Flexible polyimide fiber-based aerogel as well as preparation method and application thereof
Polyimide fiber-based aerogel with a multi-level hierarchical pore structure was prepared by electrospinning, which solved the problems of low flexibility and porosity of polyimide aerogel, achieved high flexibility and excellent thermal insulation performance, and is suitable for lightweight thermal insulation materials.
Patent Information
- Application Number
- CN202510980724.5
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-16
- Publication Date
- 2025-10-10
AI Technical Summary
Existing polyimide aerogels have problems such as poor flexibility, poor fatigue resistance and low porosity when used in flexible devices.
Polyamic acid nanofiber membrane is prepared by electrospinning, and then freeze-dried and imidized to form a flexible polyimide fiber-based aerogel. The multi-level hierarchical pore structure and physical forces between fibers are used to improve flexibility and stability.
It achieves high flexibility, low density and excellent thermal insulation performance, has good dimensional stability and compression resistance, and is suitable for lightweight thermal insulation materials.
Smart Images
Figure CN120757848A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of aerogels and relates to a flexible polyimide fiber-based aerogel and a preparation method and application thereof. Background Art
[0002] Aerogel materials have attracted widespread attention due to their special properties such as high specific surface area, high porosity, and low density. As a type of organic aerogel, polyimide aerogel has high toughness and high thermal stability compared to traditional inorganic aerogels, and has a wider range of application prospects. Although polyimide aerogels prepared using the traditional sol-gel method have good mechanical strength and low density, their rigid structure, low porosity, and large shrinkage rate limit their application in deformable devices. In order to improve the application of polyimide aerogels in the field of flexible devices, a large number of studies have used cross-linking agents, carbon nanotubes, polymers and other materials mixed with polyimide or modified polyimide molecular chains to improve the flexibility of polyimide aerogels. However, there are still problems with poor fatigue resistance and low porosity. Summary of the Invention
[0003] In view of the above-mentioned deficiencies in the prior art, the present invention provides a flexible polyimide fiber-based aerogel and a preparation method and application thereof. The flexible polyimide fiber-based aerogel has better flexibility, dimensional stability and thermal insulation effect.
[0004] A first aspect of the present invention provides a method for preparing a flexible polyimide fiber-based aerogel, comprising the following steps:
[0005] S1. 4,4-diaminodiphenyl ether and 1,2,4,5-pyromellitic dianhydride are added to a first solvent and reacted in a nitrogen environment to obtain a polyamic acid solution;
[0006] S2. Electrospinning was performed using a polyamic acid solution as the spinning solution to obtain a polyamic acid nanofiber membrane;
[0007] S3. The polyamic acid nanofiber membrane is dispersed in a second solvent to obtain a polyamic acid fiber dispersion;
[0008] S4. The polyamic acid fiber dispersion is freeze-dried to obtain a polyamic acid fiber aerogel;
[0009] S5. The polyamic acid fiber aerogel is subjected to imidization treatment to obtain a flexible polyimide fiber-based aerogel.
[0010] In some embodiments of the present invention, the first solvent is N,N-dimethylformamide.
[0011] In some embodiments of the present invention, in step S1, the usage ratio of 4,4-diaminodiphenyl ether, 1,2,4,5-pyromellitic dianhydride and the first solvent is 12 mmol:12 mmol:(20-40) mL.
[0012] In some embodiments of the present invention, the electrospinning conditions are: spinning speed 0.5 ml / h, spinning voltage 18 kV, collecting roller speed 50 rad / min, and receiving distance 15 cm.
[0013] In some embodiments of the present invention, the second solvent is a mixed solution of water and tert-butyl alcohol. Adding tert-butyl alcohol is conducive to better dispersion of the polyamic acid fiber.
[0014] In some embodiments of the present invention, the mass percentage of tert-butanol in the second solvent is 20-40 wt %.
[0015] In some embodiments of the present invention, the concentration of the polyamic acid fiber in the polyamic acid fiber dispersion is 0.1 wt% to 3 wt%.
[0016] In some embodiments of the present invention, the imidization treatment is specifically: subjecting the polyamic acid fiber aerogel to a graded temperature treatment from low to high.
[0017] In some embodiments of the present invention, the step of temperature treatment from low to high is as follows: first treatment at 100° C. for 1 hour, then treatment at 200° C. for 1 hour, and then treatment at 300° C. for 2 hours.
[0018] The second aspect of the present invention provides a flexible polyimide fiber-based aerogel prepared according to the above preparation method.
[0019] A third aspect of the present invention provides a use of the flexible polyimide fiber-based aerogel in a thermal insulation material.
[0020] Compared with the prior art, the flexible polyimide fiber-based aerogel skeleton structure provided by the present invention, which is composed of physically entangled nanofibers, has a multi-level hierarchical pore structure, which provides the aerogel with more compression space and resilience, thereby maintaining high resilience; at the same time, the binding force of the aerogel mainly comes from the physical force between the fibers, and the ductility between the fibers improves the dimensional stability of the aerogel during freeze-drying and heat treatment, making the flexible polyimide fiber-based aerogel have better flexibility and dimensional stability; due to the characteristics of the nanofiber structure in the aerogel, it has a low density, which enables it to be used in fields with lightweight requirements. In addition, the large amount of static air inside the low-density aerogel gives the aerogel a low thermal conductivity, thereby obtaining excellent thermal insulation performance. BRIEF DESCRIPTION OF THE DRAWINGS
[0021] These and / or other aspects and advantages of the present invention will become apparent and readily understood from the following description of the preferred embodiments taken in conjunction with the accompanying drawings, in which:
[0022] Figure 1 This is a physical photo of the aerogel PINF-1 prepared in Example 1;
[0023] Figure 2 Shows photos of the aerogel PINF-1 after applying pressure and removing pressure, as well as schematic diagrams of the fiber morphology in each state;
[0024] Figure 3 Scanning electron microscope images of aerogels prepared in Examples 1-3 and Comparative Example 1 are shown;
[0025] Figure 4 Shows actual photographs of the aerogel PINF-1 prepared in Example 1 and the PI aerogel prepared in Comparative Example 1 when bent 180°;
[0026] Figure 5 These are stress-strain curves of the fiber-based aerogels prepared in Examples 1, 2, and 3 and the lamellar aerogel prepared in Comparative Example 1, wherein (a) is a stress-strain curve of the lamellar aerogel prepared in Comparative Example 1 at different strains, (b) is a stress-strain curve of the fiber-based aerogel prepared in Example 1 at different strains, (c) is a stress-strain curve of the fiber-based aerogel prepared in Example 2 at different strains, and (d) is a stress-strain curve of the fiber-based aerogel prepared in Example 3 at different strains;
[0027] Figure 6 The stress-strain curves, residual stress and residual strain diagrams of the fiber-based aerogel PINF-1 prepared in Example 1 after 1, 10, 50 and 100 compression cycles;
[0028] Figure 7 Infrared thermal imaging of the fiber-based aerogel PINF-1 prepared by the method shown in Example 1 on a hot stage at 100° C. at different times. DETAILED DESCRIPTION
[0029] The technical solution of the present invention will be further described in detail below through examples and in conjunction with the accompanying drawings. In the specification, the same or similar reference numerals indicate the same or similar components. The following description of the embodiments of the present invention with reference to the accompanying drawings is intended to explain the overall inventive concept of the present invention and should not be construed as limiting the present invention.
[0030] It should be noted that the terms used in this application are generally terms commonly used by those skilled in the art. If there is any inconsistency with commonly used terms, the terms in this application shall prevail.
[0031] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.
[0032] Example 1
[0033] This embodiment 1 provides a method for preparing a fiber-based polyimide aerogel 1, and the specific steps are as follows:
[0034] (1) A three-necked flask was selected as the reaction container, and nitrogen was introduced to expel the air therein. Then, 20 ml of DMF was added to the three-necked flask, 2.4 g of ODA was weighed and added to the container and stirred for 0.5 h until it was completely dissolved. Then, 2.6 g of PMDA was added to the three-necked flask in equal parts, with an interval of 0.5 h each time, and reacted for 6 h in a nitrogen atmosphere at 4 ° C. After the reaction was completed, 13.5 ml of DMF solution was added and ultrasonically stirred for 2 h under the same conditions to obtain polyamic acid (PAA) spinning solution.
[0035] (2) PAA nanofiber membrane was prepared using an electrospinning machine. The spinning process was as follows: spinning speed 0.5 ml / h, spinning voltage 18 kV, collecting roller speed 50 rad / min, receiving distance 15 cm, and PAA nanofiber (PAANF) membrane was obtained.
[0036] (3) The PAANF membrane was mixed with a 30 wt% aqueous solution of tert-butyl alcohol and homogenized at 15,000 rad / min for 20 min using a homogenizer to obtain a dispersion with a PAANF concentration of 1 wt%. The dispersion was freeze-dried to obtain a PAANF aerogel, which was then subjected to imidization (using a graded temperature treatment from low to high: 100°C for 1 h, then 200°C for 1 h, then 300°C for 2 h, and then cooled) to obtain aerogel PINF-1 (named based on the PAA nanofiber content in the dispersion).
[0037] The actual picture of the PINF-1 aerogel prepared in Example 1 is as follows: Figure 1 As shown, the prepared aerogel is yellow in color and has a regular morphology. It can be stably placed on the pistil, indicating its lightweight characteristics.
[0038] The aerogel PINF-1 prepared in Example 1 was compressed using a universal material analyzer to 20% of its original height, and then the pressure was removed. Photos of the aerogel PINF-1 after and after the pressure was applied and removed, as well as schematic diagrams of the fiber morphology in each state, are shown in FIG. Figure 2 It can be seen that the aerogel PINF-1 can still recover after the pressure is removed after the pressure is applied, indicating that the aerogel provided by the embodiment of the present invention has excellent compression resistance.
[0039] Example 2
[0040] This embodiment 2 provides a method for preparing a fiber-based polyimide aerogel 2, and the specific steps are as follows:
[0041] (1) A three-necked flask was selected as the reaction container, and nitrogen was introduced to expel the air therein. Then, 20 ml of DMF was added to the three-necked flask, 2.4 g of ODA was weighed and added to the container and stirred for 0.5 h until it was completely dissolved. Then, 2.6 g of PMDA was added to the three-necked flask in equal parts, with an interval of 0.5 h each time, and reacted for 6 h in a nitrogen atmosphere at 4 ° C. After the reaction was completed, 13.5 ml of DMF solution was added and ultrasonically stirred for 2 h under the same conditions to obtain polyamic acid (PAA) spinning solution.
[0042] (2) PAA nanofiber membrane was prepared using an electrospinning machine. The spinning process was as follows: spinning speed 0.5 ml / h, spinning voltage 18 kV, collecting roller speed 50 rad / min, receiving distance 15 cm, and PAA nanofiber (PAANF) membrane was obtained.
[0043] (3) The PAANF membrane was mixed with a 30 wt% aqueous solution of tert-butyl alcohol and homogenized at 15,000 rad / min for 20 min to obtain a dispersion with a PAANF concentration of 0.5 wt%. The dispersion was freeze-dried to obtain a PAANF aerogel, which was then subjected to imidization (using a graded temperature treatment from low to high: 100°C for 1 h, 200°C for 1 h, 300°C for 2 h, and then cooled) to obtain the PINF-0.5 aerogel.
[0044] Example 3
[0045] This embodiment 3 provides a method for preparing a fiber-based polyimide aerogel 3, and the specific steps are as follows:
[0046] (1) A three-necked flask was selected as the reaction container, and nitrogen was introduced to expel the air therein. Then, 20 ml of DMF was added to the three-necked flask, 2.4 g of ODA was weighed and added to the container and stirred for 0.5 h until it was completely dissolved. Then, 2.6 g of PMDA was added to the three-necked flask in equal parts, with an interval of 0.5 h each time, and reacted for 6 h in a nitrogen atmosphere at 4 ° C. After the reaction was completed, 13.5 ml of DMF solution was added and ultrasonically stirred for 2 h under the same conditions to obtain polyamic acid (PAA) spinning solution.
[0047] (2) PAA nanofiber membrane was prepared using an electrospinning machine. The spinning process was as follows: spinning speed 0.5 ml / h, spinning voltage 18 kV, collecting roller speed 50 rad / min, receiving distance 15 cm, and PAA nanofiber (PAANF) membrane was obtained.
[0048] (3) The PAANF membrane was mixed with a 30 wt% aqueous solution of tert-butyl alcohol and homogenized at 15,000 rad / min for 20 min using a homogenizer to obtain a dispersion with a PAANF concentration of 2 wt%. The dispersion was freeze-dried to obtain a PAANF aerogel. The PAANF aerogel was then subjected to imidization (using a graded temperature treatment from low to high: 100°C for 1 h, then 200°C for 1 h, then 300°C for 2 h, and then cooled) to obtain a PINF aerogel. The aerogel was named PINF-2 based on the nanofiber content.
[0049] Comparative Example 1
[0050] This comparative example 1 provides a method for preparing a lamellar polyimide aerogel, and the specific steps are as follows:
[0051] (1) A three-necked flask was selected as the reaction container, and nitrogen was introduced to expel the air therein. Then, 20 ml of DMF was added to the three-necked flask, 2.4 g of ODA was weighed and added to the container and stirred for 0.5 h until it was completely dissolved. Then, 2.6 g of PMDA was added to the three-necked flask in equal parts, with an interval of 0.5 h each time, and reacted for 6 h in a nitrogen atmosphere at 4 ° C. After the reaction was completed, 13.5 ml of DMF solution was added and ultrasonically stirred for 2 h under the same conditions to obtain polyamic acid (PAA) spinning solution.
[0052] (2) PAA solution and water were poured into a beaker at a volume ratio of 1:1 and magnetically stirred for 24 hours to obtain a precipitate. The solvent was then removed by vacuum filtration and freeze-dried to obtain an aqueous PAA powder. 0.5 g of PAA powder was mixed with 20 g of water, and 0.25 g of triethylamine was added dropwise to hydrophilically modify the PAA powder. The mixture was magnetically stirred for 2 hours to obtain a PAA dispersion with a concentration of 2.5 wt%.
[0053] (3) The PAA dispersion was poured into a mold and freeze-dried, and subjected to imidization treatment (graded temperature treatment from low to high: first 100°C for 1 hour, then 200°C for 1 hour, then 300°C for 2 hours, and then cooled) to obtain a lamellar PI aerogel, which was recorded as PI aerogel.
[0054] The microstructure of the aerogels prepared in Comparative Example 1, Example 1, Example 2 and Example 3 is as follows: Figure 3 As shown. Among them, Figure (a) and the figure below it are scanning electron microscope images of the PI aerogel prepared in Comparative Example 1; Figure (b) and the figure below it are scanning electron microscope images of the PINF-1 aerogel prepared in Example 1, and the fiber diameter is in the range of 200-500nm; Figure (c) is a scanning electron microscope image of the PINF-0.5 aerogel prepared in Example 2; Figure (d) is a scanning electron microscope image of the PINF-2 aerogel prepared in Example 3. It can be seen that both PI aerogel and PINF aerogel present a typical honeycomb pore structure. The pores of the PI aerogel present a uniformly arranged high-orientation structure, and its pore walls are mainly composed of sheet-like PI. The pore walls of the PINF aerogel are mainly formed by the entanglement of nanofibers, and the pores formed are of different sizes, with an obvious multi-level pore structure. With the increase of PINF concentration, the aerogel pore walls gradually become denser and the pore size decreases.
[0055] Test Example 1
[0056] The aerogel PINF-1 prepared in Example 1 and the PI aerogel prepared in Comparative Example 1 were bent 180° using tweezers. Figure 4 Figure (a) shows a photo of the PINF-1 aerogel bent 180°, showing its structural integrity and superior flexibility. Figure (b) shows a photo of the PI aerogel bent 180°, showing that it breaks when bent 180°.
[0057] Test Example 2
[0058] The volume V0 of the aerogel was measured by freezing it at -10℃, and the aerogel was dried using a freeze dryer. The volume V1 of the aerogel was measured according to the formula Calculate the shrinkage.
[0059] The shrinkage and density of the aerogels prepared by the methods shown in Comparative Example 1, Example 1, Example 2, and Example 3 were calculated according to the shrinkage formula and the density formula. The results are shown in Table 1.
[0060] Table 1 Shrinkage and density of aerogels obtained in Examples 1-3 and Comparative Example 1
[0061]
[0062] As shown in the table above, PI aerogels exhibit a volume shrinkage of up to 63.5%, indicating poor structural stability. Compared to PI aerogels, PINF aerogels, whose frameworks are composed of nanofibers, exhibit significantly improved dimensional stability, reducing their volume shrinkage from 63.5% to 34.7%. As the PINF content increases, the volume shrinkage further decreases to 26.7%.
[0063] According to the above table, since the skeleton of the sheet-like PI aerogel is continuous, the pore content of the fiber-based polyimide aerogel is higher than that of the PI aerogel, and its density is lower than that of the PI aerogel.
[0064] Test Example 3
[0065] The compression performance of the aerogels prepared by the methods shown in Comparative Example 1, Example 1, Example 2, and Example 3 was tested using a Modle E43 universal material testing machine. The compression rate of the material tester was set to 5 mm / min. After the test was completed, the force-displacement curve of the material was obtained, and the compression stress-strain curve was obtained by calculation as shown in FIG. Figure 5 shown.
[0066] Depend on Figure 5 From Figures (a), (b) and (c), we can see that the compression stress-strain curves of PI, PINF-0.5 and PINF-1 within 40% are basically the same, and they have good compression rebound properties. Figure 5 As shown in Figure (d), the stress-strain curve of PINF-2 shifts at 40% compression, indicating plastic deformation at 40% compression. When the compressive strain is increased to 80%, the curve of the PI aerogel shifts significantly, much more than that of PINF-0.5 and PINF-1, indicating that PINF-0.5 and PINF-1 aerogels have larger elastic deformation regions and are more elastic than PI and PINF-2 aerogels.
[0067] Test Example 4
[0068] (1) Plastic deformation rate test
[0069] In order to further compare the elasticity of the aerogel, the plastic deformation of the aerogel after 80% compressive strain was recorded. The test method is as follows:
[0070] The height h0 of the test sample is tested. After the sample is compressed to 80%, the pressure is unloaded and the height h1 of the test sample is restored. According to the formula Calculate the plastic deformation rate of the sample.
[0071] The plastic deformation rates of the aerogels obtained in Examples 1-3 and Comparative Example 1 are shown in Table 2.
[0072] (2) Compression strength and compression modulus test
[0073] The compressive properties of the aerogels prepared by the methods described in Comparative Example 1, Example 1, Example 2, and Example 3 were tested using a Modle E43 universal materials testing machine. The compression rate of the material analyzer was set to 5 mm / min. After the test, a force-displacement curve was obtained, and a compressive stress-strain curve was calculated. The maximum stress in the stress-strain curve was read as the compressive strength, and the slope of the linear elastic phase of the stress-strain curve was calculated as the compression modulus.
[0074] The compressive strength and compression modulus of the aerogels obtained in Examples 1-3 and Comparative Example 1 are shown in Table 2.
[0075] Table 2 Plastic deformation rate, compressive strength and compression modulus of aerogels obtained in Examples 1-3 and Comparative Example 1
[0076]
[0077] As shown in Table 2, the plastic deformations of PINF-0.5 and PINF-1 are 6.9% and 11.3%, respectively, far lower than the 29.9% of PI aerogel and 26.2% of PINF-2. This suggests that the flexible structure of the fibers contributes to their good resilience, but excessive fibers increase the stiffness of the aerogel, weakening its resilience.
[0078] As shown in Table 2, the compressive strength and compression modulus of PI aerogel are 24.7 kPa and 0.056 kPa, respectively, which are higher than those of PINF-0.5 (1.9 kPa, 0.003 kPa) and PINF-1 (12.5 kPa, 0.037 kPa), and lower than those of PINF-2 (38.1 kPa, 0.173 kPa), indicating that PI aerogel has better compressive strength than PINF. However, with the increase of PINF content, the aerogel skeleton strength increases, and the strength is gradually higher than that of PI aerogel.
[0079] Test Example 5
[0080] The aerogel PINF-1 prepared by the method shown in Example 1 was subjected to multiple cycles of compression at a strain of 40% using a Modle E43 universal material testing machine. The compression rate was set to 5 mm / min. After the test was completed, the force-displacement curve of the material was obtained, and the stress-strain curves after 1 cycle, 10 cycles, 50 cycles, and 100 cycles were calculated. The residual strain is the ratio of the strain after the compression cycle to the initial strain, and the residual stress is the ratio of the stress after the compression cycle to the initial stress. The test results are shown in Figure 2. Figure 6 As shown, (a) is the stress-strain curve diagram, and (b) is the residual stress and residual strain diagram.
[0081] according to Figure 6 As shown in the (a) graph in FIG. 10, the stress-strain curve of the PINF-1 aerogel is consistent in the 10th compression cycle and the 1st cycle, and the stress-strain curve deviates after the 50th cycle and the 100th cycle. According to the (b) graph in FIG. 10, the strain residual of the PINF-1 aerogel slightly decreases after 100 compression cycles, but can still reach 98.76% of the original aerogel. Figure 6 As shown in the (a) graph in FIG. 10, the stress-strain curve of the PINF-1 aerogel is consistent in the 10th compression cycle and the 1st cycle, and the stress-strain curve deviates after the 50th cycle and the 100th cycle. According to the (b) graph in FIG. 10, the strain residual of the PINF-1 aerogel slightly decreases after 100 compression cycles, but can still reach 98.76% of the original aerogel.
[0082] Test Example 6
[0083] The aerogel PINF-1 prepared by the method shown in Example 1 was placed on a hot stage at 100°C, and the heat insulation performance thereof was investigated by infrared thermal imaging, as shown in FIG. 11. As can be seen, the temperature of the surface of the aerogel gradually increased with time, and the temperature tended to be stable after 2 min, with the surface temperature being close to 57°C, and the temperature difference with the hot stage being 43°C, indicating that the PINF-1 aerogel has excellent heat insulation performance. Figure 7
[0084] Although some embodiments of the general inventive concept have been shown and described, it will be understood by those having ordinary skill in the art that changes can be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined by the claims and their equivalents.
Claims
1. A method for preparing a flexible polyimide fiber-based aerogel, characterized in that: The preparation method comprises the following steps: S1. 4,4-diaminodiphenyl ether and 1,2,4,5-pyromellitic dianhydride are added to a first solvent and reacted in a nitrogen environment to obtain a polyamic acid solution; S2. Electrospinning was performed using a polyamic acid solution as the spinning solution to obtain a polyamic acid nanofiber membrane; S3. The polyamic acid nanofiber membrane is dispersed in a second solvent to obtain a polyamic acid nanofiber dispersion; S4. The polyamic acid nanofiber dispersion is freeze-dried to obtain a polyamic acid fiber aerogel; S5. The polyamic acid fiber aerogel is subjected to imidization treatment to obtain a flexible polyimide fiber-based aerogel.
2. The preparation method according to claim 1, characterized in that The first solvent is N,N-dimethylformamide.
3. The preparation method according to claim 1, characterized in that In step S1, the usage ratio of 4,4-diaminodiphenyl ether, 1,2,4,5-pyromellitic dianhydride and the first solvent is 12 mmol:12 mmol:(20-40) mL.
4. The preparation method according to claim 1, characterized in that The electrospinning conditions are as follows: spinning speed 0.5 ml / h, spinning voltage 18 kV, collecting roller speed 50 rad / min, and receiving distance 15 cm.
5. The preparation method according to claim 1, characterized in that The second solvent is a mixed solution of water and tert-butanol.
6. The preparation method according to claim 1, characterized in that The concentration of the polyamic acid fiber in the polyamic acid fiber dispersion is 0.1 wt% to 3 wt%.
7. The preparation method according to claim 1, characterized in that The imidization treatment specifically includes: subjecting the polyamic acid fiber aerogel to a graded temperature treatment from low to high.
8. The preparation method according to claim 7, characterized in that The stepwise temperature treatment from low to high is specifically: first treating at 100° C. for 1 hour, then treating at 200° C. for 1 hour, and then treating at 300° C. for 2 hours.
9. A flexible polyimide fiber-based aerogel prepared according to the preparation method according to any one of claims 1 to 8.
10. Use of the flexible polyimide fiber-based aerogel according to claim 9 in thermal insulation materials.