A slow-fading thermochromic silica composite material and its preparation method and application

Abstract

The present invention belongs to the technical field of thermochromic material preparation, and discloses a slow-fading thermochromic silica composite material, a preparation method thereof, and an application thereof, wherein the silica composite material is composed of silica microspheres and polydimethylsiloxane in a volume ratio of 1 to 10:1, and the silica microspheres are hydrophobically modified nanoscale microspheres; after the silica composite material is heated, the Rayleigh scattering of the pores in the polydimethylsiloxane matrix and the multiple scattering of light by the silica microspheres produce structural colors, and after returning to normal temperature, the silica composite material slowly absorbs moisture in the air, and the Rayleigh scattering and multiple scattering are gradually suppressed, causing the color of the silica composite material to gradually fade. The silica composite material of the present invention has a fading time of up to several hours, and the fading time can be further extended as the ambient humidity decreases, and can be applied to anti-counterfeiting labels, thermal history tracking sensors, humidity monitoring, smart windows and other fields.

Description

Technical Field

[0001] The invention belongs to the technical field of thermochromic material preparation, and in particular relates to a method for preparing a slow-fading thermochromic silica gel composite material. Background Art

[0002] The color of thermochromic materials can change with temperature, and they are used in thermochromic fibers, smart windows, temperature monitoring and other fields. Traditional thermochromic materials are mostly pigment color-changing materials such as organic dyes, spiropyran compounds or vanadium dioxide, which change color by chemical or phase changes with temperature. In addition to pigment color-changing materials, thermochromic materials based on structural colors have received increasing attention in recent years due to their wide color change range, green environmental protection, and non-fading advantages. Structural colors come from the scattering of light by periodic or non-periodic nanostructures, and their color is mainly determined by the refractive index and lattice spacing of the nanostructures. Therefore, the thermochromism of structural color materials can be achieved by changes in the refractive index or lattice spacing with temperature.

[0003] Hu et al. (①Structural Color‐Based Smart Liquid Windows Address the Tradeoff Between High Optical Transparency and Brilliant Color. Adv FunctMaterials 2024, 34 (10), 2310861. ②Liquid, Transparent, and Antideformable Thermochromic Photonic Crystals for Displays. Advanced Optical Materials2022, 10 (17), 2200769.) took advantage of the fact that the refractive index of polymers decreases with increasing temperature and prepared thermochromic materials using polyethylene glycol diacrylate (PEGDA) and silica microspheres. When the temperature increased from 20 ℃ to 100 ℃, the refractive index difference between PEGDA and silica increased from 0.006 to 0.04. The higher refractive index difference produced structural color.Ge et al (①Thermochromic Photonic Crystal Paper with Integrated MultilayerStructure and Fast Thermal Response: A Waterproof and Mechanically StableMaterial for Structural-Colored Thermal Printing. Advanced Materials 2024, 36(1), 2309344. ②Ultrafast and Irreversibly Thermochromic SiO2-PC / PEG DoubleLayer for Green Thermal Printing. Small 2022, 18 (14), 2106533. ③ADisposable Thermally Triggered Photonic Crystal Anti-Counterfeiting Tag with Irreversible Response and Multi-Step Color Changes. Small 2024, 20 (30), 2311308) used meltable polymers such as paraffin wax, polyethylene glycol, etc. and silica microspheres to prepare multilayer materials. When the temperature rises, the polymer melts and fills the gaps in the silica microsphere layer, reducing the refractive index difference and causing discoloration, which can be used for one-time thermal color printing. In addition, Wu et al. (Ultrathin Photonic Crystal Film with Supersensitive Thermochromism in Air. Chemical Engineering Journal 2023, 451, 139075.) used zinc sulfide-silicon dioxide core-shell particles to assemble into a film. When the temperature dropped from 15 °C to 5 °C, the lattice spacing increased, causing the color to change from blue to orange.

[0004] The color change rate is an important parameter that needs to be considered in the practical application of thermochromic materials. Thermochromic materials based on pigment color usually have a critical color change temperature. When the temperature is reached, the color will change within a few seconds to tens of seconds. The structural color thermochromic materials reported so far also have a fast color change rate and are suitable for applications such as smart windows and temperature detection.

[0005] Existing pigment-colored thermochromic materials have the characteristics of easy fading, difficult preparation of reversible color-changing pigment molecules, and the material itself is not green and environmentally friendly; although thermochromic materials based on structural colors have great advantages in terms of green environmental protection, not easy to fade, and color change range, their preparation methods are still relatively complicated and difficult to achieve large-scale preparation and application. Moreover, there are currently no reports of reversible thermochromic materials with sufficiently long fading time (can last for more than several hours after color change before slowly fading), which also limits the wider application of thermochromic materials. Summary of the invention

[0006] In view of the above defects or improvement needs of the prior art, the present invention aims to provide a reversible thermochromic material which has a long retention time after color change and a slow fading.

[0007] To achieve the above-mentioned purpose, according to one aspect of the present invention, there is provided a thermochromic silica composite material with slow fading, wherein the silica composite material is composed of silica microspheres and polydimethylsiloxane in a volume ratio of 1 to 10:1, and the silica microspheres are hydrophobically modified nanoscale microspheres; when the silica composite material is heated, structural color is generated under the combined action of Rayleigh scattering of pores in the polydimethylsiloxane matrix and multiple scattering of light by the silica microspheres; after returning to room temperature, the silica composite material slowly absorbs moisture in the air, and Rayleigh scattering and multiple scattering are gradually suppressed, causing the color of the silica composite material to gradually fade.

[0008] Furthermore, the particle size of the silica microspheres has a decisive influence on the color of the silica composite material, and the particle size of the silica microspheres is 100-500 nm.

[0009] According to a second aspect of the present invention, there is provided an application of the slow-fading thermochromic silica gel composite material as described above, wherein the application fields of the silica gel composite material include anti-counterfeiting labels, thermal history tracking sensors, humidity monitoring or smart windows.

[0010] Furthermore, when the silicone composite material is applied to anti-counterfeiting labels or thermal history tracking sensors, heating devices of different shapes are used to generate patterns of corresponding shapes on the silicone composite material by local heating, and then the heating device is removed. The pattern on the silicone composite material is maintained for more than 8 hours.

[0011] Furthermore, when the silica gel composite material is used for humidity monitoring, the silica gel composite material is first pressed into a sheet, and then heated to change its color, and then tapes of the same shape are affixed to the upper and lower surfaces thereof. The tapes are removed after 5 to 6 hours, and the pattern corresponding to the tape shape is replicated on the composite material.

[0012] Furthermore, when the silicone composite material is applied to a smart window, the silicone composite material is coated on the window and heated using a transparent heating film. After 10 to 20 minutes, the silicone composite material turns blue-green and has a transmittance greater than 70%. The heating is then stopped, and the transmittance of the silicone composite material drops to 50% after it cools to room temperature. The color and low transmittance are maintained for 6 to 8 hours.

[0013] According to a third aspect of the present invention, a method for preparing the slow-fading thermochromic silica gel composite material is provided, the method comprising the following steps:

[0014] S100: Silica microspheres were synthesized by Stöber method and their surfaces were hydrophobically modified;

[0015] S200: stirring and mixing the modified silica microspheres, polydimethylsiloxane and an organic solvent to obtain a precursor solution, and ultrasonicating at room temperature for 30 minutes for later use;

[0016] S300: The precursor solution is allowed to stand at room temperature for 8 to 12 hours to allow the organic solvent to evaporate, and then placed at 80° C. for 30 minutes for curing to obtain a thermochromic silica gel composite material.

[0017] Furthermore, the hydrophobic modification of the silica microspheres in S100 specifically includes: modifying the hydroxyl groups on the surface of the silica microspheres with a silane coupling agent.

[0018] Furthermore, the organic solvent in S200 is n-hexane.

[0019] Compared with the prior art, the present invention can achieve the following beneficial effects:

[0020] The thermochromic silica composite material of the present invention changes color under the combined effect of Rayleigh scattering of the pores in the polydimethylsiloxane matrix and multiple scattering of light by silica microspheres, wherein the particle size of silica plays a decisive role in color, and as the silica composite material slowly absorbs moisture in the air, when the absorbed moisture increases to a certain extent, Rayleigh scattering and multiple scattering are gradually suppressed, and the color gradually fades. The fading time of the thermochromic silica composite material of the present invention is as long as several hours, and the fading time can be further extended as the ambient humidity decreases, and a continuous color change effect can be brought about by a relatively short energy input, which meets the requirements of sustainable development. The color effect of the silica composite material of the present invention will not decay with the increase of the color change and fading cycle, and a permanent service life and an unlimited number of color change and fading cycles can be achieved without destroying the material structure. The method for preparing the thermochromic silica material of the present invention is simple, and the materials PDMS and SiO2 used are both environmentally and human-friendly raw materials, and will not cause harm to people or pollute the environment during use and in the recycling process after use. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The technical roadmap of the preparation method of the slow-fading thermochromic silica gel composite material in the embodiment of the present invention.

[0022] Figure 2 1 is a characterization diagram of SiO2 microspheres before and after modification in an embodiment of the present invention, wherein A is a SEM image of SiO2 microspheres before and after modification, scale: 500 nm; B is the water contact angle of SiO2 film before and after modification; C is an infrared spectrum image of SiO2 microspheres before and after modification.

[0023] Figure 3 : is a color characterization diagram of the thermochromic silica gel composite material in the embodiment of the present invention, wherein A is a photograph of the silica gel composite material prepared using SiO2 microspheres of different particle sizes before and after color change, scale: 5 mm; B is the reflectance spectrum of the silica gel composite material prepared using 240nm SiO2 microspheres before and after color change; C is the color difference change of the silica gel composite material prepared using 240nm SiO2 microspheres compared with the initial state during the color change and fading cycle; D is a photograph of the silica gel composite material prepared using 240nm SiO2 microspheres observed at different angles and the reflectance spectrum at this angle, scale: 5 mm.

[0024] Figure 4 1 is the color change of the thermochromic silica composite material at different temperatures and in the temperature rise and fall cycle in the embodiment of the present invention, wherein A is the micrograph and reflection spectrum of the composite material when stable at different temperatures; B is the color change of the composite material in the temperature rise and fall cycle and the change of the color difference between the composite material and the initial state.

[0025] Figure 5 : The effect of environmental humidity on the fading time of the thermochromic silica gel composite material in the embodiment of the present invention, wherein A is a photo of the color change of the composite material over time at different relative humidity, scale: 5 mm; B is the change of the water absorption rate of the composite material over time at different humidity.

[0026] Figure 6 This is an application demonstration of the thermochromic silica gel composite material of an embodiment of the present invention, wherein A is an application demonstration of the composite material on anti-counterfeiting labels or thermal history tracking sensors; B is an application demonstration of the composite material on humidity monitoring; and C is an application demonstration of the composite material on smart windows. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] The present invention provides a slow-fading thermochromic silica composite material, which is composed of hydrophobically modified nano-scale silica microspheres and polydimethylsiloxane in a volume ratio of 1 to 10:1. After the silica composite material is heated, the structural color is generated under the combined action of Rayleigh scattering of the pores in the polydimethylsiloxane matrix and the multiple scattering of light by the silica microspheres. After returning to normal temperature, the silica composite material slowly absorbs moisture in the air, and Rayleigh scattering and multiple scattering are gradually suppressed, causing the color of the silica composite material to gradually fade. Based on this characteristic, the silica composite material of the present invention can be applied to anti-counterfeiting labels, thermal history tracking sensors, humidity monitoring, smart windows and other fields. The particle size of the silica microspheres has a decisive influence on the color of the silica composite material, so the particle size of the silica microspheres in the silica composite material of the present invention is controlled to be 100 to 500 nm.

[0029] like Figure 1 As shown, the preparation method of the slow-fading thermochromic silica composite material comprises the following steps:

[0030] 1. Silica microspheres with a particle size of 100-500 nm were synthesized by the Stöber method, and their surfaces were hydrophobically modified. Specifically, a silane coupling agent was used to modify the hydroxyl groups on their surfaces to obtain hydrophobic silica microspheres that can be uniformly dispersed in the oil phase.

[0031] 2. Mix the hydrophobically modified silica microspheres and polydimethylsiloxane (PDMS) in a volume ratio of 1 to 10:1, add the organic solvent n-hexane, mix them evenly by mechanical stirring to obtain a precursor solution, and ultrasonicate at room temperature for 30 minutes for use.

[0032] 3. Pour the precursor solution into a mold or a culture dish, let it stand at room temperature for 8 to 12 hours to allow the organic solvent to evaporate, and then place it in an 80°C oven for 30 minutes to cure to obtain a thermochromic silica gel composite material. Example 1

[0033] In this embodiment, a series of SiO2 microspheres with different particle sizes are prepared, and the surface of the SiO2 microspheres is hydrophobically modified for use.

[0034] The specific steps are as follows:

[0035] 1. SiO2 microspheres with an average particle size of 100 nm ~ 500 nm were prepared using the Stöber method.

[0036] 2. Dodecyltrimethoxysilane (DTMS) was used to modify the hydroxyl groups on the surface of SiO2 microspheres to obtain hydrophobic SiO2 microspheres that can be evenly dispersed in the oil phase.

[0037] In order to verify the successful preparation and modification of SiO2 microspheres, we used scanning electron microscopy (SEM) to observe the SiO2 microspheres before and after modification ( Figure 2 -A), the size of the particles remains unchanged before and after modification. At the same time, we tested the water contact angle and infrared spectrum of SiO2 microspheres before and after modification ( Figure 2 -B and 2-C), the water contact angle of the modified SiO2 microspheres increased significantly, and its infrared spectrum at 2950 cm -1 and 2860 cm -1 The methylene peak of the modified silane coupling agent DTMS appeared in the wavenumber, proving the success of its modification. Example 2

[0038] SiO2 microspheres with average particle sizes of 220 nm, 240 nm, 260 nm, and 280 nm were selected from the modified SiO2 microspheres obtained in Example 1 to prepare thermochromic silica gel composite materials. The specific steps are as follows:

[0039] 1. Mix the hydrophobically modified silica microspheres and polydimethylsiloxane (Sylgard-184) in a volume ratio of 5:1, add n-hexane as a solvent, mix evenly by mechanical stirring to obtain a precursor solution, and ultrasonicate at room temperature for 30 minutes for use.

[0040] 2. Pour the precursor solution into a mold or a culture dish, let it stand at room temperature for 10 hours to allow the n-hexane to evaporate, and then place it in an oven at 80°C for 30 minutes for curing to obtain a thermochromic silica gel composite material.

[0041] The color-changing properties of the prepared silica gel composites were studied using camera photography and reflectance spectrum measurement methods. Figure 3 -A shows the photos taken by the camera when the silica composite material was heated and cooled. The composite material turned green after being heated to 80°C for 10 minutes, but when cooled to room temperature, it took 6 to 8 hours or even longer for the color to completely fade. When the particle size of SiO2 microspheres was increased, different colors could be obtained. At the same time, our composite material achieved a reversible cycle of color change and fading. We measured the reflectance spectrum of the silica composite material prepared by SiO2 microspheres with a particle size of 240 nm when it changed color and faded, and calculated the color difference (color difference) between each spectrum and the initial state through the spectrum ( Figure 3 -B and 3-C), it can be found that after ten cycles, the reversible color change characteristics are still maintained, and the color after color change does not fade or change. At the same time, the color of this composite material has a low angle dependence, and the color of the reflectance spectrum remains unchanged when observed and measured at various angles ( Figure 3 -D).

[0042] We used a microscope-spectrometer combination to characterize the color changes of silica composites at different temperatures and during heating and cooling cycles. This method can take microscopic photos of the composites in real time and measure the reflectance spectrum. Figure 4 -A, in the stable state below 40 ℃, the composite material does not change color; when the temperature rises to 60 ℃ or higher, the composite material begins to change color, and the color change becomes more obvious as the temperature rises. Figure 4 -B), we first heated the sample at a heating rate of 2 ℃ / min (20 ℃ to 100 ℃). As the temperature increased, the composite material gradually changed color and the color difference gradually increased. At 100 ℃, the temperature was kept constant for 45 minutes to reach equilibrium, and then the sample was cooled to 20 ℃ at a cooling rate of 2 ℃ / min. During the cooling process, the color did not fade immediately, but the color difference gradually increased. Finally, it took 8 h or more for the color to gradually fade at 20 ℃.

[0043] Environmental humidity has a great influence on the fading time. We placed the composite material after discoloration in environments with different humidity and recorded the changes in its color and weight every 30 minutes. Figure 5 -A and Figure 5 -B), as the humidity increases, the fading time of the material gradually shortens. At 100% relative humidity, it only takes 1.5 to 2 hours to completely fade. And as the humidity increases, the rate of increase of the water absorption rate of the composite material and the water absorption rate when it finally reaches equilibrium also increase. This proves that composite materials can also be used for environmental humidity monitoring. Example 3

[0044] The thermochromic silica composite material prepared in Example 2 was subjected to application tests in the fields of anti-counterfeiting labels or thermal history tracking sensors, humidity monitoring, smart windows, etc.

[0045] Figure 6 The application of the silica gel composite material prepared by the present invention in different aspects is demonstrated. Figure 6 -A shows the use of heating devices of different shapes to produce different patterns on the composite material through local heating. This pattern can be maintained for a long time without disappearing and can be used as an anti-counterfeiting mark or thermal history tracking sensor. Figure 6-B demonstrates the application of this material in humidity monitoring. The composite material is first heated to change color, and then tapes of different shapes are pasted on both sides. Since the tape hinders the absorption of moisture, when the tape is removed after 5 to 6 hours, the color of the area covered by the tape still exists, so that the shape of the tape can be replicated on the composite material. Figure 6 -C demonstrates the application of the material in smart windows. Initially, the composite material is colorless and translucent. It is heated using a transparent heating film. After 10 to 20 minutes, the material turns blue-green and still has high transparency, with a transmittance of more than 70%. At this time, the heating is stopped, and the transmittance drops to 50% after the material cools to room temperature. The color and low transmittance can be maintained for 6 to 8 hours.

[0046] The above-mentioned embodiments only express the implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that for ordinary technicians in this field, several modifications and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be based on the attached claims.

Claims

1. A slow-fading thermochromic silica composite material, characterized in that: The silica composite material is composed of silica microspheres and polydimethylsiloxane in a volume ratio of 1 to 10:1, and the silica microspheres are hydrophobically modified nanoscale microspheres. When the silica composite material is heated, structural color is generated under the combined action of Rayleigh scattering of pores in the polydimethylsiloxane matrix and multiple scattering of light by the silica microspheres. After returning to room temperature, the silica composite material slowly absorbs moisture in the air, and Rayleigh scattering and multiple scattering are gradually suppressed, causing the color of the silica composite material to gradually fade.

2. The slow-fading thermochromic silica composite material according to claim 1, characterized in that: The particle size of the silica microspheres has a decisive influence on the color of the silica gel composite material, and the particle size of the silica microspheres is 100-500 nm.

3. An application of the slow-fading thermochromic silica composite material according to claim 1 or 2, characterized in that: The application fields of the silicone composite material include anti-counterfeiting labels, thermal history tracking sensors, humidity monitoring or smart windows.

4. The use of the slow-fading thermochromic silica composite material according to claim 3, characterized in that: When the silicone composite material is applied to anti-counterfeiting labels or thermal history tracking sensors, heating devices of different shapes are used to generate patterns of corresponding shapes on the silicone composite material by local heating, and then the heating device is removed. The pattern on the silicone composite material is maintained for more than 8 hours.

5. The use of the slow-fading thermochromic silica composite material according to claim 3, characterized in that: When the silica gel composite material is used for humidity monitoring, the silica gel composite material is first pressed into a sheet and then heated to change its color. Then, tapes of different shapes are affixed to the upper and lower surfaces of the sheet. The tapes are removed after 5 to 6 hours, and the pattern corresponding to the tape shape is replicated on the composite material.

6. The use of the slow-fading thermochromic silica composite material according to claim 3, characterized in that: When the silicone composite material is applied to a smart window, the silicone composite material is coated on the window and heated using a transparent heating film. After 10 to 20 minutes, the silicone composite material turns blue-green and has a transmittance greater than 70%. The heating is then stopped, and the transmittance of the silicone composite material drops to 50% after it cools to room temperature. The color and low transmittance are maintained for 6 to 8 hours.

7. A method for preparing the slow-fading thermochromic silica composite material according to claim 1 or 2, characterized in that: The method comprises the following steps: S100: Adoption The silica microspheres were synthesized by the method and their surfaces were hydrophobically modified; S200: stirring and mixing the modified silica microspheres, polydimethylsiloxane and an organic solvent to obtain a precursor solution, and ultrasonicating at room temperature for 30 minutes for later use; S300: The precursor solution is allowed to stand at room temperature for 8 to 12 hours to allow the organic solvent to evaporate, and then is placed at a constant temperature of 80° C. for 30 minutes for curing to obtain a thermochromic silica gel composite material.

8. The method for preparing the slow-fading thermochromic silica composite material according to claim 7, characterized in that: The hydrophobic modification of the surface of the silica microspheres in S100 specifically includes: modifying the hydroxyl groups on the surface of the silica microspheres using a silane coupling agent.

9. The method for preparing the slow-fading thermochromic silica composite material according to claim 7, characterized in that: The organic solvent in S200 is n-hexane.

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