties of carbon fibers and is suitable for ablation-resistant thermal insulation composite materials is one of the urgent technical challenges to be addressed. Summary of the Invention

[0004] To address the issue that polyacrylonitrile-based carbon fibers cannot simultaneously achieve both thermal insulation and ablation resistance, this invention provides a polyacrylonitrile-based hollow carbon fiber for thermal protection materials and its preparation method.

[0005] This invention first pre-oxidizes porous precursor fibers to obtain pre-oxidized fibers with a porous structure. Then, phenolic resin is coated onto the surface of the pre-oxidized fibers through solution polymerization. After impregnation in an organosilicon source solution, rapid Joule heating at high temperature yields hollow carbon fibers with a surface simultaneously modified with phenolic carbon microspheres and a dense silica coating. The Joule heating flash evaporation technology rapidly heats the carbon source to ultra-high temperatures using a burst of high current pulses. This causes a large number of gas molecules to escape in a short time, further redistributing and expanding the pores within the porous structure. After the pulse ends, the system cools rapidly, fixing the new pore structure and forming a hollow carbon fiber. Furthermore, the excellent ablation resistance of the phenolic carbon microspheres and silica coating significantly improves the ablation resistance of the hollow carbon fiber. Simultaneously, the unique hollow carbon fiber structure with excellent thermal insulation properties formed by the preparation technology of this invention achieves excellent integrated performance of carbon fiber ablation and thermal insulation. Furthermore, compared to carbon fibers obtained from conventional carbonization furnaces, the rapid Joule heating carbonization process, with its ultra-fast carbonization rate and extremely short heating time, intensifies the thermal motion of carbon atoms and rearranges them into a new crystal structure, resulting in carbon fibers with a lower degree of graphitization, thereby further reducing the thermal conductivity of the carbon fibers themselves.

[0006] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0007] A method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials includes the following steps:

[0008] Step 1: Pre-oxidize the porous polyacrylonitrile fiber precursor to obtain pre-oxidized fiber;

[0009] Step 2: Place the pre-oxidized fiber in a phenolic resin polymerization solution prepared with resorcinol and formaldehyde and heat to react to obtain phenolic resin modified pre-oxidized fiber, and then dry it.

[0010] Step 3: Immerse the phenolic resin-modified pre-oxidized fiber in an ethanol solution containing an organosilicon source and then dry it.

[0011] Step 4: The pre-oxidized fibers impregnated with silicon source are fixed on a graphite block, a draw ratio of 0.8% to 1.5% is applied, and the blocks are placed in a rapid Joule heating device and heated in a vacuum environment to react and obtain polyacrylonitrile-based hollow carbon fibers modified with phenolic carbon and silica coating.

[0012] Furthermore, the temperature of the pre-oxidation treatment in step 1 is 220~250℃, and the time is 20~40 minutes.

[0013] Furthermore, the mass ratio of resorcinol, formaldehyde, and deionized water in the phenolic resin polymerization solution is 1:0.5 to 2:20.

[0014] Furthermore, in step 2, the heating reaction temperature is 50~95℃, and the time is 0.2~2 hours.

[0015] Furthermore, in step 2, the mass ratio of pre-oxidized fiber to resorcinol is 0.2~3:1.

[0016] Furthermore, in step 2, the drying temperature is 60~120℃, and the drying time is 1~6 hours.

[0017] Furthermore, in step 3, the organosilicon source in the ethanol solution containing the organosilicon source is any one or more of methyl orthosilicate, ethyl orthosilicate, tetraethyl orthosilicate, and silane coupling agent in any proportion; the mass percentage content of the organosilicon source in the ethanol is 30% to 65%.

[0018] Furthermore, in step 3, the soaking time is 10-60 seconds; the drying temperature is 60-100℃, and the time is 0.5-4 hours.

[0019] Furthermore, the reaction temperature in step 4 is 1300~1800℃, and the reaction time is 10~60 seconds.

[0020] This invention provides a type of polyacrylonitrile-based hollow carbon fiber for use in thermal protection materials. The hollow carbon fiber has an average outer diameter of 16-20 μm and an average inner diameter (i.e., the diameter of the hollow portion) of 7-10 μm; its bulk density is 1.51-1.56 g / cm³. 3 The tensile strength of the single filament is 400~700MPa, and the axial thermal conductivity is 3.7~3.9W / m*K.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] This invention utilizes a hollow structure built inside the fiber and a phenolic carbon microsphere and silica coating deposited on the fiber surface, exhibiting excellent ablation resistance. This reduces the thermal conductivity of the carbon fiber while simultaneously improving its ablation resistance, thus balancing the thermal insulation and ablation resistance properties of polyacrylonitrile-based carbon fibers. This makes it suitable for the field of thermal protection materials. The molding process of this invention is simple, and the preparation cycle is short. The prepared polyacrylonitrile-based hollow carbon fibers can replace traditional carbon fibers as a reinforcing material for the thermal insulation of solid rocket engines, solving the problem that traditional thermal insulation materials cannot simultaneously possess low density, low thermal conductivity, ablation resistance, and erosion resistance. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 The image shows a SEM image of the hollow carbon fiber prepared in Example 1.

[0025] Figure 2 The image shows a SEM image of the hollow carbon fiber prepared in Example 1.

[0026] Figure 3 The image shows a SEM image of the porous carbon fiber prepared in Comparative Example 1.

[0027] Figure 4 The image shows a SEM image of the porous carbon fiber prepared in Comparative Example 1.

[0028] Figure 5 The image shows the EDS energy spectrum of the hollow carbon fiber prepared in Example 2.

[0029] Figure 6 The image shows the Raman spectrum of the hollow carbon fiber prepared in Example 1.

[0030] Figure 7 The image shows the Raman spectrum of the porous carbon fiber prepared in Comparative Example 1. Detailed Implementation

[0031] To gain a deeper understanding of this invention, we will provide a comprehensive and detailed description. However, this invention has various implementations and is not limited to the specific examples listed herein. These examples are presented to enhance a full understanding of the disclosure of this invention.

[0032] Example 1

[0033] A method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials includes the following steps:

[0034] Step 1: The porous polyacrylonitrile precursor fiber is pre-oxidized at a temperature of 220℃ for 30 minutes to obtain a porous pre-oxidized fiber.

[0035] Step 2: Prepare a phenolic resin polymerization solution by adding resorcinol and formaldehyde to deionized water, adjusting the mass ratio of resorcinol:formaldehyde:deionized water to 1:0.5:20. Then, place the pre-oxidized fiber in the polymerization solution and heat at 50°C for 2 hours to obtain phenolic resin-modified pre-oxidized fiber, which is then dried in a 60°C forced-air oven for 6 hours. The mass ratio of pre-oxidized fiber to resorcinol is controlled at 0.2:1.

[0036] Step 3: Immerse the dried pre-oxidized fiber in an ethanol solution with a methyl silicate content of 50% for 20 seconds, and then dry it in a 100°C forced-air oven for 0.5 hours.

[0037] Step 4: Fix the impregnated pre-oxidized fiber onto a graphite block, apply a draw ratio of 0.8%, and place it in a rapid Joule heating device. Heat it at 1300°C in a vacuum environment for 60 seconds to obtain polyacrylonitrile-based hollow carbon fibers modified with phenolic carbon and silica coating.

[0038] Example 2

[0039] A method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials includes the following steps:

[0040] Step 1: The porous polyacrylonitrile precursor fiber is pre-oxidized at a temperature of 230℃ for 40 minutes to obtain a porous pre-oxidized fiber.

[0041] Step 2: Prepare a phenolic resin polymerization solution by adding resorcinol and formaldehyde to deionized water, adjusting the mass ratio of resorcinol:formaldehyde:deionized water to 1:1:20. Then, place the pre-oxidized fiber in the polymerization solution and heat at 95°C for 0.2 hours to obtain phenolic resin-modified pre-oxidized fiber, which is then dried in a 120°C forced-air oven for 1 hour. The mass ratio of pre-oxidized fiber to resorcinol is controlled at 0.8:1.

[0042] Step 3: Immerse the dried pre-oxidized fiber in an ethanol solution with a methyl silicate content of 30% for 10 seconds, and then dry it in a 90°C forced-air oven for 1 hour.

[0043] Step 4: Fix the impregnated pre-oxidized fiber onto a graphite block, apply a draw ratio of 1.0%, and place it in a rapid Joule heating device. Heat it at 1800°C for 10 seconds in a vacuum environment to obtain polyacrylonitrile-based hollow carbon fibers modified with phenolic carbon and silica coating.

[0044] Example 3

[0045] A method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials includes the following steps:

[0046] Step 1: The porous polyacrylonitrile precursor fiber is pre-oxidized at a temperature of 240℃ for 20 minutes to obtain a porous pre-oxidized fiber.

[0047] Step 2: Prepare a phenolic resin polymerization solution by adding resorcinol and formaldehyde to deionized water, adjusting the mass ratio of resorcinol:formaldehyde:deionized water to 1:1.5:20. Then, place the pre-oxidized fiber in the polymerization solution and heat at 60°C for 2 hours to obtain phenolic resin-modified pre-oxidized fiber, which is then dried in an 80°C forced-air oven for 4 hours. Maintain the mass ratio of pre-oxidized fiber to resorcinol at 1.5:1.

[0048] Step 3: Immerse the dried pre-oxidized fiber in an ethanol solution with a tetraethyl orthosilicate content of 40% for 30 seconds, and then dry it in an 80°C forced-air oven for 2 hours.

[0049] Step 4: Fix the impregnated pre-oxidized fiber onto a graphite block, apply a draw ratio of 1.2%, and place it in a rapid Joule heating device. Heat it at 1400°C in a vacuum environment for 50 seconds to obtain polyacrylonitrile-based hollow carbon fibers modified with phenolic carbon and silica coating.

[0050] Example 4

[0051] A method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials includes the following steps:

[0052] Step 1: The porous polyacrylonitrile precursor fiber is pre-oxidized at a temperature of 230℃ for 30 minutes to obtain a porous pre-oxidized fiber.

[0053] Step 2: Prepare a phenolic resin polymerization solution by adding resorcinol and formaldehyde to deionized water, adjusting the mass ratio of resorcinol:formaldehyde:deionized water to 1:2:20. Then, place the pre-oxidized fiber in the polymerization solution and heat at 70°C for 1.5 hours to obtain phenolic resin-modified pre-oxidized fiber, which is then dried in a 90°C forced-air oven for 3 hours. Maintain the mass ratio of pre-oxidized fiber to resorcinol at 2:1.

[0054] Step 3: Immerse the dried pre-oxidized fiber in an ethanol solution with a tetraethyl orthosilicate content of 60% for 40 seconds, and then dry it in a 60°C forced-air oven for 4 hours.

[0055] Step 4: Fix the impregnated pre-oxidized fiber onto a graphite block, apply a draw ratio of 1.5%, and place it in a rapid Joule heating device. Heat it at 1500°C in a vacuum environment for 40 seconds to obtain polyacrylonitrile-based hollow carbon fibers modified with phenolic carbon and silica coating.

[0056] Example 5

[0057] A method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials includes the following steps:

[0058] Step 1: The porous polyacrylonitrile precursor fiber is pre-oxidized at a temperature of 250℃ for 20 minutes to obtain a porous pre-oxidized fiber.

[0059] Step 2: Prepare a phenolic resin polymerization solution by adding resorcinol and formaldehyde to deionized water, adjusting the mass ratio of resorcinol:formaldehyde:deionized water to 1:1:20. Then, place the pre-oxidized fiber in the polymerization solution and heat at 80°C for 1 hour to obtain phenolic resin-modified pre-oxidized fiber, which is then dried in a 100°C forced-air oven for 2 hours. Maintain the mass ratio of pre-oxidized fiber to resorcinol at 3:1.

[0060] Step 3: Immerse the dried pre-oxidized fiber in an ethanol solution with a tetraethyl orthosilicate content of 40% for 60 seconds, and then dry it in a 70°C forced-air oven for 3 hours.

[0061] Step 4: Fix the impregnated pre-oxidized fiber onto a graphite block, apply a draw ratio of 1.3%, and place it in a rapid Joule heating device. Heat it at 1600°C in a vacuum environment for 30 seconds to obtain polyacrylonitrile-based hollow carbon fibers modified with phenolic carbon and silica coating.

[0062] Comparative Example 1

[0063] Step 1: The porous polyacrylonitrile precursor fiber is pre-oxidized in air at a temperature of 255°C for 20 minutes to obtain a porous pre-oxidized fiber.

[0064] Step 2: Apply a draw ratio of 1.2% to the porous pre-oxidized fiber and perform carbonization treatment. The protective atmosphere is argon, the treatment temperature is 1400℃, and the residence time is 5min to obtain polyacrylonitrile-based porous carbon fiber.

[0065] The performance test results of polyacrylonitrile-based hollow carbon fibers used in thermal protection materials are shown in the table below.

[0066] Table 1. Performance test results of polyacrylonitrile-based hollow carbon fibers for thermal protection materials.

[0067]

[0068] Comparison of the results from the proportions and examples shows that the graphitization degree of the polyacrylonitrile-based hollow carbon fibers prepared by this invention is significantly lower than that of porous carbon fibers, and the thermal conductivity of the hollow carbon fibers is about 17% lower than that of the porous polyacrylonitrile-based carbon fibers prepared by the conventional carbonization furnace method. Furthermore, the phenolic carbon microspheres and silica coating with excellent ablation resistance deposited on the surface of the hollow carbon fibers can improve the ablation resistance of the carbon fibers, realizing the preparation of ablation-resistant and heat-insulating integrated carbon fibers.

[0069] Contents not described in detail in this specification are prior art known to those skilled in the art. Although illustrative specific embodiments of the invention have been described above to facilitate understanding by those skilled in the art, it should be understood that the invention is not limited to the scope of the specific embodiments. Various modifications are readily apparent to those skilled in the art as long as they fall within the spirit and scope of the invention as defined and determined by the appended claims, and all inventions utilizing the concept of this invention are protected.

Claims

1. A method for producing polyacrylonitrile-based hollow carbon fibers for thermal protection materials, characterized by: Includes the following steps: Step 1: Pre-oxidize the porous polyacrylonitrile fiber precursor to obtain pre-oxidized fiber; Step 2: Place the pre-oxidized fiber in a phenolic resin polymerization solution prepared with resorcinol and formaldehyde and heat to react to obtain phenolic resin modified pre-oxidized fiber, and then dry it. Step 3: Immerse the phenolic resin-modified pre-oxidized fiber in an ethanol solution containing an organosilicon source and then dry it. Step 4: The pre-oxidized fibers impregnated with silicon source are fixed on a graphite block, a draw ratio of 0.8% to 1.5% is applied, and the blocks are placed in a rapid Joule heating device and heated in a vacuum environment to react and obtain polyacrylonitrile-based hollow carbon fibers modified with phenolic carbon and silica coating.

2. The method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials according to claim 1, characterized in that: The pre-oxidation treatment in step 1 is carried out at a temperature of 220~250℃ for 20~40 minutes.

3. The method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials according to claim 1, characterized in that: The mass ratio of resorcinol, formaldehyde, and deionized water in the phenolic resin polymerization solution is 1:0.5 to 2:

20.

4. The method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials according to claim 1, characterized in that: In step 2, the heating reaction temperature is 50~95℃, and the time is 0.2~2 hours.

5. The method of claim 1, wherein the polyacrylonitrile-based hollow carbon fiber for a thermal protection material is characterized by: In step 2, the mass ratio of pre-oxidized fiber to resorcinol is 0.2~3:

1.

6. The method of claim 1, wherein the polyacrylonitrile-based hollow carbon fiber for a thermal protection material is characterized by: In step 2, the drying temperature is 60~120℃ and the drying time is 1~6 hours.

7. The method of claim 1, wherein the polyacrylonitrile-based hollow carbon fiber for a thermal protection material is characterized by: In step 3, the organosilicon source in the ethanol solution containing the organosilicon source is any one or more of methyl orthosilicate, ethyl orthosilicate, tetraethyl orthosilicate, and silane coupling agent in any proportion; the mass percentage content of the organosilicon source in the ethanol is 30%~65%.

8. The method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials according to claim 1, characterized in that: The soaking time in step 3 is 10-60 seconds; the drying temperature is 60-100℃ and the time is 0.5-4 hours.

9. The method for preparing polyacrylonitrile-based hollow carbon fibers for thermal protection materials according to claim 1, characterized in that: The reaction temperature in step 4 is 1300~1800℃, and the reaction time is 10~60 seconds.

10. A polyacrylonitrile-based hollow carbon fiber for thermal protection materials prepared by the preparation method according to any one of claims 1 to 9, characterized in that: The average outer diameter of the hollow carbon fiber is 16-20 mu m, the average inner diameter, i.e., the hollow portion diameter, is 7-10 mu m, the bulk density is 1.51-1.56 g / cm 3 , the single-filament tensile strength is 400-700 MPa, and the axial thermal conductivity is 3.7-3.9 W / m*K.

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