Preparation method of gradient density ablation-resistant material

Abstract

The invention relates to the technical field of composite material forming processes, and particularly provides a preparation method of a gradient-density ablation-resistant material, which comprises the following steps: preparing a preform which comprises a high-density fiber layer for preparing an ablation-resistant layer and a low-density fiber layer for preparing a thermal insulation layer; a preparation mold is designed according to the appearance structure of a formed product, the prefabricated body is laid in the preparation mold, modified phenolic resin is injected into the preparation mold in a vacuum mode through an RTM injection machine, and the gradient-density ablation-resistant material is obtained after curing and drying. The preparation of the ablation-resistant material with gradient density is completed with a low-cost and high-efficiency method, the integrally-formed prefabricated body is adopted, the process is simple, the preparation period is short, the influence of an adhesive on the connection reliability of the ablation-resistant layer and the thermal insulation layer in a traditional method is avoided, meanwhile, the interior of the prefabricated body is reinforced by Z-direction needling fibers, and the thermal insulation performance of the thermal insulation layer is improved. The interface strength is improved, and the buffer layer preparation process is omitted.

Description

Technical Field

[0001] The invention relates to the technical field of composite material forming technology, and specifically provides a method for preparing a gradient density ablation-resistant material. Background Art

[0002] When a high-speed aircraft flies in the atmosphere, the surrounding air temperature rises sharply due to its high flight speed, and the heat energy is quickly transferred to the surface of the aircraft, causing the temperature of the outer surface of the aircraft structure to rise sharply. Traditional materials have high density and high thermal conductivity and cannot be used in the thermal protection system of high-speed aircraft.

[0003] Ablative heat protection is the most important heat protection method in single-use spacecraft heat protection structures. It is a semi-passive heat control technology. The ablative structure is achieved by taking away a large amount of heat through the consumption of the surface mass of the material under the action of heat flow, such as decomposition, melting, evaporation, sublimation, erosion and other physical and chemical changes, and preventing external heat from transferring to the interior of the structure. The traditional thermal protection system is formed by bonding the ablative layer and the thermal insulation layer separately. The temperature resistance of the adhesive seriously affects the performance of the entire thermal protection system. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a method for preparing a gradient density ablation-resistant material, and utilizes the RTM (Resin Transfer Molding) process to inject modified phenolic resin into a 2.5D preform with a gradient density, wherein the ablation-resistant layer and the thermal insulation layer are integrally formed, and Z-direction needle punching is used between the ablation-resistant layer and the thermal insulation layer. Compared with the traditional method of forming separately and then bonding, the ablation-resistant material obtained by the present invention has the advantages of large specific heat, small thermal conductivity, high residual carbon rate, high layer shear strength, etc.

[0005] The method for preparing the gradient density ablation-resistant material provided by the present invention comprises: S1: providing a preform, the preform comprising a high-density fiber layer and a low-density fiber layer integrally formed, the high-density fiber layer being used to prepare an ablation-resistant layer of a gradient density ablation-resistant material, and the low-density fiber layer being used to prepare a heat-insulating layer of the gradient density ablation-resistant material; S2: applying a release agent to a mold for preparing a gradient density ablation-resistant material; S3: Laying the preform in the preparation mold; S4: The prepared mold is evacuated, and a modified phenolic resin that has been treated with a vacuum degassing process is injected into the prepared mold using an RTM injection machine to obtain a gradient density ablation-resistant material.

[0006] Preferably, the preform is provided with Z-direction needles to improve the layer shear strength of the ablation-resistant layer and the heat-insulating layer.

[0007] Preferably, the preform is made of high silica cloth and glass fiber mesh.

[0008] Preferably, the preparation mold is provided with at least one glue injection port and one glue discharge port.

[0009] Preferably, in S2, the release agent is applied at least twice.

[0010] Preferably, the thickness of the preform is greater than the theoretical preparation thickness of the gradient density ablation-resistant material, and the thickness difference is at least 2 mm.

[0011] Preferably, the vacuum degree during vacuum degassing of the modified phenolic resin is -0.07 MPa to -0.1 MPa.

[0012] Preferably, in S4, the process of injecting the modified phenolic resin into the preparation mold using an RTM injection machine is: The vacuum degree inside the mold was set to -0.1 MPa; Open the injection port, inject the modified phenolic resin into the preparation mold with a pressure of 0.5 MPa until the modified phenolic resin completely fills the preparation mold, and record the first injection time; Open the glue discharge port and continue to inject the modified phenolic resin into the prepared mold several times to discharge the bubbles.

[0013] Preferably, it also includes: S5: curing the gradient density ablation-resistant material in the prepared mold; S6: demolding the gradient density ablation-resistant material; S7: Drying the gradient density ablation-resistant material after demoulding.

[0014] Preferably, in S5, during curing, the gradient density ablation-resistant material in the prepared mold is heated to 60° C. for an initial heating time of 60 minutes; after curing at 60° C. for 60 minutes, the material is heated to 110° C. for a secondary heating time of 100 minutes, and then cured at 110° C. for 500 minutes; In S7, the gradient density ablation-resistant material after demoulding is heated to 110° C. during drying, and the heating time is 120 minutes; and the material is cured at 110° C. for 600 minutes.

[0015] Compared with the prior art, the present invention can achieve the following beneficial effects: The preform in the present invention is made in one piece, and the preform is divided into a high-density fiber layer and a low-density fiber layer. The high-density fiber layer is used to prepare the ablation-resistant layer. The high-density fiber arrangement can effectively absorb and disperse heat, improve thermal stability, and reduce thermal conductivity; the low-density fiber layer is used to prepare the thermal insulation layer. The low-density fiber arrangement can reduce the thermal capacity of the material and improve the thermal insulation efficiency, and can also reduce the weight of the structure. The gradient density fiber layer can take into account the different thermal properties of the ablation-resistant layer and the thermal insulation layer to meet different performance requirements. In addition, Z-direction needle punching is used between the ablation-resistant layer and the thermal insulation layer to increase the layer shear strength. The one-piece manufacturing method not only shortens the process cycle of the ablation-resistant layer and the thermal insulation layer, but also avoids the influence of adhesives on the overall thermal protection system in the traditional method.

[0016] The present invention adopts modified phenolic resin and optimizes the process of injecting modified phenolic resin, which can effectively avoid the problem of excessive porosity in ablation-resistant materials; the present invention utilizes the RTM process to inject modified phenolic resin, and the obtained ablation-resistant material has good surface state, large specific heat, small thermal conductivity, high residual carbon rate, high layer shear strength, and controllable molding size accuracy, which greatly reduces the workload of later research and preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a structural diagram of a gradient density ablation-resistant material provided according to an embodiment of the present invention; Figure 2 is a schematic structural diagram of a mold preparation method according to an embodiment of the present invention; Figure 3 It is a cross-sectional structural diagram of a preparation mold provided according to an embodiment of the present invention.

[0018] Reference numerals include: Ablation-resistant layer 1, heat-insulating layer 2, upper cover plate 3, middle frame 4, lower cover plate 5, glue injection port 6, glue discharge port 7, sealing groove 8. DETAILED DESCRIPTION

[0019] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, the same modules are represented by the same reference numerals. In the case of the same reference numerals, their names and functions are also the same. Therefore, the detailed description thereof will not be repeated.

[0020] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0021] Embodiment 1, embodiment 1 of the invention provides a method for preparing a gradient density ablation-resistant material, which specifically comprises the following steps: S1: Figure 1 As shown, first, a 2.5D preform is prepared according to the shape structure of the molded product (that is, the gradient density ablation-resistant material to be prepared). The molded product is mainly layered into an ablation-resistant layer 1 and a thermal insulation layer 2. The main function of the ablation-resistant layer 1 is to protect the structure from damage in an extremely high temperature environment. The ablation-resistant layer 1 needs to have high temperature resistance, good thermal stability and low thermal conductivity. The function of the thermal insulation layer 2 is to reduce the transfer of heat from the ablation-resistant layer 1 to the internal structure. In the traditional process, the ablation-resistant layer 1 and the thermal insulation layer 2 are prepared separately, and then the ablation-resistant layer 1 and the thermal insulation layer 2 are connected together by glue. During the bonding process of the ablation-resistant layer 1 and the thermal insulation layer 2, not only a buffer layer needs to be designed, but also the glue has poor stability in a high temperature environment, which will affect the stability of the molded product.

[0022] Therefore, the embodiment of the present invention has specially designed the preform, which is prepared by high-silica cloth and glass fiber mesh. The preform is an integrated structure, which is divided into a high-density fiber layer and a low-density fiber layer as a whole. The preform is provided with Z-direction needles, that is, short fibers are inserted in the vertical direction. The Z-direction needles can improve the layer shear strength of the ablation-resistant layer 1 and the thermal insulation layer 2. Among them, the high-density fiber layer is used to prepare the ablation-resistant layer 1, which can ensure that the prepared ablation-resistant layer 1 has a higher fiber density, so as to provide better thermal protection performance. The low-density fiber layer is used to prepare the thermal insulation layer 2, which can ensure that the prepared thermal insulation layer 2 has a lower fiber density, which can usually absorb and disperse heat more effectively, thereby protecting the internal structure.

[0023] S2: Figure 2 and Figure 3 As shown, a preparation mold for gradient density ablation-resistant material is designed according to the appearance structure of the molded product. The preparation mold is mainly composed of an upper cover plate 3, a middle frame 4 and a lower cover plate 5. At least one glue injection port 6 and a glue discharge port 7 are arranged on the upper cover plate 3, and the glue injection port 6 and the glue discharge port 7 can be plugged or opened. When the glue injection port 6 and the glue discharge port are plugged, the preparation mold can be regarded as a closed container. In order to improve the airtightness of the preparation mold, a sealing groove 8 should be arranged at the contact gap between the upper cover plate 3 and the middle frame 4, and the sealing groove 8 adopts a flexible sealing strip.

[0024] When the mold is used for the first time, it should be placed in an oven to remove oil, and then cleaned with anhydrous ethanol. Then apply a release agent to the inner surface of the mold (i.e. the molding surface of the product), and the release agent should be applied at least twice, with a time interval of at least 15 minutes. After applying the release agent for the first time, let it stand for 15 minutes until the release agent is completely cured, then apply the release agent for the second time, and let it stand until the release agent is completely cured.

[0025] After the release agent is applied, install the sealing groove 8 at the contact gap between the upper cover plate 3 and the middle frame 4, then buckle the upper cover plate 3 to close the mold, the mold gap is ≤0.05mm, and then plug the injection port 6 and the discharge port 7. Use compressed air to perform an airtightness test on the mold after closing the mold, pressurize the inside of the mold to 0.4MPa, and keep the pressure for at least 30 minutes. It is required that the pressure drop inside the mold after 30 minutes is not more than 1% of the internal air pressure of the mold after the initial pressurization. If the airtightness requirements are met, the next step can be performed.

[0026] S3: Remove the upper cover plate 3 and lay the preform in the preparation mold. The thickness of the preform is required to be greater than the theoretical preparation thickness of the gradient density ablation-resistant material, and the thickness difference between the two is at least 2 mm to ensure that the compression amount of the preform during the preparation process is met. In Invention Example 1, the preform is obtained by weaving a 5 mm thick high-silica cloth and a glass fiber mesh, and the total thickness of the preform is 12-13 mm.

[0027] The upper cover plate 3 is buckled to close the mold, and the mold gap is ≤0.05mm. The compressed air is used again to perform an airtightness test on the preparation mold after mold closing. The inside of the preparation mold is pressurized to 0.4MPa, and the pressure is maintained for at least 30 minutes. It is also required that the pressure drop inside the preparation mold after 30 minutes is not greater than 1% of the internal air pressure of the preparation mold after the initial pressurization.

[0028] S4: Provide modified phenolic resin, and take part of it to measure the viscosity coefficient of the modified phenolic resin. The viscosity coefficient should be 200-500mPa·S. Pour the modified phenolic resin into the barrel of the RTM injection machine, and perform vacuum degassing on the modified phenolic resin. Use a vacuum pump for degassing. During the degassing process using the vacuum pump, the vacuum degree should be -0.07MPa~-0.1MPa, and the vacuum degassing time should last at least 30 minutes. After the degassing is completed, some resin can be taken to observe the internal state of the resin. If there are no obvious bubbles, it means that the vacuum degassing is completed.

[0029] In order to facilitate the injection of modified phenolic resin into the preparation mold, it is necessary to establish an air pressure difference between the preparation mold and the modified phenolic resin barrel. First, use a vacuum pump to evacuate the preparation mold. Since the preform in the preparation mold is a combination of fluffy fiber mesh and fiber cloth, its internal capillary tension can cause a large amount of air to remain. Therefore, the vacuum pump is used for the vacuuming process. At least -0.1MPa pressure is used for vacuuming, and the vacuuming time is at least 60min. When the vacuum degree inside the preparation mold is -0.1Mpa, turn off the vacuum pump. Then use compressed air to pressurize the barrel storing the modified phenolic resin, and the pressure is at least 0.5Mpa.

[0030] Open the injection port 6, and use the RTM injection machine to inject the modified phenolic resin into the preparation mold. The injection pressure is 0.5Mpa. When the modified phenolic resin completely fills the preparation mold, the initial injection is completed, and the initial injection time, that is, the duration from the start to the end of the injection, is recorded. At this time, there are still bubbles in the modified phenolic resin in the preparation mold. At this time, open the glue discharge port 7, and use the extended injection time to discharge the bubbles. Continue to inject the modified phenolic resin into the preparation mold. The extended injection time should at least exceed the initial glue injection time. Repeat the above bubble discharge process more than three times, and after completion, plug the glue injection port 6 and the glue discharge port 7.

[0031] S5: Heat and cure the gradient density ablation-resistant material in the prepared mold. During the curing process, the gradient density ablation-resistant material in the prepared mold is heated several times. The temperature is first heated from room temperature to 60°C for 60 minutes. After curing at 60°C for 60 minutes, the material is heated to 110°C for the second time for 100 minutes. The material is cured at 110°C for 500 minutes. At this time, the gradient density ablation-resistant material has reached the demolding condition.

[0032] S6: Use the top screw to open the upper cover plate 3 of the preparation mold. When opening the upper cover plate 3, it is necessary to ensure that the upper cover plate 3 is stable and cannot be tilted. After the upper cover plate 3 is removed, the sealing groove 8, the lower cover plate 5 and the middle frame 4 are removed in sequence to complete the demoulding.

[0033] S7: After demoulding, the gradient density ablation-resistant material needs to be dried. The gradient density ablation-resistant material is placed in an oven and heated from room temperature to 110°C for 120 minutes. The material is then cured at 110°C for 600 minutes. After drying, the surface color of the gradient density ablation-resistant material is uniform and there is no resin cracking. The preparation of the gradient density ablation-resistant material is completed.

[0034] Example 2: The preparation process of Example 2 of the present invention is exactly the same as that of Example 1, with the only difference being that the preform is woven and wound with a 10 mm thick high-silica cloth and a glass fiber mesh, and the total thickness of the preform is 32-33 mm. Other processes and conditions are exactly the same.

[0035] The present invention successfully prepared gradient density ablation-resistant materials through Examples 1 and 2. The performance comparison of the materials is as follows: Table 1 Comparison of material properties

[0036] From the above results, it can be seen that the overall density of the gradient density ablation-resistant material prepared in Example 1 and Example 2 is much lower than that of the traditional existing thermal protection material, and the thermal conductivity is close to one-tenth of the existing thermal protection material, which can effectively achieve heat insulation, and the specific heat capacity is close to 4 times that of the existing thermal protection material, that is, when the gradient density ablation-resistant material prepared in the present invention and the existing thermal protection material of the same mass are raised to the same temperature, the gradient density ablation-resistant material prepared in the present invention can absorb 4 times the heat of the existing thermal protection material. It can be seen that the gradient density ablation-resistant material prepared in the present invention can effectively isolate heat from being transferred to the inside, and meet the requirements of the aircraft thermal protection system.

[0037] Although the embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and cannot be understood as limiting the present invention. Those skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

[0038] The above specific implementations of the present invention do not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for preparing a gradient density ablation-resistant material, characterized in that: include: S1: providing a preform, wherein the preform comprises a high-density fiber layer and a low-density fiber layer which are integrally formed, wherein the high-density fiber layer is used to prepare the ablation-resistant layer of the gradient density ablation-resistant material, and the low-density fiber layer is used to prepare the heat-insulating layer of the gradient density ablation-resistant material; S2: applying a release agent to the mold for preparing the gradient density ablation-resistant material; S3: placing the preform in the preparation mold; S4: evacuating the preparation mold, and injecting the modified phenolic resin that has been treated by a vacuum degassing process into the preparation mold using an RTM injection machine to obtain a gradient density ablation-resistant material.

2. The method for preparing a gradient density ablation-resistant material according to claim 1, characterized in that: The preform is provided with Z-direction needles for improving the layer shear strength of the ablation-resistant layer and the heat-insulating layer.

3. The method for preparing the gradient density ablation-resistant material according to claim 1, characterized in that: The preform is made of high-silica cloth and glass fiber mesh.

4. The method for preparing the gradient density ablation-resistant material according to claim 1, characterized in that: The preparation mold is provided with at least one glue injection port and one glue discharge port.

5. The method for preparing the gradient density ablation-resistant material according to claim 1, characterized in that: In the above-mentioned S2, the release agent is applied at least twice.

6. The method for preparing the gradient density ablation-resistant material according to claim 3, characterized in that: The thickness of the preform is greater than the theoretical preparation thickness of the gradient density ablation-resistant material, and the thickness difference is at least 2 mm.

7. The method for preparing the gradient density ablation-resistant material according to claim 1, characterized in that: The vacuum degree during vacuum degassing of the modified phenolic resin is -0.07MPa to -0.1MPa.

8. The method for preparing the gradient density ablation-resistant material according to claim 4, characterized in that: In S4, the process of injecting the modified phenolic resin into the preparation mold using the RTM injection machine is as follows: The vacuum degree inside the preparation mold is -0.1Mpa; Open the injection port, inject the modified phenolic resin into the preparation mold with a pressure of 0.5 MPa until the modified phenolic resin completely fills the preparation mold, and record the first injection time; The glue discharge port is opened, and the modified phenolic resin is continuously injected into the preparation mold for multiple times to discharge bubbles.

9. The method for preparing the gradient density ablation-resistant material according to claim 8, characterized in that: Also includes: S5: curing the gradient density ablation-resistant material in the prepared mold; S6: demolding the gradient density ablation-resistant material; S7: Drying the gradient density ablation-resistant material after demoulding.

10. The method for preparing the gradient density ablation-resistant material according to claim 9, characterized in that: In S5, during curing, the gradient density ablation-resistant material in the prepared mold is heated to 60° C. for an initial heating time of 60 minutes; after curing at 60° C. for 60 minutes, the material is heated to 110° C. for a secondary heating time of 100 minutes, and then cured at 110° C. for 500 minutes; In the above S7, the gradient density ablation-resistant material after demoulding is heated to 110° C. during drying, and the heating time is 120 minutes; and the material is cured at 110° C. for 600 minutes.

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