Flame-retardant polylactic acid composite aerogel integrating radiation refrigeration and thermal insulation as well as preparation method and application of flame-retardant polylactic acid composite aerogel

Abstract

The invention discloses a radiation refrigeration and thermal insulation integrated flame-retardant polylactic acid composite aerogel, a preparation method and application, and belongs to the technical field of functional materials, the composite aerogel comprises a polylactic acid matrix and a filler; the filler is of a sheet-shaped multi-layer core-shell structure and is composed of an inner core, a middle coating layer and an outer layer, the inner core is flaky gamma-Al2O3, the middle coating layer is a phosphorus-containing compound, and the outer layer is a silicon dioxide protective layer. The optical performance, the thermal stability, the flame retardant property and the mechanical strength of the material are remarkably improved; according to the preparation method, a multi-step chemical treatment process and a freeze drying process are combined, high-cost supercritical drying is effectively avoided, and meanwhile, the dispersity of the filler and the bonding performance of a matrix are improved. The material has excellent comprehensive performance and economical efficiency, and can be applied to the fields of photo-thermal management, thermal insulation, new energy automobiles, building energy conservation, aerospace, industrial high-performance thermal insulation materials and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of functional materials, and relates to an aerogel, in particular to a flame-retardant polylactic acid composite aerogel integrating radiative cooling and thermal insulation, a preparation method and an application thereof. Background Art

[0002] With the increase in global energy consumption and the intensification of environmental protection pressure, the development of materials with high-efficiency photothermal management and heat insulation performance has become an important research direction in the current energy and environmental protection fields. Due to its low density, high porosity and good thermal insulation performance, aerogel has become one of the ideal thermal insulation materials. Especially polylactic acid aerogel, due to its excellent biodegradability and environmental protection characteristics, is widely used in the research and development of environmentally friendly thermal insulation materials. However, traditional polylactic acid aerogels still have deficiencies in optical properties and thermal stability, especially in improving the reflectivity and emissivity of aerogels.

[0003] Traditional aerogel fillers mostly use silica (SiO 2 ), but its optical properties are limited and it is difficult to meet the requirements of higher applications. To solve this problem, researchers have tried to use other functional fillers, such as alumina (Al 2 O 3 ), which has higher reflectivity and stronger optical properties. Although alumina can effectively improve the optical properties of aerogels, its dispersion and stability in the matrix are still challenges in the preparation process.

[0004] In addition, existing aerogel preparation methods mostly rely on supercritical drying technology. Although this method can obtain aerogels with high porosity, due to the expensive equipment, complex operation and high energy consumption, it limits large-scale industrial production. Therefore, how to prepare aerogels with excellent optical properties, thermal stability and low cost at low cost has become a major problem in materials science research. Summary of the Invention

[0005] Aiming at the problems in the prior art, the present invention provides a flame-retardant polylactic acid composite aerogel integrating radiative cooling and thermal insulation, a preparation method and an application thereof. Through multi-layer structure design and optimization of process conditions, the optical properties, mechanical properties and thermal stability of the aerogel are significantly improved, meeting the actual requirements of photothermal management and efficient heat insulation.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] In the first aspect, the present invention provides a flame-retardant polylactic acid composite aerogel integrating radiative cooling and thermal insulation, including a polylactic acid matrix and a filler; the filler is a sheet-like multi-layer core-shell structure, composed of an inner core, an intermediate coating layer and an outer layer; the inner core is a sheet-like γ-Al 2 O 3, the middle coating layer is a phosphorus-containing composite, and the outer layer is a silica protection layer.

[0008] In a second aspect, the present invention also provides a method for preparing the above-mentioned flame-retardant polylactic acid composite aerogel integrating radiative cooling and thermal insulation, comprising the following steps:

[0009] S1. Remove surface impurities and activate surface hydroxyl groups through an acidic solution and ultrasonic treatment of flaky γ-Al 2 O 3 ;

[0010] S2. Add the cleaned and activated flaky γ-Al 2 O 3 to a mixed solution of ethanol and water, add a phosphorus-containing composite, heat and stir to react to form a middle coating layer, and obtain flaky γ-Al 2 O 3 coated with a phosphorus-containing composite;

[0011] S3. Add the flaky γ-Al 2 O 3 coated with a phosphorus-containing composite to ethanol, then add water and a catalyst, dropwise add a siloxane compound, react, and perform surface coating treatment to obtain the filler;

[0012] S4. Dissolve polylactic acid in a mixed solvent, heat and stir, add the filler, continue to stir and mix evenly, then dropwise add water to form a stable emulsion; let stand; freeze; then spray deionized water on the surface of the frozen emulsion for solvent replacement, after the replacement is completed, freeze and shape, and then freeze-dry to obtain the composite aerogel.

[0013] Further, the specific method of S1 is: add flaky γ-Al 2 O 3 to absolute ethanol, add an acidic solution under stirring conditions, perform ultrasonic treatment to remove surface impurities and activate surface hydroxyl groups, and then centrifuge, wash and dry with absolute ethanol to obtain the cleaned and activated flaky γ-Al 2 O 3 ; the mass ratio of flaky γ-Al 2 O 3 to absolute ethanol is 1-10:80-100; the particle size range of flaky γ-Al 2 O 3 is 5-10 μm, and the thickness is 50-100 nm; the pH value of the acidic solution is 2-4; the acidic solution is one of hydrochloric acid, acetic acid or nitric acid; the ultrasonic time is 20-50 min.

[0014] Further, in S2, the phosphorus-containing complex is composed of triethyl phosphate and ammonium metaphosphate, and the mass ratio of triethyl phosphate to ammonium metaphosphate is 1:0.05 - 0.2; the mass ratio of flaky γ-Al 2 O 3 , ethanol, water and the phosphorus-containing complex is 1:40 - 50:10 - 12:0.05 - 0.2.

[0015] Further, in S2, the heating temperature is 50 - 80 °C and the time is 3 - 6 h.

[0016] Further, in S3, the catalyst is ammonia water or ethylenediamine; the siloxane compound is one of tetraethoxysilane and tetramethoxysilane; the mass ratio of flaky γ-Al 2 O 3 , ethanol, water, the catalyst and the siloxane compound is 1 - 20:80 - 90:8 - 12:0.01 - 0.1:1 - 10;

[0017] Further, in S3, the dropping rate of the siloxane compound is 0.05 - 0.15 mL / min; the reaction time is 1 - 3 h.

[0018] Further, in S4, the polylactic acid is L-polylactic acid and the range of the weight-average molecular weight is 80000 - 100000; the mixed solvent is composed of 1,4-dioxane and N,N-dimethylacetamide, and the mass ratio of 1,4-dioxane to N,N-dimethylacetamide is 3 - 6:1. The mass ratio of polylactic acid, the mixed solvent, the filler and water is 30 - 60:90 - 120:1 - 5:5 - 10.

[0019] Further, in S4, the heating temperature is 40 - 60 °C and the time is 2 - 6 h; the time of the second stirring is 1 - 2 h; the standing time is 30 - 60 min; the freezing temperature is -20 - -10 °C and the time is 8 - 16 h; the pressure of freeze-drying is 1 - 10 Pa and the temperature is -60 - -40 °C and the time is 10 - 24 h.

[0020] Thirdly, the present invention also provides the application of the above-mentioned integrated radiation refrigeration and thermal insulation flame-retardant polylactic acid composite aerogel in low-temperature fluid transportation, photothermal management, heat insulation, new energy vehicles, building energy conservation, aerospace and industrial heat-insulating materials.

[0021] The beneficial effects of the present invention are as follows:

[0022] 1. The present invention mainly uses polylactic acid to prepare an aerogel with a micro-nano hierarchical porous structure, with a porosity exceeding 95%, a low thermal conductivity, and can be used in the fields of building energy conservation, food preservation, cryogenic liquid transportation, etc. Polylactic acid is a biodegradable thermoplastic polymer material of biological origin, with a wide source and low cost. During use, its structure is stable, and after the service period, it can be degraded by biological enzymes and microorganisms, with little environmental pollution, greatly reducing the generation of building or packaging solid waste.

[0023] 2. In the present invention, a dense phosphorus-containing polymer layer is constructed on the surface of flaky γ-Al 2 O 3 through a polycondensation reaction, and then a silica layer is coated by the sol-gel method to prepare an aerogel filler. During the preparation of the aerogel, due to its gravity sedimentation effect, large-size particles are enriched at the bottom surface of the aerogel, significantly improving the solar reflectance of the bottom surface of the aerogel. At the same time, the Al-O, P=O, and Si-O chemical bonds rich in the filler all exhibit high infrared emissivity at 8-13 μm, and when combined with the polylactic acid matrix, an emissivity higher than 0.9 in the atmospheric window can be achieved. The excellent spectral selectivity presented by the bottom surface of the aerogel enables it to be used as a radiative cooling surface to achieve daytime cooling. At the same time, the single-sided deposition effect of the filler avoids the decline in the mechanical properties of the main skeleton of the material caused by the traditional uniformly filled filler, reduces the filler dosage, and lowers the cost.

[0024] 3. Adding the filler of the present invention to the polylactic acid material also endows it with good flame retardant properties. Flaky γ-Al 2 O 3 is a kind of good flame retardant filler material. By coating the phosphorus-containing polymer and silica layer, the flame retardancy of the alumina filler can be significantly enhanced. During the heating and combustion process of the material, the phosphorus-containing polymer decomposes to generate phosphorus compounds, and these compounds form a dense protective layer on the surface of the composite aerogel, blocking the oxygen supply and reducing the heat conduction, thereby effectively inhibiting further thermal decomposition reactions. In addition, the decomposition of phosphoric acid may also promote the formation of a char layer, which, as an efficient thermal barrier, can significantly improve the high-temperature resistance and flame retardant properties of the material. The coating of the silica layer can effectively reduce the water absorption effect of the phosphorus-containing polymer layer and avoid its deliquescence behavior during long-term use.

[0025] IV. The present invention regulates the internal pore size of the aerogel through a mixed solvent of 1,4-dioxane and N,N-dimethylacetamide. By utilizing the difference in compatibility between water molecules and 1,4-dioxane and N,N-dimethylacetamide, the polylactic acid solution is transformed into a water-in-oil emulsion, in which nano-sized water droplets are formed. The change in the ratio of 1,4-dioxane and N,N-dimethylacetamide can control the size of the nano water droplets, and the regulation of this size can meet the requirements of Mie scattering for the pore size in the solar light band (0.3 - 2.5 μm). During the preparation process, the organic solvents in the frozen emulsion are replaced with deionized water, and then the aerogel is obtained by freeze-drying. This method has low cost and does not require expensive supercritical drying equipment.

[0026] V. The present invention provides a preparation method of a polylactic acid composite aerogel, which realizes the comprehensive improvement of the aerogel material in terms of optical properties, thermal stability, mechanical properties, and economy. At the same time, it has sustainability in material selection and preparation process, and also takes into account economy and operability, making it suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the SEM image of the filler prepared in Example 1;

[0028] Figure 2 is the SEM image of the surface of the composite aerogel prepared in Example 1;

[0029] Figure 3 is the SEM image of the cross-section of the composite aerogel prepared in Example 1;

[0030] Figure 4 is the comparison chart of the compression test of the aerogels prepared in Example 1 and Comparative Example 1;

[0031] Figure 5 is the TG comparison chart of the aerogels prepared in Example 1 and Comparative Example 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] The present invention will be further described below in conjunction with specific embodiments.

[0033] Example 1

[0034] This example provides a flame-retardant polylactic acid composite aerogel integrated with radiative cooling and thermal insulation, and the preparation method is as follows:

[0035] S1. Add 5 g of flaky γ-Al 2 O 3The mixture was added to 90 g of anhydrous ethanol, and an acidic solution with a concentration of 2 wt.% and a pH value of 3 was added under stirring. The surface impurities were removed and the surface hydroxyl groups were activated by ultrasonic treatment for 30 min. The mixture was then centrifuged and washed three times with anhydrous ethanol and dried to obtain a clean and activated flaky γ-Al 2 O 3 .

[0036] S2, 1g clean activated flaky γ-Al 2 O 3 Add to a mixed solution of 45g ethanol and 11g deionized water, add 0.1g of triethyl phosphate and ammonium metaphosphate in a mass ratio of 1:0.1. Stir at 60°C for 4h to complete the reaction, forming an intermediate phosphorus-containing complex coating layer. Then wash with anhydrous ethanol three times and dry to obtain a phosphorus-containing complex-coated flaky γ-Al 2 O 3 .

[0037] S3, 10g of γ-Al sheet coated with phosphorus-containing composite 2 O 3 Add to 85g of ethanol, add 9g of deionized water and 0.05g of ammonia water, and add 3g of tetraethoxysilane by sol-gel method. Stir for 2h at room temperature to complete the reaction and form an outer silica coating layer. Then wash with anhydrous ethanol 3 times and dry to obtain a multilayer core-shell structure flaky filler.

[0038] S4, dissolve 50g of L-polylactic acid in 110g of 1,4-dioxane and N,N-dimethylacetamide solvent mixed in a mass ratio of 4:1, and stir at 50°C for 4h. Add 2g of multilayer core-shell flaky γ-Al 2 O 3 , continue stirring for 1.5h to make it evenly dispersed. Then add 8g of deionized water at a rate of 0.1mL / min to form a stable emulsion, and let it stand for 30min. Transfer the emulsion to -15℃ and freeze it for 12h. After taking it out, spray the surface of the emulsion with deionized water for solvent replacement, and then freeze-dry it at 5Pa and -50℃ for 24h to obtain the final polylactic acid-based composite aerogel.

[0039] Example 2

[0040] This embodiment provides a flame-retardant polylactic acid composite aerogel integrating radiation cooling and thermal insulation, and the preparation method is as follows:

[0041] S1, 6g of γ-Al flakes 2 O 3Add it to 92 g of absolute ethanol, add an acidic solution with a concentration of 3 wt.% and a pH value of 2.5 under stirring conditions, and remove surface impurities and activate surface hydroxyl groups by ultrasonic treatment for 35 min. Subsequently, centrifuge and wash with absolute ethanol 3 times and dry to obtain clean and activated flaky γ-Al 2 O 3 。

[0042] S2. Add 1 g of clean and activated flaky γ-Al 2 O 3 to a mixed solution of 46 g of ethanol and 12 g of deionized water, and add 0.15 g of triethyl phosphate and ammonium metaphosphate with a mass ratio of 1:0.15. Stir at 65 °C for 3.5 h to complete the reaction and form a phosphorus-containing composite coating layer in the middle. Subsequently, wash with absolute ethanol 3 times and dry to obtain flaky γ-Al coated with a phosphorus-containing composite 2 O 3 。

[0043] S3. Add 12 g of flaky γ-Al coated with a phosphorus-containing composite 2 O 3 to 88 g of ethanol, add 9.5 g of deionized water and 0.06 g of ethylenediamine, and dropwise add 4 g of tetraethoxysilane by the sol-gel method. Stir at room temperature for 2.5 h to complete the reaction and form an outer silica coating layer. Subsequently, wash with absolute ethanol 3 times and dry to obtain a multi-layer core-shell structured flaky filler.

[0044] S4. Dissolve 55 g of polylactic acid in 112 g of a solvent mixture of 1,4-dioxane and N,N-dimethylacetamide mixed in a mass ratio of 5:1, and stir at 45 °C for 4.5 h. Add 3 g of multi-layer core-shell structured flaky γ-Al 2 O 3 , continue to stir for 1.5 h to make it uniformly dispersed. Subsequently, dropwise add 9 g of deionized water at a rate of 0.1 mL / min to form a stable emulsion, and let it stand for 40 min. Transfer the emulsion to -18 °C and freeze for 14 h, take it out and spray the surface of the emulsion with deionized water for solvent replacement, and then freeze-dry at 4 Pa and -45 °C for 24 h to obtain the final polylactic acid-based composite aerogel.

[0045] Example 3

[0046] This example provides a flame-retardant polylactic acid composite aerogel integrating radiative cooling and thermal insulation, and the preparation method is as follows:

[0047] S1. Add 7 g of flaky γ-Al 2 O 3Add it to 93 g of absolute ethanol, and add an acidic solution with a concentration of 2 wt.% and a pH value of 3 under stirring conditions. Remove surface impurities and activate surface hydroxyl groups by ultrasonic treatment for 30 min. Subsequently, centrifuge and wash with absolute ethanol 3 times and dry to obtain clean and activated flaky γ-Al 2 O 3 .

[0048] S2. Add 1 g of clean and activated flaky γ-Al 2 O 3 to a mixed solution of 47 g of ethanol and 10 g of deionized water, and add 0.12 g of triethyl phosphate and ammonium metaphosphate with a mass ratio of 1:0.2. Stir at 50 °C for 4 h to complete the reaction and form a phosphorus-containing composite coating layer in the middle. Subsequently, wash with absolute ethanol 3 times and dry to obtain flaky γ-Al coated with a phosphorus-containing composite 2 O 3 .

[0049] S3. Add 12 g of flaky γ-Al coated with a phosphorus-containing composite 2 O 3 to 87 g of ethanol, add 10 g of deionized water and 0.05 g of ammonia water, and dropwise add 4 g of tetraethoxysilane by the sol-gel method. Stir at room temperature for 3 h to complete the reaction and form an outer silica coating layer. Subsequently, wash with absolute ethanol 3 times and dry to obtain a flaky filler with a multi-layer core-shell structure

[0050] S4. Dissolve 52 g of polylactic acid in 115 g of a solvent mixture of 1,4-dioxane and N,N-dimethylacetamide mixed in a mass ratio of 4:1, and stir at 50 °C for 4 h. Add 2.5 g of flaky γ-Al with a multi-layer core-shell structure 2 O 3 , continue to stir for 1 h to make it evenly dispersed. Subsequently, dropwise add 7 g of deionized water at a rate of 0.1 mL / min to form a stable emulsion, and let it stand for 30 min. Transfer the emulsion to -17 °C and freeze for 12 h. Take it out and spray the surface of the emulsion with deionized water for solvent replacement, and then freeze-dry at 6 Pa and -50 °C for 24 h to obtain the final polylactic acid-based composite aerogel

[0051] Example 4

[0052] This example provides a flame-retardant polylactic acid composite aerogel integrating radiative cooling and thermal insulation, and the preparation method is as follows

[0053] S1. Add 8 g of flaky γ-Al 2 O 3Add it to 95 g of absolute ethanol, and add an acidic solution with a concentration of 4 wt.% and a pH value of 2 under stirring conditions. Remove surface impurities and activate surface hydroxyl groups by ultrasonic treatment for 40 min. Subsequently, centrifuge and wash with absolute ethanol 3 times and dry to obtain clean and activated flaky γ-Al 2 O 3 .

[0054] S2. Add 1 g of clean and activated flaky γ-Al 2 O 3 to a mixed solution of 48 g of ethanol and 11 g of deionized water, and add 0.15 g of triethyl phosphate and ammonium metaphosphate with a mass ratio of 1:0.1. Stir at 70 °C for 3 h to complete the reaction and form a phosphorus-containing composite coating layer in the middle. Subsequently, wash with absolute ethanol 3 times and dry to obtain flaky γ-Al coated with a phosphorus-containing composite 2 O 3 .

[0055] S3. Add 15 g of flaky γ-Al coated with a phosphorus-containing composite 2 O 3 to 88 g of ethanol, add 9 g of deionized water and 0.08 g of ethylenediamine, and dropwise add 5 g of tetraethoxysilane by the sol-gel method. Stir at room temperature for 2 h to complete the reaction and form an outer silica coating layer. Subsequently, wash with absolute ethanol 3 times and dry to obtain a flaky filler with a multi-layer core-shell structure.

[0056] S4. Dissolve 58 g of polylactic acid in 120 g of a solvent mixture of 1,4-dioxane and N,N-dimethylacetamide mixed in a mass ratio of 5:1, and stir at 55 °C for 5 h. Add 4 g of flaky γ-Al with a multi-layer core-shell structure 2 O 3 , continue to stir for 2 h to make it evenly dispersed. Subsequently, dropwise add 10 g of deionized water at a rate of 0.15 mL / min to form a stable emulsion, and let it stand for 40 min. Transfer the emulsion to -20 °C and freeze for 10 h. Take it out and spray the surface of the emulsion with deionized water for solvent replacement, and then freeze-dry at 8 Pa and -45 °C for 24 h to obtain the final polylactic acid-based composite aerogel.

[0057] Example 5

[0058] This example provides a flame-retardant polylactic acid composite aerogel integrating radiative cooling and thermal insulation, and the preparation method is as follows:

[0059] S1. Add 10 g of flaky γ-Al 2 O 3The mixture was added to 100 g of anhydrous ethanol, and an acidic solution with a concentration of 5 wt.% and a pH value of 4 was added under stirring. The surface impurities were removed and the surface hydroxyl groups were activated by ultrasonic treatment for 50 min. The mixture was then centrifuged and washed three times with anhydrous ethanol and dried to obtain a clean and activated flaky γ-Al 2 O 3 .

[0060] S2, 1g clean activated flaky γ-Al 2 O 3 Add to a mixed solution of 50g ethanol and 12g deionized water, add 0.2g of triethyl phosphate and ammonium metaphosphate in a mass ratio of 1:0.2. Stir at 80°C for 6h to complete the reaction, forming an intermediate phosphorus-containing complex coating layer. Then wash with anhydrous ethanol three times and dry to obtain a phosphorus-containing complex-coated flaky γ-Al 2 O 3 .

[0061] S3, 20g of flaky γ-Al coated with phosphorus-containing composite 2 O 3 Add to 90g ethanol, add 10g deionized water and 0.1g ammonia water, and add 6g tetraethoxysilane by sol-gel method. Stir for 3h at room temperature to complete the reaction and form an outer silica coating layer. Then wash with anhydrous ethanol three times and dry to obtain a multilayer core-shell structure flaky filler. SEM shows Figure 1 shown.

[0062] S4, dissolve 60g of polylactic acid in 115g of 1,4-dioxane and N,N-dimethylacetamide solvent mixed in a mass ratio of 3:1, and stir at 50°C for 6h. Add 5g of multilayer core-shell flaky γ-Al 2 O 3 , continue stirring for 2 hours to make it evenly dispersed. Then, 8g of deionized water was added dropwise at a rate of 0.15mL / min to form a stable emulsion, and it was allowed to stand for 50 minutes. The emulsion was transferred to -15℃ and frozen for 12 hours. After taking it out, the surface of the emulsion was sprayed with deionized water for solvent replacement, and then freeze-dried at 10Pa and -50℃ for 24 hours to obtain the final polylactic acid-based composite aerogel. The SEM images of the surface and cross-section are shown as follows: Figure 2 and Figure 3 shown.

[0063] Comparative Example 1

[0064] This comparative example provides an aerogel, and the preparation method is basically the same as that of Example 1, except that S1 to S3 are not performed, and multilayer core-shell structured flaky γ-Al is not used in S4. 2 O 3 , directly prepare pure polylactic acid aerogel.

[0065] The compression test results of the two aerogels prepared in Example 1 and Comparative Example 1 are as Figure 4 shown.

[0066] Comparative Example 2

[0067] This comparative example provides a composite aerogel, and the preparation method is basically the same as that of Example 1, except that: S1-S3 are not carried out, and the filler used in S4 is flaky γ-Al 2 O 3 , and a composite aerogel is prepared.

[0068] The TG results of the two aerogels prepared in Example 1 and Comparative Example 1 are as Figure 5 shown.

[0069] The performance of the aerogels prepared in each example and comparative example was tested, and the results are shown in Table 1.

[0070] Table 1 Performance test results of each example and comparative example

[0071]

[0072] The test results of each example and comparative example in Table 1 clearly show the significant influence of flaky γ-Al 2 O 3 and its multi-layer coating on the performance of the polylactic acid composite aerogel. The results show that the introduction of flaky γ-Al 2 O 3 , whether it is coated or not, and the content of the multi-layer coated flaky γ-Al 2 O 3 fillers all play an important role in the optical, flame retardant and mechanical properties of the aerogel.

[0073] Comparative Example 1 without the addition of multi-layer coated flaky γ-Al 2 O 3 showed the worst results in terms of optical, flame retardant and mechanical properties, with a bottom reflectivity of 86.78%, a bottom emissivity of 0.78, and an oxygen index of only 21.4%. This indicates that pure polylactic acid aerogel has significant deficiencies in light and heat reflection, high temperature oxidation resistance and mechanical enhancement. In contrast, the performance was significantly improved after adding multi-layer coated flaky γ-Al 2 O 3 in the examples. The oxygen index increased significantly, especially at the bottom surface. Due to the sedimentation effect of flaky γ-Al 2 O 3 , the reflectivity exceeded 95%. This improvement is mainly due to the high light reflection characteristics of flaky γ-Al 2 O 3 and the formation of a dense reflection layer on the bottom surface.

[0074] Comparison between Comparative Example 1 and Comparative Example 2 shows that although the uncoated flaky γ-Al 2 O 3 It contributes to the improvement of performance, but the effect is limited. 2 O 3 The bottom reflectivity of the aerogel is 93.72%, the oxygen index is increased to 25.8%, and the elastic modulus is increased to 38.9 MPa. 2 O 3 The dispersion is poor and the bonding strength with the matrix is ​​insufficient, and its performance is still significantly lower than that of multi-layer coated flaky γ-Al 2 O 3 The multi-layer coating design further improves the dispersibility, chemical stability and matrix bonding of aluminum oxide by introducing phosphorus-containing compounds and silicon dioxide outer layers, thereby significantly improving the overall performance of the material.

[0075] In the embodiment, the multilayer coated flaky γ-Al 2 O 3 The change of content has a certain regular effect on various properties. In terms of optical properties, the reflectivity of the front and bottom surfaces increases with the increase of the number of multi-layer coated γ-Al sheets. 2 O 3 The bottom reflectivity is slightly improved with the increase of the content, wherein the bottom reflectivity is gradually increased from 95.67% in Example 1 to 96.45% in Example 3, and is stable in Examples 4 and 5. This shows that the appropriate amount of multilayer coated flaky γ-Al 2 O 3 A dense sedimentation layer can be formed more effectively, but too much filler may destroy the pore uniformity, limiting further improvement. In terms of flame retardancy, the oxygen index gradually increases from 29.8% in Example 1 to 31.2% in Example 3, but at a higher content (such as Example 5), the oxygen index decreases slightly, which may be because excessive filler weakens the pore uniformity of the aerogel, thereby affecting the flame retardant effect.

[0076] The changes in mechanical properties also show a similar pattern. 2 O 3 With the increase of content, the elastic modulus steadily increased from 44.3MPa in Example 1 to 46.2MPa in Example 3, which is mainly attributed to the multi-layer coated flaky γ-Al 2 O 3 However, in Example 5, the elastic modulus is only 43.8 MPa, which is only slightly improved compared with Example 3, indicating that the increase of fillers has a saturation effect on the improvement of mechanical properties.

[0077] In general, uncoated γ-Al2 O 3 Although it improves the material properties to some extent, its performance in optical properties, flame retardancy, and mechanical properties is significantly inferior to that of the multi-layer coated flaky γ-Al 2 O 3 in the examples. The test results show that the multi-layer coated flaky γ-Al 2 O 3 The change in content has an important impact on the properties of the aerogel: an appropriate amount of filler can effectively improve the material properties, but too much filler may lead to a decrease in the porosity of the material, which in turn has an adverse effect on some properties. The multi-layer coated flaky γ-Al 2 O 3 effectively optimizes the properties of the polylactic acid aerogel by enhancing its bonding force and dispersibility with the matrix, while reducing the influence of the photothermal and chemical environment on the matrix. The appropriate content of flaky γ-Al 2 O 3 in combination with the multi-layer coating design provides strong technical support for the application of polylactic acid aerogels in the fields of photothermal management, flame retardancy and heat preservation, etc., showing excellent application potential.

[0078] In the present invention, unless otherwise specified, the scientific and technical terms used herein have the meanings commonly understood by those skilled in the art. And the reagents, materials, and operation steps used herein are all widely used reagents, materials, and conventional steps in the corresponding fields.

[0079] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation, characterized in that: including a polylactic acid matrix and a filler; The filler is a flaky multilayer core-shell structure, consisting of an inner core, an intermediate coating layer and an outer layer; the inner core is a flaky γ-Al2O3, the intermediate coating layer is a phosphorus-containing composite, and the outer layer is a silicon dioxide protective layer.

2. The method for preparing the flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation according to claim 1, characterized in that: The following steps are involved: S1, removing impurities on the surface of flake γ-Al2O3 and activating surface hydroxyl groups by acidic solution and ultrasonic treatment; S2, adding clean activated flaky γ-Al2O3 to a mixed solution of ethanol and water, adding a phosphorus-containing compound, heating and stirring to react to form an intermediate coating layer, and obtaining a flaky γ-Al2O3 coated with the phosphorus-containing compound; S3, adding the flaky γ-Al2O3 coated with the phosphorus-containing complex into ethanol, then adding water and a catalyst, dropping a siloxane compound, reacting, and performing a surface coating treatment to obtain the filler; S4, dissolving polylactic acid in a mixed solvent, heating and stirring, adding the filler, continuing to stir and mix evenly, then adding water dropwise to form a stable emulsion; allowing to stand; freezing; then spraying deionized water on the surface of the frozen emulsion to replace the solvent, and after the replacement is completed, freezing to shape, and then freeze-drying to obtain the composite aerogel.

3. The method for preparing the flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation according to claim 2, characterized in that: The specific method of S1 is: adding flaky γ-Al2O3 to anhydrous ethanol, adding an acidic solution under stirring conditions, ultrasonicating, removing surface impurities and activating surface hydroxyls, then centrifugally washing with anhydrous ethanol and drying to obtain clean and activated flaky γ-Al2O3; The mass ratio of flaky γ-Al2O3 to anhydrous ethanol is 1-10:80-100; The particle size of the flake γ-Al2O3 ranges from 5 to 10 μm, and the thickness ranges from 50 to 100 nm; The pH value of the acidic solution is 2 to 4; the acidic solution is one of hydrochloric acid, acetic acid or nitric acid; The ultrasonic time is 20 to 50 minutes.

4. The method for preparing the flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation according to claim 2, characterized in that: In S2, the phosphorus-containing complex is composed of triethyl phosphate and ammonium metaphosphate, and the mass ratio of triethyl phosphate to ammonium metaphosphate is 1:0.05-0.2; The mass ratio of flaky gamma-Al2O3, ethanol, water and phosphorus-containing complex is 1:40-50:10-12:0.05-0.

2.

5. The method for preparing the flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation according to claim 2, characterized in that: In S2, the heating temperature is 50-80°C and the heating time is 3-6 hours.

6. The method for preparing the flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation according to claim 2, characterized in that: In S3, the catalyst is aqueous ammonia or ethylenediamine; The siloxane compound is one of tetraethoxysilane and tetramethoxysilane; The mass ratio of the phosphorus-containing composite coated flaky gamma-Al2O3, ethanol, water, catalyst and siloxane compound is 1-20:80-90:8-12:0.01-0.1:1-10.

7. The method for preparing the flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation according to claim 2, characterized in that: In S3, the dropping speed of the siloxane compound is 0.05 to 0.15 mL / min; The reaction time is 1 to 3 hours.

8. The method for preparing the flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation according to claim 2, characterized in that: In S4, the polylactic acid is L-polylactic acid, and the weight average molecular weight ranges from 80,000 to 100,000; The mixed solvent consists of 1,4-dioxane and N,N-dimethylacetamide, and the mass ratio of 1,4-dioxane to N,N-dimethylacetamide is 3-6:

1. The mass ratio of polylactic acid, mixed solvent, filler and water is 30-60:90-120:1-5:5-10.

9. The method for preparing the flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation according to claim 2, characterized in that: In S4, the heating temperature is 40-60°C for 2-6 hours; the second stirring time is 1-2 hours; The standing time is 30 to 60 minutes; The freezing temperature is -20 to -10°C and the freezing time is 8 to 16 hours; The freeze-drying pressure is 1 to 10 Pa, the temperature is -60 to -40°C, and the time is 10 to 24 hours.

10. The flame-retardant polylactic acid composite aerogel with integrated radiation cooling and thermal insulation as claimed in claim 1 is used in low-temperature fluid transportation, light and heat management, thermal insulation, new energy vehicles, building energy conservation, aerospace and industrial insulation materials.

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