A method for synthesizing a POSS-modified phenolic resin and preparing a composite material based on the resin

Abstract

This invention belongs to the field of resin synthesis and composite material preparation technology. Specifically, it provides a method for synthesizing POSS-modified phenolic resin and preparing composite materials based on this resin. By preparing isocyanate-containing POSS and copolymerizing it with phenol, formaldehyde, etc., POSS-modified phenolic resin is obtained. POSS can be uniformly dispersed in phenolic resin, solving the problem of easy agglomeration and difficulty in uniform dispersion of conventional POSS in phenolic resin. The low-density composite material prepared by RTM process using isocyanate-containing POSS-modified phenolic resin as matrix has significantly improved heat resistance, oxidation resistance, and ablation resistance compared with materials prepared by conventional POSS-modified phenolic resin, meeting the requirements of the new generation of ultra-high speed and ultra-long distance rockets for efficient heat insulation, controllable ablation and gasification, and stable ablation under oxygen.

Description

Technical Field

[0001] This invention belongs to the field of resin synthesis and composite material preparation technology, specifically relating to the synthesis of cage-type polysilsesquioxane (hereinafter referred to as POSS) modified phenolic resin and its low-density composite material preparation method. Background Technology

[0002] Phenolic resins are widely used in aerospace thermal protection materials due to their simple preparation process, inexpensive and readily available raw materials, and good heat resistance and ablation resistance. However, the methylene bridges (-CH2-) in the structure of phenolic resins are easily oxidized and degraded, resulting in poor oxidation resistance of phenolic resin-based composites, making them difficult to use for extended periods in aerobic environments. Therefore, improving the oxidation resistance of phenolic resins has become a new research goal for phenolic resin composites.

[0003] In the 1990s, the U.S. Air Force Laboratory synthesized a series of POSS (Polyphenolic esters) and found that introducing POSS into polymers improved their heat resistance, oxidation resistance, ablation resistance, and mechanical properties. Since then, research on POSS-modified polymers has attracted widespread attention. POSS is a class of surface-functionalizable nanoparticle compounds with a three-dimensional polyhedral structure. Its molecules exist internally in the form of Si-O-Si bonds, with organic groups (R groups) attached to each silicon atom, extending outwards and surrounding an inorganic core. There are many types and properties of R groups, allowing researchers to synthesize POSS molecules with varying properties according to different needs. Furthermore, the compatibility between polymers and POSS can be improved by changing the type of R groups. If POSS cannot chemically react with the resin matrix, it can only be introduced into the polymer through physical doping. However, POSS is difficult to disperse uniformly in polymers and is prone to agglomeration. Agglomeration can prevent the material from achieving the expected performance and even cause a decrease in mechanical properties. Therefore, improving the compatibility between phenolic resin and POSS is key to improving the performance of POSS-modified phenolic resin composites. Summary of the Invention

[0004] This invention addresses the problems of existing technologies by providing a method for synthesizing POSS-modified phenolic resin and a method for preparing composite materials based on this resin. The prepared isocyanate-based POSS solves the problem of easy aggregation of conventional POSS in phenolic resin, ensuring that POSS is uniformly dispersed in the phenolic resin. The uniform dispersion of POSS in phenolic resin maximizes the improvement of various resin properties. Furthermore, the low-density composite material prepared using isocyanate-based POSS-modified phenolic resin as a matrix via RTM process exhibits significantly improved heat resistance, oxidation resistance, and ablation resistance compared to composite materials prepared from conventionally POSS-modified phenolic resin.

[0005] The objective of this invention is achieved by preparing isocyanate-containing POSS and copolymerizing it with phenol, formaldehyde, etc., to obtain POSS-modified phenolic resin. Since the isocyanate groups can react with the phenolic hydroxyl groups, POSS is introduced into the chemical structure of the phenolic resin through in-situ polymerization, enhancing the compatibility between POSS and phenol, allowing POSS to be uniformly dispersed in the phenolic resin. This solves the problem of conventional POSS easily agglomerating and being difficult to disperse uniformly in phenolic resin, further improving the antioxidant properties and heat resistance of the POSS-modified phenolic resin and its composites.

[0006] The specific technical solution adopted in this invention is as follows: The synthesis of a POSS-modified phenolic resin involves the following specific steps: 1) Synthesis of POSS containing isocyanate groups Isocyanate-based siloxane, heptaphenyl-deficient POSS and tetrahydrofuran were mixed and a certain amount of hydrochloric acid aqueous solution was added under nitrogen atmosphere protection. The polycondensation reaction was carried out at 60~100℃. After the reaction was completed, the reaction solution was concentrated and mixed with an alcohol solvent. The precipitate obtained was POSS containing isocyanate groups. The isocyanate-containing POSS structure is as follows: 2) Preparation of POSS-modified phenolic resin POSS containing isocyanate groups, phenol, formaldehyde and catalyst are added to a reaction flask in a certain proportion and stirred evenly; the reactants are heated to a certain temperature and reacted for a period of time, then distilled under reduced pressure. When the temperature in the reaction flask rises back to 100℃~120℃ and there is no distillate, the reduced pressure distillation is stopped to obtain POSS modified phenolic resin.

[0007] Further, in step 1), the molar ratio of isocyanate-based siloxane to heptaphenyl-deficient POSS is (1.1~1.3):1; the amount of tetrahydrofuran added is 1~3 times the weight of the siloxane (isocyanate-based siloxane and heptaphenyl-deficient POSS); the concentration of the hydrochloric acid aqueous solution is 0.01~0.05 mol / L, and the amount added is 2~5 times the molar mass of isocyanate-based siloxane (the content of hydrochloric acid is negligible and is calculated based on the molar mass of water).

[0008] Preferably, the alcohol solvent in step 1) is one of methanol, ethanol, and isopropanol; the polycondensation reaction temperature is 60~100℃, and the reaction time is 15~17 hours.

[0009] Preferably, the phenol in step 2) is one or more of phenol, p-cresol, and resorcinol; the formaldehyde in step 2) is a 37% (w / w) aqueous solution of formaldehyde.

[0010] Further, in step 2), the molar mass ratio of the isocyanate-containing POSS to the phenol is (0.1~0.5):1; the molar mass ratio of the total molar mass of the isocyanate-containing POSS and the phenol to the molar mass of formaldehyde is 1:(0.8~0.95).

[0011] Preferably, the catalyst in step 2) is one of oxalic acid, hydrochloric acid, phosphoric acid, and sulfuric acid, and the amount added is 0.01% to 2% of the weight of phenol.

[0012] Preferably, the reaction temperature in step 2) is 80℃~100℃ and the reaction time is 1h~4h.

[0013] This invention uses POSS-modified phenolic resin as a matrix and also provides a method for preparing POSS-modified phenolic resin composite materials, the specific steps of which are as follows:

[0014] 1) POSS modified phenolic resin and alcohol solvent are mixed to prepare a resin solution with a solid content of 30wt%~60wt%, a certain amount of curing agent hexamethylenetetramine is added, and the mixture is stirred evenly to obtain the resin matrix; 2) The fiber fabric is placed in a mold and sealed, and injection molded using RTM process. Then it is placed in an oven for curing and post-curing to obtain a low-density POSS modified phenolic resin composite material.

[0015] Preferably, the alcohol solvent in step 1) is one of methanol, ethanol, and isopropanol.

[0016] Preferably, the amount of hexamethylenetetramine added in step 1) is 10% to 20% of the weight of POSS modified phenolic resin (pure resin).

[0017] Furthermore, in step 2), the curing temperature is 100~120℃ and the curing time is 8~12h; the post-curing temperature is 80℃~100℃ and the post-curing time is 15~25h.

[0018] In step 2), the fiber fabric is one or more of quartz fiber fabric, glass fiber fabric, and carbon fiber fabric.

[0019] The isocyanate-containing POSS involved in this invention can also be used to prepare thermosetting phenolic resins by in-situ polymerization with the addition of an alkaline catalyst.

[0020] Compared with the prior art, the beneficial effects of the present invention are: This invention prepares a POSS containing isocyanate groups and copolymerizes it with formaldehyde and phenol to prepare a POSS-modified phenolic resin. The POSS is introduced in situ into the phenolic resin system. The isocyanate groups in the POSS can directly react with the phenolic hydroxyl groups of phenols, and the POSS is firmly bound to the phenolic resin molecular chain through chemical bonds. This solves the aggregation problem caused by the introduction of physical doping and improves the heat resistance and oxidation resistance of the phenolic resin.

[0021] Conventional POSS particles are too large when they agglomerate in resin. During RTM injection molding, they accumulate at the injection port, resulting in uneven POSS distribution and affecting material properties. The method provided by this invention allows POSS to be uniformly dispersed in the resin without agglomeration. The resulting low-density POSS-modified phenolic resin composite material exhibits significantly improved heat resistance, oxidation resistance, and ablation resistance. Furthermore, this preparation method is simple and easy to operate.

[0022] The POSS-modified phenolic resin involved in this invention has excellent heat resistance, oxidation resistance, and ablation resistance, and can be used as a high-performance resin matrix in various materials with ablation and heat protection requirements.

[0023] The low-density composite material prepared by the method of this invention has thermal insulation, ablation resistance and other properties that can meet the requirements of the new generation of ultra-high speed and ultra-long range rockets for efficient heat insulation, controllable ablation and vaporization, and stable ablation in the presence of oxygen. Attached Figure Description

[0024] Figure 1 Scanning electron microscope image of the cured POSS modified phenolic resin containing isocyanate groups prepared in Example 1; Figure 2 The image shows a scanning electron microscope image of the cured octaphenyl POSS-modified phenolic resin prepared in Comparative Example 1. Detailed Implementation

[0025] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The following embodiments are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0026] Example 1: A method for preparing a POSS-modified phenolic resin composite material, the specific steps of which are as follows: 1) Synthesis of POSS containing isocyanate groups Isocyanate-based siloxane and heptaphenyl-deficient POSS were placed in a four-necked flask at a molar ratio of 1.2:1. Tetrahydrofuran with the same weight as the siloxane (isocyanate-based siloxane and heptaphenyl-deficient POSS) was added and mixed thoroughly. Under a nitrogen atmosphere, a hydrochloric acid aqueous solution with a molar mass of 3 times that of the isocyanate-based siloxane (the concentration of the hydrochloric acid aqueous solution was 0.01 mol / L, but the amount added was calculated based on the molar mass of water, and the hydrochloric acid was ignored) was added. The reaction was carried out at 60°C for 16 hours. After the reaction was completed, the reaction solution was concentrated and mixed with ethanol. The precipitate obtained was POSS containing isocyanate groups.

[0027] 2) Preparation of POSS-modified phenolic resin POSS containing isocyanate groups, phenol, and formaldehyde were added to a reaction flask in a molar ratio of 0.3:1:1.04 and stirred until homogeneous. Phosphoric acid (2% by weight of phenol) was added. The reactants were heated to 80°C and reacted for 2 hours. Then, vacuum distillation was carried out. When the temperature inside the reaction flask rose back to 120°C and no distillate was obtained, vacuum distillation was stopped to obtain POSS-modified phenolic resin.

[0028] 3) Preparation of resin matrix for composite materials POSS-modified phenolic resin was prepared into a resin solution with a solid content of 45 wt% by adding isopropanol. Then, 13% by weight of hexamethylenetetramine, a curing agent, was added to the POSS-modified phenolic resin and stirred evenly to obtain the resin matrix.

[0029] 4) Molding of composite materials Quartz fiber fabric was placed in a mold and sealed, then injection molded using RTM process, and then placed in an oven for curing at 100℃ for 12 hours; the post-curing temperature was 80℃ and the post-curing time was 25 hours, resulting in POSS modified phenolic resin composite material.

[0030] The POSS-modified phenolic resin prepared according to the above method has a residual weight of 65.42% at 800°C under a nitrogen atmosphere and 13.82% at 800°C under an air atmosphere. POSS is uniformly dispersed in the resin without agglomeration or phase separation. (See attached...) Figure 1 The electron microscope image is shown.

[0031] The density of the prepared POSS-modified phenolic resin composite material was 0.695 g / cm³. 3 The linear ablation rate is 0.206 mm / s, the mass ablation rate is 0.075 g / s, the room temperature thermal conductivity is 0.056 W / m·K, the tensile strength is 48.3 MPa, the flexural strength is 67.1 MPa, and the compressive strength is 27.7 MPa.

[0032] Example 2: A method for preparing a POSS-modified phenolic resin composite material, the specific steps of which are as follows: 1) Synthesis of POSS containing isocyanate groups Isocyanate-based siloxane and heptaphenyl-deficient POSS were placed in a four-necked flask at a molar ratio of 1.1:1. Tetrahydrofuran was added in an amount twice the weight of the siloxane (isocyanate-based siloxane and heptaphenyl-deficient POSS) and mixed thoroughly. Under a nitrogen atmosphere, hydrochloric acid aqueous solution with a molar mass of 5 times that of the isocyanate-based siloxane was added (the concentration of the hydrochloric acid aqueous solution was 0.05 mol / L, but the amount added was calculated based on the molar mass of water, and the hydrochloric acid was ignored). The reaction was carried out at 70°C for 15 hours. After the reaction was completed, the reaction solution was concentrated and mixed with ethanol. The precipitate obtained was POSS containing isocyanate groups.

[0033] 2) Preparation of POSS-modified phenolic resin Polysilsesquioxane (POSS) containing isocyanate groups, o-cresol, and formaldehyde were added to a reaction flask in a molar ratio of 0.1:1:1.045 and stirred until homogeneous. Then, sulfuric acid at 0.01% by weight of o-cresol was added. The reaction mixture was heated to 80°C and reacted for 4 hours. Then, vacuum distillation was carried out. When the temperature inside the reaction flask returned to 120°C and no distillate was obtained, vacuum distillation was stopped to obtain POSS-modified phenolic resin.

[0034] 3) Preparation of resin matrix for composite materials POSS-modified phenolic resin was prepared into a resin solution with a solid content of 60 wt% by adding ethanol. Then, 18% by weight of hexamethylenetetramine, a curing agent, was added to the POSS-modified phenolic resin and stirred evenly to obtain the resin matrix. 4) Molding of composite materials Quartz fiber fabric was placed in a mold and sealed, then injection molded using RTM process, and then placed in an oven for curing at 100℃ for 8 hours; the post-curing temperature was 95℃ and the post-curing time was 15 hours, resulting in POSS modified phenolic resin composite material.

[0035] The POSS-modified phenolic resin prepared according to the above method has a residual weight of 62.17% at 800°C under nitrogen atmosphere and 4.18% at 800°C under air atmosphere. POSS is uniformly dispersed in the resin without agglomeration or phase separation.

[0036] The density of the prepared POSS-modified phenolic resin composite material is 1.08 g / cm³. 3 The linear ablation rate is 0.209 mm / s, the mass ablation rate is 0.082 g / s, and the room temperature thermal conductivity is 0.104 W / m·K.

[0037] Example 3: A method for preparing a POSS-modified phenolic resin composite material, the specific steps of which are as follows: 1) Synthesis of POSS containing isocyanate groups Isocyanate-based siloxane and heptaphenyl-deficient POSS were placed in a four-necked flask at a molar ratio of 1.3:1. Tetrahydrofuran was added in an amount three times the weight of the siloxane (isocyanate-based siloxane and heptaphenyl-deficient POSS) and mixed thoroughly. Under a nitrogen atmosphere, hydrochloric acid aqueous solution with a molar mass four times that of the isocyanate-based siloxane was added (the concentration of the hydrochloric acid aqueous solution was 0.03 mol / L, but the amount added was calculated based on the molar mass of water, and the hydrochloric acid was ignored). The reaction was carried out at 100°C for 17 hours. After the reaction was completed, the reaction solution was concentrated and mixed with methanol. The precipitate obtained was POSS containing isocyanate groups.

[0038] 2) Preparation of POSS-modified phenolic resin Polysilsesquioxane (POSS) containing isocyanate groups, resorcinol, and formaldehyde were added to a reaction flask in a molar ratio of 0.5:1:1.35 and stirred until homogeneous. Oxalic acid at 1% by weight of resorcinol was added. The reactants were heated to 100°C and reacted for 2.5 hours. Then, vacuum distillation was carried out. When the temperature inside the reaction flask returned to 105°C and no distillate was obtained, vacuum distillation was stopped to obtain POSS-modified phenolic resin.

[0039] 3) Preparation of resin matrix for composite materials POSS-modified phenolic resin was prepared into a resin solution with a solid content of 30 wt% by adding isopropanol. Then, 20% of the weight of POSS-modified phenolic resin was added as a curing agent, hexamethylenetetramine, and the mixture was stirred evenly to obtain the resin matrix. 4) Molding of composite materials Glass fiber fabric was placed in a mold and sealed, then injection molded using RTM process, and then placed in an oven for curing at 115℃ for 10 hours; the post-curing temperature was 100℃ and the post-curing time was 20 hours, resulting in POSS modified phenolic resin composite material.

[0040] The POSS-modified phenolic resin prepared according to the above method has a residual weight of 68.86% at 800°C under nitrogen atmosphere and 22.31% at 800°C under air atmosphere. POSS is uniformly dispersed in the resin without agglomeration or phase separation.

[0041] The density of the prepared POSS-modified phenolic resin composite material was 0.49 g / cm³. 3 The linear ablation rate is 0.273 mm / s, the mass ablation rate is 0.089 g / s, and the room temperature thermal conductivity is 0.048 W / m·K.

[0042] Example 4: A method for preparing a POSS-modified phenolic resin composite material, the specific steps of which are as follows: 1) Synthesis of POSS containing isocyanate groups Isocyanate-based siloxane and heptaphenyl-deficient POSS were placed in a four-necked flask at a molar ratio of 1.25:1. Tetrahydrofuran was added in an amount twice the weight of the siloxane (isocyanate-based siloxane and heptaphenyl-deficient POSS) and mixed thoroughly. Under a nitrogen atmosphere, hydrochloric acid aqueous solution with a molar mass twice that of the isocyanate-based siloxane (the concentration of the hydrochloric acid aqueous solution was 0.02 mol / L, but the amount added was calculated based on the molar mass of water, and the hydrochloric acid was ignored) was added. The reaction was carried out at 90°C for 16 hours. After the reaction was completed, the reaction solution was concentrated and mixed with isopropanol. The precipitate obtained was POSS containing isocyanate groups.

[0043] 2) Preparation of POSS-modified phenolic resin Polysilsesquioxane (POSS) containing isocyanate groups, phenol, resorcinol, o-cresol, and formaldehyde were added to a reaction flask in a molar ratio of 0.2:0.8:0.1:0.1:1.02 and stirred until homogeneous. 0.05% hydrochloric acid by weight of phenol was then added. The reactants were heated to 90°C and reacted for 3 hours. Vacuum distillation was then carried out until the temperature in the reaction flask returned to 110°C and no more distillate was observed. Vacuum distillation was then stopped to obtain POSS-modified phenolic resin.

[0044] 3) Preparation of resin matrix for composite materials POSS-modified phenolic resin was prepared into a resin solution with a solid content of 40 wt% by adding isopropanol. Then, 15% of the curing agent hexamethylenetetramine by weight of POSS-modified phenolic resin was added and stirred evenly to obtain the resin matrix. 4) Molding of composite materials Quartz fiber fabric was placed in a mold and sealed, then injection molded using RTM process, and then placed in an oven for curing at 105℃ for 11 hours; the post-curing temperature was 90℃ and the post-curing time was 23 hours, resulting in POSS modified phenolic resin composite material.

[0045] The POSS-modified phenolic resin prepared according to the above method has a residual weight of 63.78% at 800°C under nitrogen atmosphere and 8.26% at 800°C under air atmosphere. POSS is uniformly dispersed in the resin without agglomeration or phase separation.

[0046] The density of the POSS-modified phenolic resin composite material is 0.63 g / cm³. 3 The linear ablation rate is 0.241 mm / s, the mass ablation rate is 0.083 g / s, and the room temperature thermal conductivity is 0.055 W / m·K.

[0047] Example 5: A method for preparing a POSS-modified phenolic resin composite material, the specific steps of which are as follows: 1) Synthesis of POSS containing isocyanate groups Isocyanate-based siloxane and heptaphenyl-deficient POSS were placed in a four-necked flask at a molar ratio of 1.2:1. Tetrahydrofuran was added in an amount three times the weight of the siloxane (isocyanate-based siloxane and heptaphenyl-deficient POSS) and mixed thoroughly. Under a nitrogen atmosphere, hydrochloric acid aqueous solution with a molar mass three times that of the isocyanate-based siloxane was added (the concentration of the hydrochloric acid aqueous solution was 0.04 mol / L, but the amount added was calculated based on the molar mass of water, and the hydrochloric acid was ignored). The reaction was carried out at 80°C for 17 hours. After the reaction was completed, the reaction solution was concentrated and mixed with ethanol. The precipitate obtained was POSS containing isocyanate groups.

[0048] 2) Preparation of POSS-modified phenolic resin Polysilsesquioxane (POSS) containing isocyanate groups, resorcinol, phenol, and formaldehyde were added to a reaction flask in a molar ratio of 0.4:0.6:0.4:1.2 and stirred until homogeneous. Oxalic acid at 1.5% by weight of phenol was added. The reactants were heated to 100°C and reacted for 1 hour. Then, vacuum distillation was carried out. When the temperature inside the reaction flask rose back to 115°C and no distillate was obtained, vacuum distillation was stopped to obtain POSS-modified phenolic resin.

[0049] 3) Preparation of resin matrix for composite materials POSS-modified phenolic resin was prepared into a resin solution with a solid content of 50 wt% by adding methanol. Then, 10% by weight of hexamethylenetetramine, a curing agent, was added to the POSS-modified phenolic resin and stirred evenly to obtain the resin matrix. 4) Molding of composite materials Carbon fiber fabric was placed in a mold and sealed, then injection molded using RTM process, and then placed in an oven for curing at 110℃ for 9 hours; the post-curing temperature was 85℃ and the post-curing time was 17 hours, resulting in POSS modified phenolic resin composite material.

[0050] The POSS-modified phenolic resin prepared according to the above method has a residual weight of 67.31% at 800°C under nitrogen atmosphere and 16.89% at 800°C under air atmosphere. POSS is uniformly dispersed in the resin without agglomeration or phase separation.

[0051] The density of the POSS-modified phenolic resin composite material is 0.81 g / cm³. 3 The linear ablation rate is 0.185 mm / s, the mass ablation rate is 0.064 g / s, and the room temperature thermal conductivity is 0.082 W / m·K.

[0052] Comparative Example 1 is a method for preparing a POSS phenolic resin composite material. The difference between this method and Example 1 is that the isocyanate-containing POSS synthesized in Example 1 is replaced with octaphenyl polysilsesquioxane, i.e., the molar ratio of octaphenyl polysilsesquioxane (POSS), phenol, and formaldehyde is 0.3:1.3:1.04. The other resin synthesis conditions and composite material preparation methods are the same as in Example 1.

[0053] The POSS phenolic resin prepared according to the above method has a residual weight of 59.94% in a nitrogen atmosphere at 800°C and a residual weight of 12.01% in an air atmosphere at 800°C. Octaphenyl POSS exhibits agglomeration in the resin and cannot be uniformly dispersed. Figure 2 The electron microscope image shows that the density of the POSS phenolic resin composite material is 0.694 g / cm³. 3 The linear ablation rate was 0.314 mm / s, the mass ablation rate was 0.116 g / s, the room temperature thermal conductivity was 0.073 W / m·K, the tensile strength was 34.5 MPa, the flexural strength was 47.2 MPa, and the compressive strength was 13.6 MPa. A comparison of the performance with the example is shown in Table 1.

[0054] Table 1 Performance comparison between Example 1 and Comparative Example 1 From Table 1 and Figure 1 It can be seen that, compared with Example 1, the residual weight of the resin cured product of Comparative Example 1 at 800°C under both nitrogen atmosphere and air atmosphere is lower. This is mainly because POSS agglomerates, making it difficult to disperse evenly in the resin, and thus the heat resistance cannot be effectively improved.

[0055] Compared with Comparative Example 1, the composite material prepared in Example 1 has significantly improved performance. This is mainly because the POSS particles in Comparative Example 1 are larger after agglomeration, making it impossible to impregnate into the fiber. Some POSS are blocked by the fiber and gather near the injection port, failing to be evenly distributed in the composite material, resulting in poor performance of the composite material and a significant difference from Example 1.

Claims

1. A method for synthesizing POSS-modified phenolic resin, characterized in that: The specific steps are as follows: 1) Synthesis of POSS containing isocyanate groups Isocyanate-based siloxane, heptaphenyl-deficient POSS, and tetrahydrofuran were mixed, and a certain amount of hydrochloric acid aqueous solution was added under nitrogen atmosphere. A polycondensation reaction was carried out at 60–100°C. After the reaction was completed, the reaction solution was concentrated and mixed with an alcohol solvent. The resulting precipitate was isocyanate-based POSS. The structure of the isocyanate-based POSS was as follows: 2) Preparation of POSS-modified phenolic resin POSS containing isocyanate groups, phenol, formaldehyde and catalyst are added to a reaction flask in a certain proportion and stirred evenly; the reactants are heated to a certain temperature and reacted for a period of time, then distilled under reduced pressure. When the temperature in the reaction flask rises back to 100℃~120℃ and there is no distillate, the reduced pressure distillation is stopped to obtain POSS modified phenolic resin.

2. The method for synthesizing POSS-modified phenolic resin according to claim 1, characterized in that: In step 1), the molar ratio of isocyanate-based siloxane to heptaphenyl-deficient POSS is (1.1-1.3):1; the amount of tetrahydrofuran added is 1-3 times the total weight of isocyanate-based siloxane and heptaphenyl-deficient POSS; the concentration of the hydrochloric acid aqueous solution is 0.01-0.05 mol / L, and the amount added is 2-5 times the molar mass of isocyanate-based siloxane.

3. The method for synthesizing POSS-modified phenolic resin according to claim 1, characterized in that: In step 1), the alcohol solvent is one of methanol, ethanol, and isopropanol; the polycondensation reaction time is 15 to 17 hours.

4. The method for synthesizing POSS-modified phenolic resin according to claim 1, characterized in that: In step 2), the phenol is one or more of phenol, p-cresol, and resorcinol; the catalyst is one of oxalic acid, hydrochloric acid, phosphoric acid, and sulfuric acid, and the amount added is 0.01% to 2% of the weight of the phenol; the formaldehyde is a 37% aqueous solution of formaldehyde.

5. The method for synthesizing POSS-modified phenolic resin according to claim 1, characterized in that: In step 2), the molar mass ratio of POSS containing isocyanate groups to phenol is (0.1-0.5):1; the molar mass ratio of the total molar mass of POSS containing isocyanate groups and phenol to the molar mass of formaldehyde is 1:(0.8-0.95).

6. The method for synthesizing POSS-modified phenolic resin according to claim 1, characterized in that: In step 2), the reaction temperature is 80-100℃ and the reaction time is 1-4h.

7. The method for preparing composite materials from the POSS-modified phenolic resin obtained by the synthesis method of POSS-modified phenolic resin according to claim 1, characterized in that: The specific steps are as follows: 1) POSS modified phenolic resin and alcohol solvent are mixed to prepare a resin solution with a solid content of 30wt% to 60wt%. A certain amount of curing agent hexamethylenetetramine is added and stirred evenly to obtain the resin matrix. 2) The fiber fabric is placed in a mold and sealed, and then injection molded using RTM process. It is then placed in an oven for curing and post-curing to obtain a low-density POSS modified phenolic resin composite material.

8. The method for preparing composite materials according to claim 7, characterized in that: The alcohol solvent in step 1) is one of methanol, ethanol, and isopropanol; the amount of hexamethylenetetramine added is 10% to 20% of the weight of the POSS modified phenolic resin.

9. The method for preparing composite materials according to claim 7, characterized in that: The curing temperature in step 2) is 100-120℃ and the curing time is 8-12h; the post-curing temperature is 80-100℃ and the post-curing time is 15-25h.

10. The method for preparing composite materials according to claim 7, characterized in that: In step 2), the fiber fabric is one or more of quartz fiber fabric, glass fiber fabric, and carbon fiber fabric.

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