Flexible silicone thermal protection material and preparation method thereof

Abstract

The present application relates to a kind of flexible silicone heat protection material and preparation method thereof, belong to heat protection material technical field, preparation method includes: the pre-oxidized PAN fiber is woven and prepared PAN fiber braided body;Based on epoxy group modified RTV silicone rubber, the modified PAN fiber braided body with Si-O-C on PAN fiber surface is prepared;Modified PAN fiber braided body impregnated with silicon resin composite system is obtained flexible silicone heat protection material by heating and curing treatment.The present application is modified by epoxy group modified RTV silicone rubber to pre-oxidized PAN fiber, realizes the good compatibility of PAN fiber and RTV silicone rubber, reduces interface delamination, improves the bending strength and stability of flexible silicone heat protection material at high temperature, effectively inhibits the oxidation damage of fiber, reduces linear ablation rate, overcomes the defects of existing flexible silicone heat protection material in prior art, such as poor high-temperature strength and stability, low high-temperature interfacial strength, easy high-temperature delamination failure, low ablation resistance.

Description

Technical Field

[0001] This invention relates to the field of thermal protection materials technology, and in particular to a flexible organosilicon thermal protection material and its preparation method. Background Technology

[0002] Traditional thermal protection materials include: silicone thermal protection materials and rigid ceramic matrix composites.

[0003] Although rigid ceramic matrix composites have excellent temperature resistance, they are brittle, have poor impact resistance, and have high thermal conductivity, making it impossible to achieve a balance between lightweighting and efficient thermal insulation.

[0004] Organosilicon thermal protection materials are mostly based on single silicone rubber or short fiber reinforced composite materials. Although they have a certain degree of flexibility, their high temperature resistance is limited, and they are prone to thermal decomposition in high-speed and high-temperature environments, resulting in a significant increase in ablation rate. This makes it difficult to meet the synergistic requirements of ablation resistance, long-term high-temperature stability, and structural flexibility in the field of thermal protection materials technology.

[0005] Although an improved solution for silicone thermal protection materials has recently emerged—fiber-reinforced silicone materials—which has improved the high-temperature strength of silicone thermal protection materials to some extent, the interfacial bonding strength of fiber-reinforced silicone materials is low, and the difference in physical properties (thermal expansion coefficient) between fibers and resins can easily lead to high-temperature delamination failure. In addition, existing fiber-reinforced silicone materials are difficult to simultaneously possess high ablation resistance and low thermal conductivity. Summary of the Invention

[0006] Based on the above analysis, the present invention aims to provide a flexible organosilicon thermal protection material and its preparation method, in order to solve at least one of the technical problems existing in the flexible organosilicon thermal protection materials in the prior art, such as poor high-temperature strength and stability, low high-temperature interface strength, easy high-temperature delamination failure, low ablation resistance, low thermal conductivity, and difficulty in simultaneously possessing high ablation resistance and low thermal conductivity.

[0007] A method for preparing a flexible organosilicon thermal protection material includes:

[0008] S1: Prepare PAN fiber braid by weaving pre-oxidized PAN fibers;

[0009] S2: Modified PAN fiber braids with Si-OC on the surface of PAN fibers were prepared by modifying PAN fiber braids with epoxy-modified RTV silicone rubber;

[0010] S3: A flexible organosilicon thermal protection material is obtained by heating and curing a modified PAN fiber braid impregnated with a silicone resin composite system.

[0011] Preferably, the porosity of the PAN fiber braid decreases in a stepwise manner from one side to the other in the thickness direction.

[0012] Preferably, the porosity of the PAN fiber braid is 25% to 35%.

[0013] Preferably, the PAN fiber braid is configured with three fiber braid layers from the inside to the outside, and the porosity of each cross section inside each fiber braid layer is equal;

[0014] The porosity of the lower fiber braided layer is 25%–28%, the porosity of the middle fiber braided layer is 30%–32%, and the porosity of the higher fiber braided layer is 35%–38%.

[0015] Preferably, the heating and curing treatment in step S2 can be a three-stage gradient heating and curing process, with the heating temperature and holding time satisfying the following: 55℃~65℃ / 1.5h~2.5h / -0.07MPa~-0.09MPa→85℃~90℃ / 3.5h~5h / -0.03MPa~-0.06MPa→115℃~135℃ / 2.5h~4h / -0.015MPa~-0.028MPa;

[0016] The heating rate must meet the following requirements: 2℃ / min~5℃ / min.

[0017] Preferably, the silicone resin composite system in step S3 includes amino-modified methylphenyl silicone resin and necessary solvents, with a solid content of 22wt% to 40wt%.

[0018] Preferably, step S3, the impregnation of the silicone resin composite system, employs a cyclic pulsating pressure impregnation process, which satisfies the following requirements:

[0019] The pressure range is 0.5MPa~2.0MPa, the pulsation frequency is 0.1Hz~0.5Hz, and the pressure waveform is a trapezoidal wave, which varies according to the pressure increase rate of 0.2±0.1MPa / s → pressure holding for 30±5s → pressure decrease rate of 0.1±0.05MPa / s.

[0020] Preferably, step S3 includes: impregnating modified PAN fiber braids in batches using silicone resin composite systems with different solid contents and performing heat curing treatment; the solid content of the silicone resin composite system gradually increases from the inner high-porosity layer to the outer low-porosity layer of the PAN fiber braids.

[0021] Preferably, step S3 includes:

[0022] S301: Place the modified PAN fiber braid with the inner side facing down and the outer side facing up, and use a silicone resin composite system with a low solid content to impregnate the modified PAN fiber braid for the first time.

[0023] S302: The modified PAN fiber braid that has completed the first impregnation is cured and hot-pressed to obtain the first-stage preform;

[0024] S303: Select a silicone resin composite system with a high solid content for the second impregnation of the first-stage preform;

[0025] S304: The second-stage preform is obtained by curing and hot-pressing the first-stage preform after the second impregnation.

[0026] S305: Referring to steps S303-S304, select a silicone resin composite system with higher solid content to impregnate, cure, and hot-press the second-stage preform. Repeat the above steps, selecting silicone resin composite systems with higher solid content to impregnate, cure, and hot-press the preforms of each stage in sequence to obtain the final flexible organosilicon thermal protection material.

[0027] A flexible organosilicon thermal protection material, characterized in that it is prepared by the preparation method of the flexible organosilicon thermal protection material according to any one of claims 1-9, comprising:

[0028] The PAN fiber braid, the silicone resin dispersed in the pores of the PAN fiber braid, and the RTV silicone rubber layer connecting the silicone resin and the PAN fibers, wherein the RTV silicone rubber layer is connected to the PAN fibers through Si-OC bonds.

[0029] An application of a flexible organosilicon thermal protection material: The flexible organosilicon thermal protection material prepared using the above method is used as a high-temperature thermal insulation layer in the temperature range of -60℃ to 300℃.

[0030] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0031] (1) The present invention uses epoxy-modified RTV silicone rubber to modify pre-oxidized PAN fibers, which achieves good compatibility between PAN fibers and RTV silicone rubber. This not only improves the bonding strength and reduces interfacial delamination, but also improves the bending strength and high-temperature stability of flexible organosilicon thermal protection materials. At the same time, the Si-OC chemical bonds at the modified interface can form a continuous Si-OC ceramic layer during high-temperature ablation, which effectively inhibits fiber oxidation damage and reduces the linear ablation rate. This overcomes the defects of existing flexible organosilicon thermal protection materials, such as poor high-temperature strength and stability, low high-temperature interfacial strength, easy high-temperature delamination failure, and low ablation resistance.

[0032] (2) The present invention forms a heat flow channel between the pores by gradually decreasing the porosity of the PAN fiber braid from the inside to the outside in the thickness direction, thereby realizing the directional dispersion and guidance of heat flow from the low porosity side to the high porosity side, and further improving the stability and bending strength at high temperature.

[0033] (3) The present invention uses gradient heating curing, which is beneficial to forming a uniform and dense RTV silicone rubber modified layer on the pre-oxidized PAN fiber, reducing interface defects and improving mechanical properties.

[0034] (4) The present invention uses a silicone resin composite system with different solid content to impregnate and modify PAN fiber braids in batches and heat-curing process, which helps to construct a thermal conductivity gradient structure from the inside to the outside of the organosilicon thermal protection material. Combined with the porosity gradient of the PAN fiber braid, it helps to construct a unidirectional heat channel, improve the ablation and oxidation resistance of PAN fibers, enable the material to achieve cross-scale synergistic bearing of thermal-mechanical load, improve the high temperature stability and bending strength of the material, reduce the linear ablation rate, and overcome the defect of existing flexible organosilicon thermal protection materials that are prone to high temperature delamination failure.

[0035] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0036] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0037] Figure 1 The infrared spectra of PAN fibers modified and unmodified in Example 1 of this invention are shown. Detailed Implementation

[0038] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0039] Terminology definition:

[0040] The outer side of flexible silicone thermal protection materials or PAN fiber braids refers to the side that is in contact with the outside environment.

[0041] The inner side of flexible silicone thermal protection materials or PAN fiber braids refers to the side that is in contact with the substrate that needs to be protected.

[0042] In a first aspect, the present invention provides a method for preparing a flexible organosilicon thermal protection material, comprising:

[0043] S1: Prepare PAN fiber braid by weaving pre-oxidized PAN fibers;

[0044] S2: Modified PAN fiber braids with Si-OC on the surface of PAN fibers were prepared by modifying PAN fiber braids with epoxy-modified RTV silicone rubber;

[0045] S3: A flexible organosilicon thermal protection material is obtained by heating and curing a modified PAN fiber braid impregnated with a silicone resin composite system.

[0046] It should be noted that RTV silicone rubber is mainly composed of polysiloxane, with its main chain consisting of silicon-oxygen bonds (Si-O-Si), and side chains typically consisting of organic groups such as methyl or phenyl groups. Polyacrylonitrile (PAN) is a polymer containing cyano groups (-CN), with its main chain composed of carbon atoms and low surface energy. The significant differences in the chemical structures of these two materials result in weak interactions and poor compatibility between them. Furthermore, it is difficult for the cyano groups in PAN to form effective chemical bonds with the silicon-oxygen bonds in RTV silicone rubber, making the modification of PAN with RTV silicone rubber extremely challenging under normal circumstances.

[0047] In practice, the pre-oxidized PAN fibers and epoxy-modified RTV silicone rubber used in this invention not only have better compatibility and achieve more thorough wetting, but also the pre-oxidized PAN fiber surface has more fiber active groups (-C=O, -OH), which have good reactivity with epoxy groups and can form Si-OC chemical bonds to achieve stable chemical connection, thus realizing the surface modification of PAN by RTV silicone rubber.

[0048] Compared with existing technologies, this invention uses epoxy-modified RTV silicone rubber to modify pre-oxidized PAN fibers, achieving good compatibility between PAN fibers and RTV silicone rubber. This not only improves the bonding strength and reduces interfacial delamination, but also enhances the flexural strength and high-temperature stability of the flexible organosilicon thermal protection material. Simultaneously, the Si-OC chemical bonds at the modified interface can form a continuous Si-OC ceramic layer during high-temperature ablation, effectively inhibiting fiber oxidation damage and reducing the linear ablation rate. This overcomes the shortcomings of existing flexible organosilicon thermal protection materials, such as poor high-temperature strength and stability, low high-temperature interfacial strength, easy high-temperature delamination failure, and low ablation resistance.

[0049] Preferably, the porosity of the PAN fiber braid decreases in a stepwise manner from one side to the other in the thickness direction.

[0050] Specifically, the porosity decreases in a stepwise manner from the inside to the outside.

[0051] Specifically, the porosity of the PAN fiber braid is 25%–35%, the thickness of the PAN fiber braid is 2 mm–5 mm, and the fiber volume content is 40%–60%.

[0052] It should be noted that fiber volume content is related to porosity and fiber diameter. When the porosity is constant, the larger the fiber diameter, the greater the fiber volume content.

[0053] More preferably, the PAN fiber braid can be configured as three fiber braid layers from the inside to the outside, with each cross section of each fiber braid layer having equal porosity.

[0054] The porosity of the lower fiber braided layer is 25%–28%, the porosity of the middle fiber braided layer is 30%–32%, and the porosity of the higher fiber braided layer is 35%–38%.

[0055] During implementation, the PAN fiber braid with a stepped porosity can form heat flow channels between the pores after being filled with a silicone resin composite system, enabling the heat flow to be directionally dispersed and guided from the low porosity side to the high porosity side; the high porosity side buffers thermal strain through the pores, enabling the material to achieve cross-scale synergistic bearing of thermal and mechanical loads.

[0056] Compared with the prior art, the present invention improves the stability and bending strength at high temperature by making the porosity of the PAN fiber braid decrease in a stepwise manner from the inside to the outside in the thickness direction, forming heat flow channels between the pores, and realizing the directional dispersion and guidance of heat flow from the low porosity side to the high porosity side.

[0057] Specifically, the pre-oxidized PAN fibers in step S1 can be obtained by oxidizing PAN fibers at 200℃~230℃ for 80min~100min.

[0058] Specifically, in step S1, the PAN fiber braid is a 2.5D or three-dimensional braided structure.

[0059] Specifically, in step S2, the epoxy-modified RTV silicone rubber is used to modify the PAN fiber braid by vacuum impregnation followed by heat curing. The thickness of the RTV silicone rubber layer is 10μm to 50μm, which can be 10μm, 15μm, 20μm, 26μm, 30μm, 35μm, 40μm, 45μm, or 50μm.

[0060] Specifically, the vacuum impregnation vacuum degree is -0.09MPa to -0.07MPa, and RTV silicone rubber is injected at a rate of 5mL / min to 8mL / min to immerse the fiber braid for 30min to 60min.

[0061] It should be noted that good wetting and flow can be achieved when the vacuum degree, injection rate, and impregnation time are within the above range.

[0062] Preferably, the heating and curing process in step S2 is a gradient temperature rise curing process.

[0063] Specifically, the heating and curing treatment in step S2 can be a three-stage gradient heating and curing process, with the heating temperature and holding time meeting the following conditions: 55℃~65℃ / 1.5h~2.5h / -0.07MPa~-0.09MPa → 85℃~90℃ / 3.5h~5h / -0.03MPa~-0.06MPa → 115℃~135℃ / 2.5h~4h / -0.015MPa~-0.028MPa. The heating rate for each stage of the gradient heating is 2℃ / min~5℃ / min.

[0064] The applicant's research found that the self-curing of RTV silicone rubber is mainly affected by moisture, and it cures from the surface inward. The reaction temperature is lower than that of pre-oxidized PAN fiber and epoxy-modified RTV silicone rubber. The initial reaction temperature of pre-oxidized PAN fiber and epoxy-modified RTV silicone rubber is about 80℃, and the complete reaction requires about 150℃. Direct high-temperature curing at a single temperature or too fast a heating rate will, on the one hand, affect the entry of moisture into RTV silicone rubber, making it difficult for RTV silicone rubber to cure fully; on the other hand, the reaction will be too violent, and small molecule products will not be able to be fully discharged, which will easily produce micropores and interface defects.

[0065] Simultaneously, by combining the viscosity and surface tension of RTV silicone rubber, the thickness of the RTV silicone rubber layer can be kept within a suitable range, allowing the modified RTV silicone rubber layer to play a better role as a flexible transition layer and reduce interfacial separation at high temperatures.

[0066] Compared with existing technologies, the present invention uses gradient temperature curing, which is beneficial to forming a uniform and dense RTV silicone rubber modified layer on pre-oxidized PAN fibers, reducing interface defects and improving mechanical properties.

[0067] Specifically, epoxy-modified RTV silicone rubber can be introduced in the following ways:

[0068] (1) Using γ-glycidyl etheroxypropyltrimethoxysilane or 3,4-epoxycyclohexylethylmethyldimethoxysilane as a modifier, the amount of which is added is 5% to 15% of the mass of the RTV silicone rubber matrix.

[0069] (2) Under inert gas protection, the modifier and hydroxyl-terminated polydimethylsiloxane are reacted at 60℃~80℃ for 4h~6h, and the content of epoxy functional groups is controlled to be 0.3mol / kg~0.8mol / kg, which can be 0.3mol / kg, 0.4mol / kg, 0.5mol / kg, 0.6mol / kg, 0.7mol / kg or 0.8mol / kg; the epoxy-modified RTV silicone rubber has a viscosity range of 5000mPa·s~15000mPa·s at 25℃, which can be 5000mPa·s, 5600mPa·s, 6000mPa·s, 7000mPa·s, 7500mPa·s, 8000mPa·s, 10000mPa·s, 11000mPa·s, 12000mPa·s or 15000mPa·s.

[0070] Specifically, the silicone resin composite system in step S3 includes amino-modified methylphenyl silicone resin and necessary solvents, with a solid content of 22wt% to 40wt%, which can be 22wt%, 24wt%, 25wt%, 26wt%, 28wt%, 30wt%, 32wt%, 34wt%, 35wt%, 36wt%, 37wt%, 38wt%, 39wt%, or 40wt%.

[0071] Specifically, the amino-modified methylphenyl silicone resin can be obtained by reacting a modifier with methylphenyl silicone resin.

[0072] Specifically, the amino content in the amino-modified methylphenyl silicone resin is controlled at 0.3wt% to 0.8wt%, and can be 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, or 0.8wt%; the viscosity at 25°C is not higher than 3000 mPa·s.

[0073] Specifically, the modifier may be γ-aminopropyltriethoxysilane or N-β-aminoethyl-γ-aminopropylmethyldimethoxysilane.

[0074] It should be noted that the amino groups can react chemically with RTV silicone rubber to form a bond, which can enhance the adhesion between methylphenyl silicone resin and RTV silicone rubber and improve compatibility. The phenyl groups in methylphenyl silicone resin can inhibit high-temperature chain segment untangling through steric hindrance effect. Combined with the interfacial compatibilization effect of amino modifier, the material is endowed with flexibility in a wide temperature range of -60 to 300℃, breaking through the mechanical property limitations of traditional silicone resins.

[0075] Specifically, the methylphenyl silicone resin can be a commercially available product, with a phenyl content controlled between 15wt% and 25wt%, which can be 15wt%, 18wt%, 20wt%, 22wt%, 23wt%, or 25wt%; a rotational viscosity range of 500mPa·s to 2000mPa·s at 25°C, which can be 500mPa·s, 600mPa·s, 700mPa·s, 800mPa·s, 900mPa·s, 1000mPa·s, 1200mPa·s, 1300mPa·s, 1500mPa·s, 1800mPa·s, or 2000mPa·s; and a glass transition temperature (Tg) ≥180°C.

[0076] Preferably, step S3, the impregnation of the silicone composite system, employs a cyclic pulsating pressure impregnation process.

[0077] The pressure range is 0.5MPa~2.0MPa, the pulsation frequency is 0.1Hz~0.5Hz, and the pressure waveform is a trapezoidal wave, which varies according to the pressure increase rate of 0.2±0.1MPa / s → pressure holding for 30±5s → pressure decrease rate of 0.1±0.05MPa / s.

[0078] It should be noted that the cyclic pulsating pressure impregnation process was selected:

[0079] On the one hand, the cyclic pulsed pressure impregnation process can improve impregnation efficiency: by periodically applying and releasing pressure, the cyclic pulsed pressure impregnation process can significantly accelerate the speed at which the impregnation liquid enters the preform and shorten the impregnation time;

[0080] On the other hand, the cyclic pulsating pressure impregnation process can improve the impregnation uniformity: the periodic change of pressure helps the impregnation liquid to be distributed more evenly throughout the preform, reducing incomplete or uneven impregnation caused by uneven capillary force.

[0081] In addition, the cyclic pulsating pressure impregnation process can improve material properties; pulsating pressure can promote the chemical reaction between the impregnation liquid and the preform, which helps to refine the microstructure of the matrix, thereby improving the mechanical properties and durability of the material; by controlling the pressure and time, the porosity and defects inside the material can be reduced, and the density and strength of the material can be improved.

[0082] More preferably, step S3 includes: impregnating the modified PAN fiber braid in batches using a silicone resin composite system with different solid contents and then performing a heat curing treatment.

[0083] Specifically, the solid content of the silicone resin composite system gradually increases from the inner high-porosity layer to the outer low-porosity layer in the PAN fiber braid.

[0084] Specifically, step S3 includes:

[0085] S301: Place the modified PAN fiber braid with the inner side facing down and the outer side facing up, and use a silicone resin composite system with a low solid content to impregnate the modified PAN fiber braid for the first time.

[0086] S302: The modified PAN fiber braid that has completed the first impregnation is cured and hot-pressed to obtain the first-stage preform;

[0087] S303: Select a silicone resin composite system with a high solid content for the second impregnation of the first-stage preform;

[0088] S304: The second-stage preform is obtained by curing and hot-pressing the first-stage preform after the second impregnation.

[0089] S305: Referring to steps S303-S304, select a silicone resin composite system with higher solid content to impregnate, cure, and hot-press the second-stage preform. Repeat the above steps, selecting silicone resin composite systems with higher solid content to impregnate, cure, and hot-press the preforms of each stage in sequence to obtain the final flexible organosilicon thermal protection material.

[0090] During implementation, the silicone resin composite system with the lowest solid content after the first impregnation preferentially fills the high-porosity layer inside the modified PAN fiber braid under the action of gravity and cyclic pulsating pressure. After curing and hot pressing, channels with low cross-linking degree, relatively softness, and low thermal conductivity are formed in the high-porosity layer. The relatively softness allows the resin and the modified RTV silicone rubber layer to have better adhesion, improving stability and mechanical properties at high temperatures. The low thermal conductivity can form a heat insulation layer and improve the PAN fiber's resistance to ablation and oxidation. The silicone resin composite system with higher solid content fills the high-porosity layer on the outside of the PAN fiber braid, and the high degree of cross-linking helps to form a dense silicon-based ceramic layer.

[0091] Compared with existing technologies, this invention uses a silicone resin composite system with different solid content to impregnate and modify PAN fiber braids in batches and then heat-curing them. This helps to construct a structure in which the thermal conductivity of the organosilicon thermal protection material changes from the inside to the outside. Combined with the porosity gradient of the PAN fiber braid, this helps to construct a unidirectional heat channel, improve the ablation and oxidation resistance of the PAN fiber, enable the material to achieve cross-scale synergistic bearing of thermal and mechanical loads, improve the material's high-temperature stability and bending strength, reduce the linear ablation rate, and overcome the defect of existing flexible organosilicon thermal protection materials that are prone to high-temperature delamination failure.

[0092] It should be noted that the unique process of batch impregnation, heating and curing of silicone resin composite systems with different solid contents in this invention needs to solve the problem of the difficulty in dispersing high solid content silicone resin in PAN fiber braid during impregnation. The above-mentioned cyclic pulsating pressure impregnation process can achieve full dispersion of silicone resin composite system.

[0093] Preferably, the heating and curing process in step S3 is a gradient heating curing process, with the heating temperature and holding time satisfying the following sequence: 80℃~90℃ / 1.0h~2.5h → 110℃~130℃ / 1.0h~2.5h → 150℃~160℃ / 1.0h~3h. The heating rate for each stage of the gradient heating is 2℃ / min~5℃ / min.

[0094] On the other hand, the present invention provides a flexible organosilicon thermal protection material, comprising:

[0095] The PAN fiber braid, the silicone resin dispersed in the pores of the PAN fiber braid, and the RTV silicone rubber layer connecting the silicone resin and the PAN fibers, wherein the RTV silicone rubber layer is connected to the PAN fibers through Si-OC bonds.

[0096] Thirdly, the application of a flexible organosilicon thermal protection material as described above, wherein the composite material is used in the field of thermal protection materials, specifically as a high-temperature insulation layer in the temperature range of -60 to 300°C.

[0097] To better illustrate the present invention, the following embodiments and comparative examples are provided:

[0098] The RTV silicone rubber used is Ronsil RTV 6338 from Shanghai Rongtai Chemical New Materials Co., Ltd., with a viscosity of 2000 mPa·s.

[0099] Example 1

[0100] This embodiment discloses a method for preparing a flexible organosilicon thermal protection material, the specific preparation steps of which are as follows:

[0101] S1: Pre-oxidized PAN fibers are woven to prepare PAN fiber braids; three-dimensional orthogonal woven PAN pre-oxidized fibers are selected as reinforcements, the fiber volume content is controlled at 50%, the braid thickness is 3mm, and the interlayer porosity is designed along the thickness direction as 28% (outer layer) → 30% (middle) → 32% (inner layer) with equal thickness in three layers. The fiber braids are placed in a 100℃ oven to dry for 2 hours to remove adsorbed moisture.

[0102] PAN pre-oxidized fibers were obtained by oxidizing three-dimensional orthogonally woven PAN fibers at 200℃ for 80 minutes.

[0103] S2: Modified PAN fiber braids with Si-OC on the surface of PAN fibers were prepared by modifying PAN fiber braids with epoxy-modified RTV silicone rubber; Figure 1 The red curve represents the infrared spectrum of the PAN fiber surface prepared by modifying PAN fiber braid with epoxy-modified RTV silicone rubber as described in Example 1, at 1020 cm⁻¹. -1The presence of Si-OC characteristic peaks indicates that Si-OC is present at the connection between RTV silicone rubber and PAN fiber braid. Figure 1 The black curve in the middle is the infrared spectrum of the surface of PAN fibers prepared by modifying unmodified blank PAN fiber braids; no 1020 cm⁻¹ curve appears. -1 The characteristic peaks of Si-OC indicate that no Si-OC was formed.

[0104] Epoxy-modified RTV silicone rubber was injected into a vacuum impregnation tank. The impregnation tank was pre-evacuated to -0.08MPa. The silicone rubber was injected at a rate of 6mL / min to immerse the fiber braid. After impregnation for 40 minutes, it was heat-cured.

[0105] The heat curing process involved a gradient temperature increase, with the heating temperature, holding time, and vacuum level set as follows: 55℃ / 1.5h / -0.07MPa → 85℃ / 4h / -0.04MPa → 125℃ / 3h / -0.015MPa. The heating rate for each stage of the gradient temperature increase was 3℃ / min; the thickness of the prepared RTV silicone rubber layer was 22μm.

[0106] Epoxy-modified RTV silicone rubber can be introduced in the following ways:

[0107] (1) γ-glycidyl etheroxypropyltrimethoxysilane is used as a modifier, and its addition amount is 15% of the mass of RTV silicone rubber matrix;

[0108] (2) Under inert gas protection, the modifier and hydroxyl-terminated polydimethylsiloxane were reacted at 60°C for 4 hours, and the content of epoxy functional groups was controlled to be 0.3 mol / kg; the viscosity of epoxy-modified RTV silicone rubber at 25°C was 5000 mPa·s.

[0109] S3: A flexible organosilicon thermal protection material is obtained by heating and curing a modified PAN fiber braid impregnated with a silicone resin composite system.

[0110] Amino-modified methylphenyl silicone resin toluene solutions with concentrations of 28%, 32%, 36%, and 40% were sequentially injected into the fiber preform.

[0111] The amino-modified methylphenyl silicone resin is a commercially available γ-aminopropyltriethoxysilane-modified methylphenyl silicone resin, wherein the amino content is controlled at 0.3wt% to 0.8wt%; and the viscosity at 25℃ is 3000 mPa·s.

[0112] Step S3: Select the cyclic pulsating pressure impregnation process for the silicone resin composite system: The impregnation pressure range of the silicone resin composite system is 0.8MPa, the pulsation frequency is 0.3Hz, and the pressure waveform is a trapezoidal wave. The pressure is increased at a rate of 0.2MPa / s, held for 30s, and decreased at a rate of 0.1MPa / s. The injection time is 8h.

[0113] Perform a stepped curing process: 80℃ / 2h → 120℃ / 2h → 150℃ / 2h, with a heating rate of 3℃ / min for each stage; preheat the hot press to 180℃, apply a pressure of 8MPa, and hold the pressure for 60 minutes.

[0114] This embodiment discloses a flexible organosilicon thermal protection material prepared by the above-described preparation method.

[0115] An application of the flexible organosilicon thermal protection material as described above, wherein the organosilicon thermal protection material is applied in the field of thermal protection materials, specifically as a high-temperature insulation layer in the temperature range of -60 to 300°C.

[0116] Example 2

[0117] This embodiment discloses a method for preparing a flexible organosilicon thermal protection material, the specific preparation steps of which are as follows:

[0118] S1: Pre-oxidized PAN fibers are woven to prepare PAN fiber braids; three-dimensional orthogonal woven PAN pre-oxidized fibers are selected as reinforcements, the fiber volume content is controlled at 45%, the braid thickness is 4mm, and the interlayer porosity is designed along the thickness direction as 28% (outer layer) → 31% (middle) → 34% (inner layer) with equal thickness in three layers. The fiber braids are placed in a 100℃ oven to dry for 2 hours to remove adsorbed moisture.

[0119] PAN pre-oxidized fibers were obtained by oxidizing three-dimensional orthogonally woven PAN fibers at 200℃ for 80 minutes.

[0120] S2: Modified PAN fiber braids with Si-OC on the surface of PAN fibers were prepared by modifying PAN fiber braids with epoxy-modified RTV silicone rubber;

[0121] Epoxy-modified RTV silicone rubber was injected into a vacuum impregnation tank. The impregnation tank was pre-evacuated to -0.08MPa. The silicone rubber was injected at a rate of 6mL / min to immerse the fiber braid. After impregnation for 40 minutes, it was heat-cured.

[0122] In step S2, the heating and curing process is a gradient temperature curing process, with the heating temperature, holding time, and vacuum degree meeting the following requirements: 60℃ / 1h / -0.08MPa → 90℃ / 3h / -0.05MPa → 130℃ / 2h / -0.02MPa. The heating rate for each stage of the gradient temperature increase is 3℃ / min; the thickness of the prepared RTV silicone rubber layer is 20μm.

[0123] Epoxy-modified RTV silicone rubber can be introduced in the following ways:

[0124] (1) 3,4-epoxycyclohexylethylmethyldimethoxysilane was used as a modifier, and its addition amount was 15% of the mass of the RTV silicone rubber matrix;

[0125] (2) Under inert gas protection, the modifier and hydroxyl-terminated polydimethylsiloxane were reacted at 60°C for 4 hours, and the content of epoxy functional groups was controlled to be 0.5 mol / kg; the viscosity of epoxy-modified RTV silicone rubber at 25°C was 8000 mPa·s.

[0126] S3: A flexible organosilicon thermal protection material is obtained by heating and curing a modified PAN fiber braid impregnated with a silicone resin composite system.

[0127] Amino-modified methylphenyl silicone resin toluene solutions with concentrations of 24%, 30%, and 36% were sequentially injected into the fiber preform.

[0128] The amino-modified methylphenyl silicone resin is a commercially available γ-aminopropyltriethoxysilane-modified methylphenyl silicone resin, wherein the amino content is controlled at 0.5 wt% and the viscosity at 25°C is 2500 mPa·s.

[0129] Step S3: Select the cyclic pulsating pressure impregnation process for the silicone resin composite system: The impregnation pressure range of the silicone resin composite system is 0.8MPa, the pulsation frequency is 0.2Hz, and the pressure waveform is a trapezoidal wave. The pressure is increased at a rate of 0.2MPa / s, held for 35s, and decreased at a rate of 0.15MPa / s. The injection time is 6h.

[0130] Perform a stepped curing process: 80℃ / 3h → 115℃ / 2h → 145℃ / 2h, with a heating rate of 4℃ / min for each stage; preheat the hot press to 160℃, apply a pressure of 6MPa, and hold the pressure for 60 minutes.

[0131] This embodiment discloses a flexible organosilicon thermal protection material prepared by the above-described preparation method.

[0132] An application of the flexible organosilicon thermal protection material as described above, wherein the organosilicon thermal protection material is applied in the field of thermal protection materials, specifically as a high-temperature insulation layer in the temperature range of -60 to 300°C.

[0133] Example 3

[0134] This embodiment discloses a method for preparing a flexible organosilicon thermal protection material, the specific preparation steps of which are as follows:

[0135] S1: Pre-oxidized PAN fibers are woven to prepare PAN fiber braids; three-dimensional orthogonal woven PAN pre-oxidized fibers are selected as reinforcements, the fiber volume content is controlled at 45%, the braid thickness is 4mm, and the interlayer porosity is designed along the thickness direction as 28% (outer layer) → 32% (middle) → 35% (inner layer) with equal thickness in three layers. The fiber braids are placed in a 100℃ oven to dry for 2 hours to remove adsorbed moisture.

[0136] PAN pre-oxidized fibers were obtained by oxidizing three-dimensional orthogonally woven PAN fibers at 200℃ for 80 minutes.

[0137] S2: Modified PAN fiber braids with Si-OC on the surface of PAN fibers were prepared by modifying PAN fiber braids with epoxy-modified RTV silicone rubber;

[0138] Epoxy-modified RTV silicone rubber was injected into a vacuum impregnation tank. The impregnation tank was pre-evacuated to -0.08MPa. The silicone rubber was injected at a rate of 6mL / min to immerse the fiber braid. After impregnation for 40 minutes, it was heat-cured.

[0139] In step S2, the heating and curing process is a gradient temperature curing process, with the heating temperature, holding time, and vacuum degree meeting the following requirements: 50℃ / 2h / -0.06MPa → 90℃ / 3h / -0.03MPa → 120℃ / 2h / -0.01MPa. The heating rate for each stage of the gradient temperature increase is 3℃ / min; the thickness of the prepared RTV silicone rubber layer is 20μm.

[0140] Epoxy-modified RTV silicone rubber can be introduced in the following ways:

[0141] (1) 3,4-epoxycyclohexylethylmethyldimethoxysilane was used as a modifier, and its addition amount was 16% of the mass of the RTV silicone rubber matrix.

[0142] (2) Under inert gas protection, the modifier and hydroxyl-terminated polydimethylsiloxane were reacted at 60°C for 5 hours, and the content of epoxy functional groups was controlled to be 0.5 mol / kg; the viscosity of epoxy-modified RTV silicone rubber at 25°C was 12000 mPa·s.

[0143] S3: A flexible organosilicon thermal protection material is obtained by heating and curing a modified PAN fiber braid impregnated with a silicone resin composite system.

[0144] Amino-modified methylphenyl silicone resin toluene solutions with concentrations of 26%, 32%, and 38% were sequentially injected into the fiber preform.

[0145] The amino-modified methylphenyl silicone resin is a commercially available γ-aminopropyltriethoxysilane-modified methylphenyl silicone resin, wherein the amino content is controlled at 0.4 wt% and the viscosity at 25°C is 2500 mPa·s.

[0146] Step S3: Select the cyclic pulsating pressure impregnation process for the silicone resin composite system: The impregnation pressure range of the silicone resin composite system is 0.8MPa, the pulsation frequency is 0.2Hz, and the pressure waveform is a trapezoidal wave. The pressure is increased at a rate of 0.2MPa / s, held for 35s, and decreased at a rate of 0.15MPa / s. The injection time is 6h.

[0147] Perform a stepped curing process: 90℃ / 1h → 120℃ / 1h → 150℃ / 1h, with a heating rate of 4℃ / min for each stage; preheat the hot press to 200℃, apply a pressure of 10MPa, and hold the pressure for 30 minutes.

[0148] This embodiment discloses a flexible organosilicon thermal protection material prepared by the above-described preparation method.

[0149] An application of the flexible organosilicon thermal protection material as described above, wherein the organosilicon thermal protection material is applied in the field of thermal protection materials, specifically as a high-temperature insulation layer in the temperature range of -60 to 300°C.

[0150] Comparative Example 1

[0151] This comparative example discloses a method for preparing flexible organosilicon thermal protection materials. The difference between this method and Example 1 is that ordinary methylphenyl silicone resin that has not undergone amino modification is selected as the matrix resin, and the viscosity is the same as in Example 1.

[0152] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0153] Comparative Example 2

[0154] This comparative example discloses a method for preparing flexible organosilicon thermal protection material. The difference between this method and Example 1 is that the volume content of PAN pre-oxidized filament fiber is 65%, while the rest is the same as in Example 1.

[0155] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0156] Comparative Example 3

[0157] This comparative example discloses a method for preparing flexible organosilicon thermal protection material. The difference between this method and Example 1 is that ordinary RTV silicone rubber that has not undergone epoxy modification is selected as the surface modification resin for PAN pre-oxidized filament fibers. The viscosity of the ordinary RTV silicone rubber is the same as that of the modified RTV silicone rubber in Example 1.

[0158] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0159] Comparative Example 4

[0160] The comparative example discloses a method for preparing flexible organosilicon thermal protection material. The difference between this method and Example 1 is that the porosity of the PAN fiber braid is uniformly set to 30%, while the rest is the same as in Example 1.

[0161] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0162] Comparative Example 5

[0163] This comparative example discloses a method for preparing flexible organosilicon thermal protection material. The difference between this method and Example 1 is that the PAN fiber braid is modified with RTV silicone rubber and then cured at a constant temperature of 85°C. The total curing time is the same as in Example 1, and the rest is the same as in Example 1.

[0164] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0165] Comparative Example 6

[0166] This comparative example discloses a method for preparing flexible organosilicon thermal protection material. The difference between this method and Example 1 is that the methylphenyl silicone resin is replaced with a methyl resin of the same viscosity, while the rest is the same as in Example 1.

[0167] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0168] Comparative Example 7

[0169] This comparative example discloses a method for preparing flexible organosilicon thermal protection materials. The difference between this method and Example 1 is that the porosity of the PAN fiber braid is uniformly set to 40%, while the rest is the same as in Example 1.

[0170] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0171] Comparative Example 8

[0172] This comparative example discloses a method for preparing flexible organosilicon thermal protection materials. The difference between this method and Example 1 is that the porosity of the PAN fiber braid is uniformly set to 25%, while the rest is the same as in Example 1.

[0173] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0174] Comparative Example 9

[0175] This comparative example discloses a method for preparing flexible organosilicon thermal protection material. The difference between this method and Example 1 is that the thickness of the PAN fiber braid is the same as in Example 1, and it is set to have 4 layers with uniform thickness: 28% (top layer) → 30% (middle layer 1) → 32% (middle layer 2) → 34% (bottom layer), and the rest is the same as in Example 1.

[0176] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0177] Comparative Example 10

[0178] The comparative example discloses a method for preparing flexible organosilicon thermal protection materials. The difference between this method and Example 1 is that the solid content of the matrix resin system is uniformly selected as 40%, while the rest is the same as in Example 1.

[0179] Flexible organosilicon thermal protection materials were prepared using the above-mentioned preparation method.

[0180] The flexible silicone composite materials obtained in the examples and comparative examples were subjected to corresponding performance tests. Table 1 summarizes the performance of the flexible silicone composite materials obtained in the examples and comparative examples. Among them, the density test method was GB 1463-2005, the thermal conductivity test method was GBT 10295-2008, the linear ablation rate test method was GJB 323B-2018, the flexural modulus test method was GBT 9341-2008, and the 800℃ residual rate was GBT 27761-2011.

[0181] Table 1 Performance of Flexible Organosilicon Composites

[0182]

[0183]

[0184] As can be seen from the above, the flexural modulus of the embodiments is between 360 MPa and 400 MPa, the thermal conductivity is between 0.15 W / m·K and 0.20 W / m·K, the linear ablation rate is between 0.032 and 0.045 mm / s, and the density is 1.30 g / cm³. 3 ~1.38g / cm 3 The residue rate at 800℃ is 83%–92.5%.

[0185] Comparing the examples with Comparative Examples 1 and 6, it can be seen that adding phenyl groups to the matrix resin can improve the flexural strength, residual rate, and high-temperature stability, and reduce the linear ablation rate; the introduction of amino groups significantly improves the flexural strength and high-temperature ablation resistance of the matrix resin.

[0186] Comparative examples with Comparative Examples 1 and 3 show that epoxy-modified RTV silicone rubber can better bond with PAN fibers, significantly improving flexural strength and high-temperature ablation resistance.

[0187] Comparing the examples with Comparative Examples 1 and 5, it can be seen that when epoxy-modified RTV silicone rubber is used to modify PAN fibers, a suitable curing process can improve the interfacial bonding force and significantly improve the flexural strength and high-temperature ablation resistance.

[0188] Comparative examples with Comparative Examples 2, 4, 6, and 7-8 show that setting a porosity gradient in the thickness direction of the PAN fiber braid can greatly improve thermal conductivity and ablation resistance.

[0189] Comparative examples and Comparative Example 10 show that the gradient change of the matrix resin solid content in the thickness direction of the PAN fiber braid can significantly improve the high temperature ablation resistance and reduce the thermal conductivity.

[0190] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a flexible organosilicon thermal protection material, characterized in that, include: S1: Prepare PAN fiber braid by weaving pre-oxidized PAN fibers; In the thickness direction of PAN fiber braids, the porosity decreases stepwise from one side to the other; the porosity of the lower layer of fiber braid is 25% to 28%, the porosity of the middle layer is 30% to 32%, and the porosity of the higher layer is 35% to 38%. S2: Modified PAN fiber braids with Si-OC on the surface of PAN fibers were prepared by modifying PAN fiber braids with epoxy-modified RTV silicone rubber; S3: A flexible organosilicon thermal protection material is obtained by heating and curing a modified PAN fiber braid impregnated with a silicone resin composite system. The silicone resin composite system in step S3 includes amino-modified methylphenyl silicone resin and necessary solvents, with a solid content of 22wt% to 40wt%. PAN fiber braids were impregnated and modified in batches using silicone resin composite systems with different solid contents, and the PAN fiber braids were heat-cured from the inner high-porosity layer to the outer low-porosity layer, with the solid content of the silicone resin composite system gradually increasing. The heating and curing process in step S3 is a three-stage gradient heating and curing process. The heating temperature and holding time meet the following requirements: 55℃ ~ 65℃ / 1.5h ~ 2.5h / -0.07MPa ~ -0.09MPa → 85℃ ~ 90℃ / 3.5h ~ 5h / -0.03MPa ~ -0.06MPa → 115℃ ~ 135℃ / 2.5h ~ 4h / -0.015MPa ~ -0.028MPa; the heating rate meets the following requirement: 2℃ / min ~ 5℃ / min.

2. The method for preparing the flexible organosilicon thermal protection material according to claim 1, characterized in that, The porosity of PAN fiber braids is 25%~35%.

3. The method for preparing the flexible organosilicon thermal protection material according to claim 1, characterized in that, Step S3: Impregnation of the silicone composite system using a cyclic pulsating pressure impregnation process. The process meets the following requirements: The pressure range is 0.5MPa ~ 2.0MPa, the pulsation frequency is 0.1Hz ~ 0.5Hz, and the pressure waveform is a trapezoidal wave, which varies according to the pressure increase rate of 0.2±0.1MPa / s → pressure holding for 30±5s → pressure decrease rate of 0.1±0.05MPa / s.

4. The method for preparing the flexible organosilicon thermal protection material according to claim 1, characterized in that, Step S3 includes: S301: Place the modified PAN fiber braid with the inner side facing down and the outer side facing up, and use a silicone resin composite system with a low solid content to impregnate the modified PAN fiber braid for the first time. S302: The modified PAN fiber braid that has completed the first impregnation is cured and hot-pressed to obtain the first-stage preform; S303: Select a silicone resin composite system with a high solid content for the second impregnation of the first-stage preform; S304: The second-stage preform is obtained by curing and hot-pressing the first-stage preform after the second impregnation. S305: Referring to steps S303-S304, select a silicone resin composite system with higher solid content to impregnate, cure, and hot-press the second-stage preform. Repeat the above steps, selecting silicone resin composite systems with higher solid content to impregnate, cure, and hot-press the preforms of each stage in sequence to obtain the final flexible organosilicon thermal protection material.

5. A flexible organosilicon thermal protection material, characterized in that, Prepared by the method for preparing the flexible organosilicon thermal protection material according to any one of claims 1-4, comprising: The PAN fiber braid, the silicone resin dispersed in the pores of the PAN fiber braid, and the RTV silicone rubber layer connecting the silicone resin and the PAN fibers, wherein the RTV silicone rubber layer is connected to the PAN fibers through Si-OC bonds.

6. An application of a flexible organosilicon thermal protection material, characterized in that, The flexible organosilicon thermal protection material prepared by the preparation method of the flexible organosilicon thermal protection material according to any one of claims 1-4 is used as a high-temperature thermal insulation layer in the temperature range of -60℃ to 300℃.

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