CN117659320A - Ablation-resistant phenolic aerogel and preparation method thereof - Google Patents

Abstract

The invention relates to an ablation-resistant phenolic aerogel and a preparation method thereof, and belongs to the technical field of aerogel preparation. The method comprises the following steps: bisphenol POSS, phenol and formaldehyde are utilized, oxalic acid is used as a catalyst for polycondensation reaction, and linear phenolic resin with a main chain containing a POSS structure is generated; carrying out sol-gel reaction by using linear phenolic resin with a main chain containing a POSS structure and an alcohol solvent and using hexamethylenetetramine as a cross-linking agent to obtain phenolic wet gel with the main chain containing the POSS structure; and (3) drying the phenolic wet gel at normal pressure to obtain the phenolic aerogel with the main chain containing the POSS structure. The invention can prepare the ablation-resistant phenolic aerogel with a nanoscale multi-stage microstructure and a POSS structure in a main chain without using high-cost preparation processes such as solvent replacement, supercritical drying, freeze drying and the like, and has the characteristics of wide applicability, simple reaction process, low process cost and the like.

Description

A kind of ablation-resistant phenolic aerogel and preparation method thereof

Technical field

The invention relates to an ablation-resistant phenolic aerogel and a preparation method thereof, and belongs to the technical field of aerogel preparation.

Background technique

Phenolic resin has become the most commonly used resin matrix for ablative heat-proof composite materials due to its excellent properties such as high temperature resistance, high carbon residue, and flame retardancy. With the rapid development of aerospace technology, deep space exploration aircraft have put forward lightweight and efficient thermal insulation requirements for ablative heat-proof composite materials. While reducing the density of fiber reinforcements, sol-gel technology is used to convert phenolic resin into Preparing it into an aerogel is an important way to reduce the density of the resin matrix and improve the thermal insulation performance. Phenolic aerogel is a gel material with a three-dimensional network space structure. The nano-scale porous structure of the aerogel limits the gas phase heat conduction and solid phase heat conduction of heat, so the material itself has good thermal insulation properties. In addition, Pyrolysis gas produces a thermal blocking effect, and the carbon layer formed by pyrolysis can reversely radiate and dissipate heat. Therefore, phenolic aerogel also has an ablative and heat-proof effect.

However, a large number of phenolic hydroxyl groups and methylene groups in the molecular structure of phenolic airgel are easily oxidized and decomposed under high temperature conditions. When used for a long time under aerobic conditions, the ablation resistance is poor, resulting in the collapse of the porous structure of the airgel and the isolation of the airgel. The thermal performance is severely reduced. At the same time, once the phenolic airgel is ablated, the mechanical properties of the airgel composite material will be significantly reduced. Under the conditions of high airflow erosion, the airgel anti-insulation material is at risk of damage and failure. . Therefore, it is necessary to carry out ablation-resistant modification on phenolic airgel to ensure that the phenolic airgel maintains a high residual weight rate during use, so that the anti-heat insulation material maintains good structural strength.

POSS is the general name for oligomeric silsesquioxanes with a cage structure, consisting of a silicon-oxygen skeleton alternately connected by Si-O, and its three-dimensional size is between 1-3nm. The exterior of each POSS molecule can be designed with active or inert functional groups, so POSS materials can be copolymerized or grafted into the macromolecular network of polymers, which solves the problem of poor compatibility between traditional nanomaterials and polymer systems. Since POSS has a nanometer size similar to that of the polymer chain segments, it can inhibit the movement of the polymer chain segments and improve the thermal and mechanical properties of the resin. Modifying phenolic aerogels with POSS is expected to significantly improve the ablation resistance of phenolic aerogels and significantly expand the service limit of existing phenolic aerogel anti-insulation materials in aerodynamic thermal environments.

In the master's degree thesis of Beijing University of Chemical Technology, "Preparation and Performance Research of Special Hybrid Phenolic Aerogels", a method of POSS graft-modified phenolic aerogels is disclosed: ① Octacarboxylic POSS graft-modified phenolic aerogels Preparation of the glue: First, use resorcinol and formaldehyde as raw materials, water as the reaction solvent, and sodium carbonate as the catalyst. After a 45°C prepolymerization process, a uniform and transparent solution is obtained. Then a certain amount of octacarboxy POSS is added, and the carboxyl group is used to undergo an esterification reaction with the hydroxyl group of the phenolic prepolymer, so that POSS is grafted to the side chain of the phenolic prepolymer. Finally, after gelling, aging, and solvent replacement with ethanol and cyclohexane , dried under normal pressure to obtain octacarboxyl POSS graft-modified phenolic aerogel. ②Preparation of hydroxyl POSS graft-modified phenolic aerogel: First, pre-polymerize the ethyl acetate solution of hydroxyl POSS and the methanol solution of formaldehyde and barium hydroxide by heating, and then rotary evaporate to remove the solvent to obtain the hydroxyl POSS grafted phenolic aerogel. Phenolic solution. Finally, after gelation, aging, solvent replacement with ethanol and tert-butanol, and freeze-drying, the hydroxyl POSS graft-modified phenolic aerogel was obtained. Although this method successfully introduced the POSS structure into the phenolic aerogel, the density of the modified phenolic aerogel increased significantly because POSS was grafted on the side chain of the phenolic molecule but not in the main chain of the phenolic molecule; and Due to reaction steric hindrance, only a small part of the reactive functional groups (eg, carboxyl group, hydroxyl group) on POSS are used for grafting reaction, and the utilization rate is low. However, these reactive functional groups (eg, carboxyl group, hydroxyl group) have poor thermal stability. It offsets part of the effect of introducing POSS structure on improving the ablation resistance of phenolic resin aerogels; this method requires multiple solvent replacements and even freeze-drying, which results in low modification efficiency and is not economical. How to improve the ablation resistance and economy of POSS-modified phenolic resin aerogels is an urgent technical problem that needs to be solved in this field.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings of the existing technology and provide an ablation-resistant phenolic aerogel whose molecular backbone contains a POSS structure and a preparation method thereof.

In order to achieve the above objects, the technical solutions adopted by the present invention are as follows:

A preparation method of ablation-resistant phenolic aerogel, including the following steps:

Utilize bisphenol POSS, phenol and formaldehyde, and use oxalic acid as a catalyst to perform a polycondensation reaction to generate a linear phenolic resin containing a POSS structure in the main chain;

Utilize novolac phenolic resin containing POSS structure in the main chain and alcohol solvent, and use hexamethylenetetramine as the cross-linking agent to perform sol-gel reaction to obtain a phenolic wet gel containing POSS structure in the main chain;

The phenolic wet gel is dried under normal pressure to obtain a phenolic aerogel containing a POSS structure in the main chain.

Further, the method uses bisphenol POSS, phenol and formaldehyde to perform a polycondensation reaction using oxalic acid as a catalyst to generate a novolac resin with a POSS structure in the main chain, including: mixing bisphenol POSS, phenol and formaldehyde in a reaction solvent to obtain a uniform Solution, add oxalic acid solution dropwise under stirring and heating conditions, react at a constant temperature for a period of time, cool down (preferably to 60-80°C), dehydrate and solvent in vacuum, and cool to produce a yellow transparent solid, which is a linear linear substance containing a POSS structure in the main chain Phenolic Resin.

Further, the above-mentioned method uses novolac resin and alcohol solvent whose main chain contains POSS structure, and uses hexamethylenetetramine as the cross-linking agent to perform sol-gel reaction to obtain a phenolic wet gel containing POSS structure in its main chain, The method includes: mixing a novolac resin with a POSS structure in its main chain and an appropriate amount of alcohol solvent, and uniformly mixing it to obtain a novolac resin solution with a POSS structure in its main chain; and stirring at room temperature, adding the linear phenolic resin with a POSS structure in its main chain to the solution. Add a certain amount of cross-linking agent hexamethylenetetramine to the phenolic resin solution. After the hexamethylenetetramine is completely dissolved, add the solution to the mold, heat it for gelation reaction and aging, and obtain the POSS structure in the main chain. of phenolic wet gel.

Further, the molecular structure of the bisphenol POSS is as shown below, R ₁ is one of methyl (Me) or phenyl (Ph), R ₂ is a functional group containing a phenol structure, such as R _2-1 , R _2-2 and R _2-3 . This molecular structure contains two R ₂ , so it is called bisphenol POSS.

Further, among the bisphenol POSS, phenol and formaldehyde, the molar ratio of the addition of bisphenol POSS and phenol (bisphenol POSS + phenol) to formaldehyde is 100:50~90, and the molar ratio of bisphenol POSS and phenol is 100:50~90. It is 1~100:99~0. The addition amount of catalyst oxalic acid is 1 to 3% of the mass sum of bisphenol POSS and phenol. The addition amount of catalyst oxalic acid is too low, resulting in insufficient polycondensation reaction. The addition amount of catalyst oxalic acid is too high, resulting in violent polycondensation reaction, poor molecular weight uniformity, too broad molecular weight distribution, and poor quality stability of the novolac resin.

Further, the reaction solvent is one or a mixture of one or more of butanone and ethyl acetate in any proportion. The added amount of the reaction solvent is 1 to 3 times the mass sum of bisphenol POSS, phenol, and formaldehyde.

Further, the temperature of the isothermal reaction is 75-100°C, and the reaction time is 6-10 hours.

Further, the alcohol solvent is one or a mixture of more than one of methanol, ethanol, n-propanol, isopropanol, n-amyl alcohol and isoamyl alcohol in any proportion.

Further, the solid content of the novolac resin solution whose main chain contains POSS structure is 8% to 50%. If the solid content is too high, the prepared aerogel will have a high density and lose the advantage of low density; if the solid content is too low, the resulting wet gel will have poor strength and will easily break when dried under normal pressure, and will shrink severely.

Further, the added amount of the cross-linking agent hexamethylenetetramine is 10% to 20% of the mass of the novolac resin containing POSS structure in the main chain in terms of mass. If the amount of cross-linking agent added is too high, unreacted cross-linking agent will remain, resulting in a decrease in the thermal stability of the airgel; if the amount of cross-linking agent added is too low, the resulting wet gel will have poor strength and will easily break when dried under normal pressure. , and severely contracted.

Further, the temperature of the gelation reaction is 70-150°C, and the time is 12-48 hours. If the reaction temperature is too low, the degree of reaction of the wet gel will be insufficient, and the resulting gel will have poor strength, will easily break when dried under normal pressure, and will shrink severely. If the reaction temperature is too high, the vapor pressure of the solvent will be high, which will pose a greater challenge to the sealing of the mold and pose safety risks. If the reaction time is too short, the degree of reaction of the wet gel will be insufficient, and the resulting gel will have poor strength, will easily break when dried under normal pressure, and will shrink severely. If the reaction time is too long, the efficiency of gel solidification will be affected.

Further, the conditions for normal pressure drying include airing at room temperature for 24 to 72 hours, and then maintaining the temperature at 100 to 120°C for 12 to 24 hours.

An ablation-resistant phenolic aerogel whose molecular main chain contains POSS structure is prepared by the above method.

The beneficial effects of the present invention compared with the prior art:

(1) The invention provides a method for preparing an ablation-resistant phenolic aerogel, which uses bisphenol POSS, phenol and aldehyde to undergo a polycondensation reaction under the catalysis of oxalic acid to synthesize a linear phenolic resin containing a POSS structure in the main chain, and uses alcohols Using hexamethylenetetramine as the solvent and hexamethylenetetramine as the cross-linking agent, through sol-gel reaction and drying under normal pressure, an ablation-resistant phenolic aerogel containing a POSS structure in the main chain was successfully prepared. By introducing the POSS structure through the main chain, the phenolic aerogel density can be achieved without grafting POSS, and the proportion of the POSS structure can be greatly increased without increasing the density. This solves the problem of POSS being grafted on the side chains of phenolic molecules, resulting in modified The problem of obvious increase in the density of phenolic aerogels; the bisphenols of bisphenol POSS are all involved in the polycondensation reaction. Compared with eight reactive functional groups (eg, carboxyl, hydroxyl), the utilization rate of reactive functional groups is high, which solves the problem of residual reactive functional groups (eg, Carboxyl group, hydroxyl group) has poor thermal stability, which offsets part of the problem of improving the ablation resistance of phenolic resin aerogels by introducing POSS structure.

(2) The present invention does not require the use of high-cost preparation processes such as solvent replacement, supercritical drying, freeze-drying, etc., and can produce ablation-resistant phenolic gas with nanoscale multi-level microstructure and POSS structure in the main chain by drying under normal pressure. Gel has the characteristics of wide applicability, simple reaction process, and low process cost.

(3) Compared with ordinary phenolic aerogels, the ablation-resistant phenolic aerogels containing POSS structure in the main chain prepared by the present invention have a higher initial decomposition temperature and higher decomposition temperature under nitrogen and air atmospheres. The residual weight rate greatly improves the ablation resistance of phenolic aerogels. It can be used as a resin matrix for exterior heat-proof materials of aerospace vehicles and greatly improves the anti-heat insulation performance of exterior heat-proof materials.

Detailed ways

In order to make the technical solution of the present invention more obvious and easy to understand, examples are given and described in detail below in conjunction with the attached table.

Example 1

The preparation method is achieved through the following steps:

1) Dissolve bisphenol POSS, phenol, and formaldehyde in ethyl acetate (mass equal to bisphenol Mix 2 times the mass sum of POSS, phenol and formaldehyde to obtain a homogeneous solution, add oxalic acid solution dropwise under stirring and 80°C conditions (the mass of oxalic acid is 2% of the mass sum of bisphenol POSS and phenol, the concentration is 10%, the solvent (Ethyl acetate), react at 80°C for 8 hours, cool to 70°C, dehydrate and solvent in vacuum, cool to produce a yellow transparent solid, which is a novolac resin with a POSS structure in the main chain. Add an appropriate amount of ethanol according to the solid content of 30%. After mixing evenly, a novolac resin solution (solid content 30%) containing POSS structure in the main chain is obtained;

2) Under stirring at room temperature, add 15% of the cross-linking agent hexamethylenetetramine by mass to the novolac resin solution containing POSS structure in the main chain, and wait until After the hexamethylenetetramine is completely dissolved, the solution is put into the mold, heated for gelation reaction and aging. The heating treatment conditions are 90°C for 6h, 120°C for 8h, and 150°C for 8h to obtain a phenolic wetted phenolic product with a POSS structure in the main chain. gel.

3) Dry the phenolic wet gel under normal pressure. The normal pressure drying conditions are to leave it at room temperature for 24 hours, and then keep it at 100°C for 12 hours to obtain an ablation-resistant phenolic aerogel containing a POSS structure in the main chain.

The test results of the ablation-resistant phenolic aerogel containing a POSS structure in the main chain obtained in this example are listed in Table 1.

Example 2

Except that the molar ratio of bisphenol POSS and phenol is 100:0, the preparation conditions and process of the ablation-resistant phenolic aerogel containing POSS structure in the main chain are the same as in Example 1. The ablation-resistant phenolic aerogel containing POSS structure in the main chain is Gel performance test result data are listed in Table 1.

Example 3

Except that the molar ratio of bisphenol POSS and phenol is 1:99, the preparation conditions and process of the ablation-resistant phenolic aerogel containing POSS structure in the main chain are the same as in Example 1. The ablation-resistant phenolic aerogel containing POSS structure in the main chain is Gel performance test result data are listed in Table 1.

Example 4

Except that the molar ratio of (bisphenol POSS + phenol) and formaldehyde is 100:50, the preparation conditions and process of the ablation-resistant phenolic aerogel containing POSS structure in the main chain are the same as in Example 1. The performance test results of etched phenolic airgel are listed in Table 1.

Example 5

Except that the molar ratio of (bisphenol POSS + phenol) and formaldehyde is 100:90, the preparation conditions and process of the ablation-resistant phenolic aerogel containing POSS structure in the main chain are the same as in Example 1. The performance test results of etched phenolic airgel are listed in Table 1.

Example 6

Except that the solid content of the novolac resin solution containing POSS structure in the main chain is 8%, the preparation conditions and process of the ablation-resistant phenolic aerogel containing POSS structure in the main chain are the same as in Example 1. The performance test results of ablative phenolic airgel are listed in Table 1.

Example 7

Except that the solid content of the novolac resin solution containing POSS structure in the main chain is 50%, the preparation conditions and process of the ablation-resistant phenolic aerogel containing POSS structure in the main chain are the same as in Example 1. The performance test results of ablative phenolic airgel are listed in Table 1.

Comparative example 1

1) According to the molar ratio of phenol and formaldehyde is 100:70, dissolve phenol and formaldehyde in ethyl acetate (the mass is equal to 2 times the combined mass of phenol and formaldehyde) and mix to obtain a uniform solution, and drop it under stirring and 80°C conditions. Add oxalic acid solution (the mass of oxalic acid is 2% of the sum of the mass of bisphenol POSS and phenol, the concentration is 10%, the solvent is ethyl acetate), react at 80°C for 8 hours, cool to 70°C, vacuum dehydrate and solvent, cool to produce yellow transparent The solid is novolac resin. According to the solid content of 30%, add an appropriate amount of ethanol and mix evenly to obtain a novolac resin solution (solid content of 30%);

2) Under stirring at room temperature, add 15% of the cross-linking agent hexamethylenetetramine to the novolac resin solution into the novolac resin solution. After the hexamethylenetetramine is completely dissolved, put the solution into the mold. , heat for gelation reaction and aging. The heat treatment conditions are 90°C for 6h, 120°C for 8h, and 150°C for 8h to obtain a phenolic wet gel.

3) Dry the phenolic wet gel under normal pressure. The normal pressure drying condition is to leave it at room temperature for 24 hours, and then keep it at 100°C for 12 hours to obtain ordinary phenolic aerogel.

The test result data of common phenolic aerogels obtained in this example are listed in Table 1.

Table 1 Performance test results of ablation-resistant phenolic aerogels obtained in Examples 1-7

It can be seen from the data in Table 1 that compared with Comparative Example 1, the ablation-resistant phenolic aerogel provided by the present invention has a higher initial decomposition temperature and a higher residual weight rate under nitrogen and air atmospheres, which greatly improves the The ablation resistance of phenolic aerogel makes it an ideal resin matrix for exterior thermal insulation materials of aerospace vehicles, which can greatly improve the anti-heat insulation performance of exterior thermal insulation materials.

The parts of the present invention that are not described in detail are well known to those skilled in the art.

The above are only preferred embodiments of the present invention. It should be noted that those of ordinary skill in the art can also make several improvements and substitutions without departing from the technical principles of the present invention. These improvements and substitutions It should also be regarded as the protection scope of the present invention.

Claims (10)

1. A method for preparing ablation-resistant phenolic aerogel, which is characterized by comprising the following steps: Utilize bisphenol POSS, phenol and formaldehyde, and use oxalic acid as a catalyst to perform a polycondensation reaction to generate a linear phenolic resin containing a POSS structure in the main chain; Utilize novolac phenolic resin containing POSS structure in the main chain and alcohol solvent, and use hexamethylenetetramine as the cross-linking agent to perform sol-gel reaction to obtain a phenolic wet gel containing POSS structure in the main chain; The phenolic wet gel is dried under normal pressure to obtain a phenolic aerogel containing a POSS structure in the main chain. 2. The method according to claim 1, characterized in that the described utilizes bisphenol POSS, phenol and formaldehyde, and uses oxalic acid as a catalyst to perform a polycondensation reaction to generate a linear phenolic resin whose main chain contains POSS structure, including: Mix bisphenol POSS, phenol and formaldehyde in the reaction solvent to obtain a uniform solution, add oxalic acid solution dropwise under stirring and heating conditions, react at a constant temperature for a period of time, cool down, vacuum dehydrate and solvent, and cool to form a yellow transparent solid, which is the main Novolac resin with POSS structure in the chain. 3. The method according to claim 2, characterized in that the reaction solvent is a mixture of one or more of butanone and ethyl acetate in any proportion, and the addition amount of the reaction solvent is bisphenol POSS, phenol, 1 to 3 times the mass of formaldehyde; The cooling is to lower the temperature to 60-80°C. 4. The method according to claim 1, characterized in that the use of novolac resin and alcohol solvents containing POSS structures in the main chain, and hexamethylenetetramine as a cross-linking agent to carry out sol-gel reaction, Obtaining a phenolic wet gel with a POSS structure in the main chain includes: uniformly mixing a novolac resin with a POSS structure in the main chain and an appropriate amount of alcohol solvent to obtain a novolac resin solution with a POSS structure in the main chain, the main chain containing The solid content of the novolac resin solution with POSS structure is 8% to 50%; under stirring at room temperature, add the cross-linking agent hexamethylenetetramine to the novolac resin solution with POSS structure in the main chain. The cross-linking agent The amount of hexamethylenetetramine added is 10% to 20% of the mass of the novolac resin containing POSS structure in the main chain; after the hexamethylenetetramine is completely dissolved, add the solution to the mold and heat to perform the gelation reaction. and aging to obtain a phenolic wet gel containing a POSS structure in the main chain. 5. method according to claim 1, is characterized in that, the molecular structure of described bisphenol POSS is as follows: Among them, R ₁ is one of methyl or phenyl, and R ₂ is a functional group containing a phenol structure. 6. The method according to claim 5, characterized in that said _R2 is one of the following: 7. The method according to claim 1, characterized in that, among the bisphenol POSS, phenol and formaldehyde, the molar ratio of bisphenol POSS, phenol plus formaldehyde is 100:50-90; bisphenol POSS , the molar ratio of phenol is 1~100:99~0; The addition amount of catalyst oxalic acid is 1 to 3% of the mass sum of bisphenol POSS and phenol. 8. The method according to claim 1, wherein the alcoholic solvent is one or more of methanol, ethanol, n-propanol, isopropanol, n-amyl alcohol and isoamyl alcohol in any proportion. mixture. 9. The method according to claim 1, characterized in that the temperature of the isothermal reaction is 75-100°C and the reaction time is 6-10h; the temperature of the gelation reaction is 70-150°C and the time is 12~48h; the normal pressure drying condition is to leave it at room temperature for 24~72h, and then keep it at 100~120°C for 12~24h. 10. An ablation-resistant phenolic aerogel, characterized in that it is prepared by the method described in any one of claims 1 to 9.