Polyimide composite material as well as preparation method and application thereof

Abstract

The invention discloses a polyimide composite material and a preparation method and application thereof, and relates to the field of aerogel, the preparation method of the polyimide composite material comprises the following steps: S1, under the condition of introducing nitrogen, adding aromatic diamine into a solvent, dissolving, adding aromatic dianhydride, and reacting to obtain a polyamide acid solution; s2, adding a cross-linking agent, performing stirring reaction, adding acetic anhydride and triethylamine, and performing gelation and aging to obtain polyimide wet gel; and S3, carrying out solvent replacement on the polyimide wet gel, and carrying out CO2 supercritical drying treatment to obtain the polyimide aerogel. According to the invention, a three-dimensional network structure is formed by a flexible chain segment of the long-chain multi-amino cross-linking agent and a polyimide molecular chain, and the polyimide aerogel composite material with low density and excellent mechanical properties is prepared by adjusting the solid content and the ratio of the cross-linking agent.

Description

Technical Field

[0001] The present invention relates to the field of aerogels, and particularly to a polyimide composite material, a preparation method thereof, and an application thereof. Background Art

[0002] Polyimide (PI) aerogel materials are one of the organic polymer aerogels with the best comprehensive properties, having excellent properties such as high temperature resistance and extremely low temperature resistance, high flexibility, and high-efficiency heat insulation. They show broad application prospects in many fields such as aerospace, energy, shipbuilding, and the microelectronics industry. Both at home and abroad, they are regarded as one of the key strategic materials for the iterative upgrading of future high-end equipment. In recent years, they have attracted the interest of researchers. Factors such as the backbone structure of the molecular chain, the type of crosslinking agent, and the crosslinking density in polyimide aerogel materials have important influences on the shrinkage rate, porosity, density, and mechanical properties of the aerogel. Breakthroughs in this field will enable such materials to be widely used in extreme environments, space exploration, aerospace equipment, etc., and solve the problems of poor high-temperature resistance, radiation resistance, and mechanical properties of traditional aerogel materials. Therefore, how to effectively regulate the rigidity and flexibility of the polyimide main-chain molecules and achieve effective control of the internal microstructure of polyimide aerogels, and prepare polyimide aerogels integrating characteristics such as high porosity, low thermal conductivity, high temperature resistance, excellent mechanical properties, low water absorption, and excellent dielectric properties is an important development direction in this field.

[0003] Chinese Patent CN114891211 B announced an ultra-thin polyimide aerogel and a preparation method. In the invention, an aromatic dianhydride structure of 4,4'-(hexafluoroisopropylidene) diphthalic anhydride and a fluorine-containing and silicon-containing aliphatic silicon diamine are used. Utilizing the superhydrophobicity of the fluorine-containing unit, a good weak interaction effect between the material and the aqueous solution can be achieved. Since there are no chemical crosslinking points between linear polyamic acids, the cured polyimide aerogel prepared with insufficient crosslinking density has low mechanical strength. In the absence of chemical crosslinking, the polyimide molecular chains are mainly combined through physical interactions, and this combination method is easily damaged when subjected to external forces, resulting in a decrease in the overall strength of the material. This makes it difficult for polyimide aerogels to meet the requirements in some application scenarios with high strength requirements. Summary of the Invention

[0004] The purpose of the present invention is to provide a polyimide composite material, a preparation method thereof, and an application thereof to solve the following technical problems:

[0005] How to effectively regulate the rigidity and flexibility of the polyimide composite material molecular chains and prepare a polyimide aerogel with excellent mechanical properties.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] In a first aspect, the present invention discloses a method for preparing a polyimide composite material, comprising the following steps:

[0008] S1. Under the condition of passing nitrogen, 4,4-diaminodiphenyl ether and 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)benzene are added to a solvent. After complete dissolution, 3,3',4,4'-biphenyltetracarboxylic dianhydride is added. After the stirring reaction is completed, an anhydride-terminated polyamic acid solution is obtained;

[0009] S2. A crosslinking agent is added to the anhydride-terminated polyamic acid solution, stirred at room temperature for 5-20 min, acetic anhydride and triethylamine are added, gelled for 15-40 min, and aged at room temperature for 24 h to obtain a polyimide wet gel;

[0010] S3. The polyimide wet gel is subjected to solvent replacement and supercritical carbon dioxide fluid drying to obtain a polyimide aerogel;

[0011] The crosslinking agent is N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine.

[0012] Preferably, the solvent described in S1 is one or more of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and N,N-dimethylacetamide (DMAc).

[0013] Preferably, the stirring reaction temperature in S1 is 0-40 °C, and the reaction time is 8-24 h.

[0014] Preferably, the molar ratio of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4-diaminodiphenyl ether, and 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-benzene in S1 is 50:48.5-49.5:48.5-49.5.

[0015] Preferably, the molar ratio of the anhydride group in 3,3',4,4'-biphenyltetracarboxylic dianhydride to N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine is 50:0.2-0.6.

[0016] Preferably, the solid content of the polyimide wet gel in S2 is 6%-10%.

[0017] Preferably, the molar ratio of acetic anhydride to triethylamine in S2 is 4:1; the molar ratio of acetic anhydride to 3,3',4,4'-biphenyltetracarboxylic dianhydride is 7-10:1.

[0018] Preferably, the supercritical fluid drying conditions in S3 are as follows: using liquid CO2, treating for 3 - 6 h under the conditions of 8 - 13 MPa and 20 - 30 °C, and then heating to 40 - 60 °C and 8 - 13 MPa for 4 - 8 h.

[0019] Preferably, the specific operation steps of the solvent replacement in S3 are as follows:

[0020] The polyimide wet gel is placed in a mixed solution with a volume ratio of NMP to acetone of 75 / 25, 50 / 50, and 25 / 75 for replacement once each, and then replaced with pure acetone solvent three more times, with a time interval of 12 h for each solvent replacement.

[0021] In a second aspect, the present invention also discloses a polyimide composite material obtained by the above preparation method.

[0022] In a third aspect, the present invention also discloses an application of the polyimide composite material.

[0023] Advantages of the present invention:

[0024] 1. The present invention uses a long-chain polyamino crosslinking agent, and the anhydride-terminated polyamic acid reacts with the long-chain polyamine to obtain a crosslinked network. The long-chain polyamino crosslinking agent forms a three-dimensional network structure with the polyimide molecular chain through flexible chain segments. The presence of crosslinking points increases the internal crosslinking density of the aerogel, significantly improving the mechanical properties of the polyimide aerogel and endowing it with better flexibility and mechanical properties.

[0025] 2. The present invention uses N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine as a crosslinking agent. Compared with 1,3,5-triaminophenoxybenzene used as a crosslinking agent in the prior art, N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine has more active sites and contains flexible alkyl chains. It belongs to a long-chain polyamino crosslinking agent and has an obvious toughening effect. The tensile strength and elongation at break of the finally prepared polyimide aerogel are both high, and the mechanical properties are excellent. Specific embodiments

[0026] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0027] In the experimental methods of the following examples, unless otherwise specified, they are all conventional methods, carried out according to the techniques or conditions described in the literature in this field or according to the product instructions. The materials, reagents, etc. used in the following examples, unless otherwise specified, can be obtained from commercial channels.

[0028] Example 1

[0029] The preparation method of the polyimide composite material adopts the following steps:

[0030] S1. Under the condition of nitrogen passing, add 120 mL of NMP solvent into a 250 mL flask, then add 1.9826 g of 4,4-diaminodiphenyl ether, 4.2405 g of 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-benzene and 5.8844 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride. After stirring and reacting at 25 °C for 12 h, an anhydride-terminated polyamic acid solution is obtained;

[0031] S2. Add 0.0255 g of N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine to the anhydride-terminated polyamic acid solution, stir at room temperature for 20 min, add 15 mL of acetic anhydride and 5.6 mL of triethylamine, transfer to a mold to gel for 30 min and stand for 24 h for aging to obtain a polyimide wet gel;

[0032] S3. Place the polyimide wet gel in a mixed solution with a volume ratio of NMP to acetone of 75 / 25, 50 / 50, and 25 / 75 for replacement once each, and then replace it with pure acetone solvent three more times, with a time interval of 12 h for each solvent replacement. Finally, dry it with supercritical carbon dioxide fluid. The drying conditions are: maintain at 25 °C and 10 MPa for 4 h, then heat up to 45 °C and 10 MPa for 6 h, and release the gas to obtain a polyimide aerogel.

[0033] Example 2

[0034] The preparation method of the polyimide composite material adopts the following steps:

[0035] S1. Under the condition of nitrogen passing, add 120 mL of NMP solvent into a 250 mL flask, then add 1.9623 g of 4,4-diaminodiphenyl ether, 4.1976 g of 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-benzene and 5.8844 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride. After stirring and reacting at 25 °C for 12 h, an anhydride-terminated polyamic acid solution is obtained;

[0036] S2. Add 0.0510 g of N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine to the anhydride-terminated polyamic acid solution, stir at room temperature for 20 min, add 15 mL of acetic anhydride and 5.6 mL of triethylamine, transfer to a mold, gel for 30 min, and age for 24 h to obtain a polyimide wet gel;

[0037] S3. Place the polyimide wet gel in a mixed solution of NMP and acetone with a volume ratio of 75 / 25, 50 / 50, and 25 / 75 for replacement once each, and then replace it with pure acetone solvent three more times. The time interval for each solvent replacement is 12 h. Finally, dry it with supercritical carbon dioxide fluid. The drying conditions are: maintain at 25 °C and 10 MPa for 4 h, then heat up to 45 °C and 10 MPa for 6 h, and release the gas to obtain a polyimide aerogel.

[0038] Example 3

[0039] A method for preparing a polyimide composite material, comprising the following steps:

[0040] S1. Under a nitrogen atmosphere, add 120 mL of NMP solvent to a 250 mL flask, then add 1.9422 g of 4,4-diaminodiphenyl ether, 4.1548 g of 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-benzene, and 5.8844 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride. Stir and react at 25 °C for 12 h to obtain an anhydride-terminated polyamic acid solution;

[0041] S2. Add 0.0764 g of N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine to the anhydride-terminated polyamic acid solution, stir at room temperature for 20 min, add 15 mL of acetic anhydride and 5.6 mL of triethylamine, transfer to a mold, gel for 30 min, and age for 24 h to obtain a polyimide wet gel;

[0042] S3. Place the polyimide wet gel in a mixed solution of NMP and acetone with a volume ratio of 75 / 25, 50 / 50, and 25 / 75 for replacement once each, and then replace it with pure acetone solvent three more times. The time interval for each solvent replacement is 12 h. Finally, dry it with supercritical carbon dioxide fluid. The drying conditions are: maintain at 25 °C and 10 MPa for 4 h, then heat up to 45 °C and 10 MPa for 6 h, and release the gas to obtain a polyimide aerogel.

[0043] Example 4

[0044] A method for preparing a polyimide composite material, comprising the following steps:

[0045] S1. Under the condition of nitrogen passing, add 165 mL of NMP solvent into a 250 mL flask, then add 1.9422 g of 4,4-diaminodiphenyl ether, 4.1548 g of 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-benzene and 5.8844 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride. After stirring and reacting at 25 °C for 12 h, an anhydride-terminated polyamic acid solution is obtained;

[0046] S2. Add 0.0764 g of N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine into the anhydride-terminated polyamic acid solution, stir at room temperature for 20 min, add 15 mL of acetic anhydride and 5.6 mL of triethylamine, transfer to a mold, gel for 30 min and stand for 24 h for aging to obtain a polyimide wet gel;

[0047] S3. Place the polyimide wet gel in a mixed solution with a volume ratio of NMP to acetone of 75 / 25, 50 / 50, and 25 / 75 for replacement once each, and then replace it with pure acetone solvent three more times. The time interval for each solvent replacement is 12 h. Finally, dry it with supercritical carbon dioxide fluid. The drying conditions are: keep at 25 °C and 10 MPa for 4 h, then heat up to 45 °C and 10 MPa for 6 h, and release the gas to obtain a polyimide aerogel.

[0048] Example 5

[0049] A preparation method of a polyimide composite material, comprising the following steps:

[0050] S1. Under the condition of nitrogen passing, add 85 mL of NMP solvent into a 250 mL flask, then add 1.9422 g of 4,4-diaminodiphenyl ether, 4.1548 g of 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-benzene and 5.8844 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride. After stirring and reacting at 25 °C for 12 h, an anhydride-terminated polyamic acid solution is obtained;

[0051] S2. Add 0.0764 g of N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine into the anhydride-terminated polyamic acid solution, stir at room temperature for 20 min, add 15 mL of acetic anhydride and 5.6 mL of triethylamine, transfer to a mold, gel for 30 min and stand for 24 h for aging to obtain a polyimide wet gel;

[0052] S3. The polyimide wet gel was placed in a mixed solution with a volume ratio of NMP to acetone of 75 / 25, 50 / 50, and 25 / 75 for one replacement each, and then replaced with pure acetone solvent three more times. The time interval for each solvent replacement was 12 h. Finally, it was dried with supercritical carbon dioxide fluid. The drying conditions were: maintaining at 25 °C and 10 MPa for 4 h, then heating to 45 °C and 10 MPa for 6 h, and deflating to obtain the polyimide aerogel.

[0053] Comparative Example 1

[0054] The preparation method of the polyimide composite material comprises the following steps:

[0055] S1. Under the condition of nitrogen passing, 85 mL of NMP solvent was added to a 250 mL flask, and then 1.9422 g of 4,4-diaminodiphenyl ether, 4.1548 g of 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-benzene, and 5.8844 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride were added. After stirring and reacting at 25 °C for 12 h, an anhydride-terminated polyamic acid solution was obtained.

[0056] S2. 0.1598 g of 1,3,5-triaminophenoxybenzene (TAB) was added to the anhydride-terminated polyamic acid solution, stirred at room temperature for 20 min, 15 mL of acetic anhydride and 5.6 mL of triethylamine were added, transferred to a mold, gelled for 30 min, and aged by standing for 24 h to obtain a polyimide wet gel.

[0057] S3. The polyimide wet gel was placed in a mixed solution with a volume ratio of NMP to acetone of 75 / 25, 50 / 50, and 25 / 75 for one replacement each, and then replaced with pure acetone solvent three more times. The time interval for each solvent replacement was 12 h. Finally, it was dried with supercritical carbon dioxide fluid. The drying conditions were: maintaining at 25 °C and 10 MPa for 4 h, then heating to 45 °C and 10 MPa for 6 h, and deflating to obtain the polyimide aerogel.

[0058] Comparative Example 2

[0059] S1. Under the condition of nitrogen passing, 85 mL of NMP solvent was added to a 250 mL flask, and then 1.9422 g of 4,4-diaminodiphenyl ether, 4.1548 g of 1,4-bis(4-amino-2-trifluoromethyl-phenoxy)-benzene, and 5.8844 g of 3,3',4,4'-biphenyltetracarboxylic dianhydride were added. After stirring and reacting at 25 °C for 12 h, an anhydride-terminated polyamic acid solution was obtained.

[0060] S2. Add 0.0585 g of tris(2-aminoethyl)amine to the anhydride-terminated polyamic acid solution, stir at room temperature for 20 min, add 15 mL of acetic anhydride and 5.6 mL of triethylamine, transfer to a mold, gel for 30 min, and let stand for 24 h for aging to obtain a polyimide wet gel;

[0061] S3. Place the polyimide wet gel in a mixed solution with a volume ratio of NMP to acetone of 75 / 25, 50 / 50, and 25 / 75 for displacement once each, and then displace with pure acetone solvent three more times. The time interval for each solvent displacement is 12 h. Finally, dry with supercritical carbon dioxide fluid. The drying conditions are: maintain at 25 °C and 10 MPa for 4 h, then heat up to 45 °C and 10 MPa for 6 h, and release the gas to obtain a polyimide aerogel.

[0062] Perform performance tests on the polyimide composites prepared in Examples 1-5 and Comparative Examples 1-2. The test methods are as follows:

[0063] Density: Detect according to GB1033-1986-1986 "Test Methods for Density and Relative Density of Plastics";

[0064] Tensile strength and elongation at break: Cut the sample into a rectangle of 1 cm × 10 cm, put it into a tensile testing machine, and perform tensile strength and elongation at break tests at a speed of 5 cm / min. The number of test samples is more than 5, and the average value is taken.

[0065] The test results are shown in Table 1:

[0066] Table 1 Preparation parameters and performance parameters of polyimide composites

[0067]

[0068]

[0069] As can be seen from Examples 1 to 3 in Table 1, when the molar ratio of the anhydride group in 3,3',4,4'-biphenyltetracarboxylic dianhydride to the amino group in the cross-linking agent N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine increases from 100:1 to 100:3, the density of the polyimide aerogel increases from 0.12 g / cm 3 to 0.18 g / cm 3, as the amount of the crosslinking agent increases, the density of the crosslinking points inside the polyimide aerogel also increases, the intermolecular cohesion increases, and the density of the sample shows an increasing trend; the tensile strength of the polyimide aerogel increases from 13.5 MPa to 15.4 MPa. As the amount of the crosslinking agent increases, the particle size and the free volume between molecular chains increase. The crosslinking agent and the monomer crosslink to form larger particles, which have higher interfacial strength and larger space for anti-deformation, thereby improving the tensile strength and elongation at break, and thus significantly increasing the mechanical properties and flexibility of the polyimide aerogel film.

[0070] It can be seen from the comparison between Example 5 and Comparative Example 1 that the tensile strength and elongation at break of the polyimide aerogel prepared from 1,3,5-triaminophenoxybenzene are lower than those of the polyimide aerogel prepared from long-chain polyamines. Since the 1,3,5-triaminophenoxybenzene crosslinking agent contains a rigid benzene ring group and a relatively large number of functional groups, a dense crosslinked network structure is formed, and the rigid group hinders the movement of the molecular chain, making the molecular movement more difficult and reducing the flexibility of the system. It can be seen from the comparison between Example 5 and Comparative Example 2 that as the alkyl chain length and amine group content increase, the mechanical properties of the polyimide aerogel all increase, indicating that the toughening method by introducing a long-chain polyamine crosslinking agent into the polyimide aerogel has excellent mechanical properties and obvious toughening effect.

[0071] In the present invention, a three-dimensional network structure is formed by the flexible chain segments of the long-chain polyamine crosslinking agent and the polyimide molecular chain. By adjusting the solid content and the ratio of the crosslinking agent, a polyimide aerogel composite material with low density and excellent mechanical properties is prepared.

[0072] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0073] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a polyimide composite material, characterized in that, It includes the following steps: S1. Under the condition of introducing nitrogen, 4,4-diaminodiphenyl ether and 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene are added into a solvent. After complete dissolution, 3,3',4,4'-biphenyltetracarboxylic dianhydride is added. After the stirring reaction is completed, an anhydride-terminated polyamic acid solution is obtained; S2. A crosslinking agent is added to the anhydride-terminated polyamic acid solution, and it is stirred at room temperature for 5 - 20 min. Acetic anhydride and triethylamine are added, and it gels for 15 - 40 min and ages at room temperature for 24 h to obtain a polyimide wet gel; S3. The polyimide wet gel is subjected to solvent replacement and supercritical carbon dioxide fluid drying to obtain a polyimide composite material; The crosslinking agent is N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine.

2. The preparation method of the polyimide composite material according to claim 1, characterized in that, The solvent described in S1 is one or more of N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide.

3. The preparation method of the polyimide composite material according to claim 1, characterized in that, The stirring reaction temperature described in S1 is 0 - 40 °C, and the reaction time is 8 - 24 h.

4. The preparation method of the polyimide composite material according to claim 1, characterized in that The molar ratio of 3,3',4,4'-biphenyltetracarboxylic dianhydride, 4,4-diaminodiphenyl ether, and 1,4-bis(4-amino-2-trifluoromethylphenoxy)benzene described in S1 is 50:48.5 - 49.5:48.5 - 49.

5.

5. The preparation method of the polyimide composite material according to claim 1, characterized in that, The molar ratio of 3,3',4,4'-biphenyltetracarboxylic dianhydride and N1,N1,N2-tris(2-aminoethyl)-N2-[2-[bis(2-aminoethyl)amino]ethyl]ethane-1,2-diamine is 50:0.2 - 0.

6.

6. The preparation method of the polyimide composite material according to claim 1, characterized in that, The solid content of the polyimide wet gel described in S2 is 6% - 10%.

7. The preparation method of the polyimide composite material according to claim 1, characterized in that, The molar ratio of acetic anhydride and triethylamine described in S2 is 4:1; the molar ratio of acetic anhydride and 3,3',4,4'-biphenyltetracarboxylic dianhydride is 7 - 10:

1.

8. The preparation method of the polyimide composite material according to claim 1, characterized in that, The supercritical carbon dioxide fluid drying conditions described in S3 are: using liquid CO2, treating at 8 - 13 MPa and 20 - 30 °C for 3 - 6 h, and then heating to 40 - 60 °C and 8 - 13 MPa for 4 - 8 h.

9. A polyimide composite material, characterized in that, It is obtained by using the preparation method described in any one of claims 1 - 8.

10. An application of the polyimide composite material as described in claim 9.

Images