Reversible thermochromic coating and preparation method thereof

Abstract

The invention discloses a reversible thermochromic coating and a preparation method thereof, and belongs to the technical field of thermochromic materials, the coating is of a sandwich structure, and the coating sequentially comprises an adhesive layer, an H-SiO2-EPCM functional layer and a protective layer from bottom to top; the H-SiO2-EPCM functional layer is formed by mixing an H-SiO2 photon nano pigment and an organic phase change material; the H-SiO2 photon nano pigment is a silicon dioxide nanosphere with a hollow structure, an amorphous carbon layer exists on the inner wall of a shell layer of the silicon dioxide nanosphere, and a rough and disordered superstructure is arranged on the surface of the shell layer; the organic phase change material is formed by mixing organic fatty acids, and the organic fatty acids comprise lauric acid, myristic acid, palmitic acid and stearic acid. The coating is prepared through spraying, has excellent mechanical and chemical stability and wide substrate adaptability, and has application value in the fields of dynamic anti-counterfeiting, personalized health monitoring, drinking water safety early warning, electronic equipment thermal risk monitoring and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of thermochromic materials, in particular to a reversible thermochromic coating and a preparation method thereof. BACKGROUND

[0002] Thermochromic materials, as a class of materials that can change their optical properties (including color, transparency and reflectivity) in response to temperature changes, have attracted continuous attention due to their potential applications in smart coatings, energy-saving materials, temperature sensors, advanced anti-counterfeiting technology, thermal monitoring and conductive elastomers. In recent years, thermoresponsive structural color materials with periodic micro-nano structures have shown superior performance to traditional thermochromic materials (such as organic dyes, inorganic compounds and ionic liquids) in terms of color tunability, temperature sensitivity, resistance to photobleaching and non-toxicity, and have made rapid progress in display, anti-counterfeiting and thermal monitoring applications.

[0003] Currently, the most representative thermoresponsive structural color material is thermochromic photonic crystal (TPC) based on Bragg diffraction principle, whose reflection wavelength can be adjusted by physical parameters such as lattice spacing, effective refractive index and observation angle. According to the structure configuration, TPCs are mainly divided into two categories: one is a hybrid system combining photonic structure with thermoresponsive material, such as polystyrene particles embedded in poly (N-isopropylacrylamide) (PNIPAM) hydrogel system, which realizes structural color blue shift through temperature-induced volume shrinkage; paraffin-based phase change material and SiO2 photonic crystal multilayer film system, which controls optical response through refractive index change; polyethylene glycol permeated SnO2 inverse opal system, which controls light scattering, transmission through crystalline-amorphous phase transition; the other is an intrinsic thermoresponsive photonic structure, such as poly (styrene-N-isopropylacrylamide-acrylic acid) microgel film, PNIPAM inverse opal hydrogel, phase change polymer inverse opal scaffold. These systems realize color transition through temperature-regulated light-matter interactions (such as scattering, absorption or reflection), but thermosensitive polymers often show limited reversible response, while phase change materials have the problem of irreversible optical transition.

[0004] Although TPCs show potential in advanced anti-counterfeiting and temperature sensors, the core bottleneck is the complex and time-consuming self-assembly process of photonic structure, and the integration of thermoresponsive material often requires multilayer preparation, physical bonding or additional ultraviolet curing steps, resulting in a cumbersome preparation process and making it difficult to achieve large-scale production, which seriously restricts the transformation from laboratory research to practical commercial application. Therefore, developing a next-generation thermoresponsive structural color material that does not require complex self-assembly, can simplify the preparation process, has the potential for large-scale production and can be seamlessly integrated with commercial products, has become an urgent need in the current field. SUMMARY

[0005] The present application aims to provide a reversible thermochromic coating and a preparation method thereof to solve the above problems.

[0006] The present application provides a reversible thermochromic coating, which has a sandwich structure and comprises, from bottom to top, an adhesive layer, an H-SiO2-EPCM functional layer and a protective layer; the H-SiO2-EPCM functional layer is formed by mixing H-SiO2 photonic nanometer pigments and organic phase change materials.

[0007] Preferably, in the reversible thermochromic coating, the H-SiO2 photonic nanometer pigments are hollow silica nanospheres, the inner wall of the shell layer of the silica nanospheres has an amorphous carbon layer, the surface of the shell layer has a rough and disordered superstructure, and the particle size range is 200-500 nm.

[0008] Preferably, in the reversible thermochromic coating, the organic phase change materials include a material formed by mixing organic fatty acids lauric acid and palmitic acid, and also include pure component organic fatty acids lauric acid, palmitic acid, myristic acid and stearic acid.

[0009] Preferably, in the reversible thermochromic coating, the adhesive layer is formed by drying a polymer aqueous solution of one of water-based acrylic resin, water-based polyurethane, polyacrylic resin, epoxy resin or polyethylene, and the mass fraction range is 40-60 wt%; The protective layer is formed by drying a polymer aqueous solution of one of water-based acrylic resin, water-based polyurethane, polyacrylic resin, epoxy resin or polyethylene, and the mass fraction range is 2-5 wt%.

[0010] Preferably, in the reversible thermochromic coating, the thickness of the adhesive layer is 15 μm, the thickness of the H-SiO2-EPCM functional layer is 40 μm, and the thickness of the protective layer is 5 μm.

[0011] A preparation method of the reversible thermochromic coating is provided, which comprises the following steps: (1) Preparation of organic phase change materials: mix organic fatty acids lauric acid and palmitic acid, heat to 50℃ and magnetically stir at 500 rpm for 30 minutes, and then cool to room temperature to obtain the organic phase change materials; or directly select one of pure component organic fatty acids lauric acid, palmitic acid, myristic acid and stearic acid, and weigh the pure reagent for standby; (2) Preparation of H-SiO2-EPCM ethanol solution: H-SiO2 photonic nano pigment and organic phase change material are dispersed in ethanol, and ultrasonic treatment is carried out at 40℃ for 3 hours; wherein the mass fraction of H-SiO2 photonic nano pigment is 30wt%, and the mass fraction of organic phase change material is 70wt%; wherein the H-SiO2 photonic nano pigment is a hollow structured silica nanosphere, the inner wall of the shell layer of the silica nanosphere has an amorphous carbon layer, the surface of the shell layer has a rough and disordered superstructure, and the particle size range is 200-500nm; (3) Spraying water-based acrylic adhesive layer: 40-60wt% of water-based polymer aqueous solution is sprayed onto the substrate, and after drying, an adhesive layer with a thickness of 15μm is formed; (4) Spraying H-SiO2-EPCM functional layer: the H-SiO2-EPCM ethanol solution prepared in step (3) is sprayed onto the adhesive layer at 50℃, and after drying, a functional layer with a thickness of 40μm is formed; (5) Spraying water-based acrylic protective layer: 2-5wt% of water-based acrylic emulsion aqueous solution is sprayed onto the functional layer, and after drying, a protective layer with a thickness of 5μm is formed, to obtain the programmable thermochromic coating.

[0012] Preferably, in the preparation method of the above-mentioned reversible thermochromic coating, the organic phase change material in step (2) is the material mixed by organic fatty acid lauric acid and palmitic acid in step (1), or is one of the pure component materials of organic fatty acid lauric acid, palmitic acid, myristic acid and stearic acid.

[0013] Preferably, in the preparation method of the above-mentioned reversible thermochromic coating, in steps (3), (4) and (5), a commercial spray gun is used in the spraying process, the nozzle diameter is 0.5mm, the pressure is 30psi, and the working distance is 5cm.

[0014] Therefore, the present application has the following beneficial effects by adopting the above-mentioned reversible thermochromic coating and its preparation method: (1) By spraying high-concentration water-based acrylic acid, H-SiO2-eutectic phase change material ethanol solution and low-concentration water-based acrylic acid on the substrate in turn, a sandwich-structured H-SiO2-EPCM thermochromic coating (HPTC) is obtained, wherein the phase change material is in a phase separation state with H-SiO2 nanospheres in the form of sheet crystals, and the top protective layer can penetrate into the surface gap to ensure good color fastness; the crystalline properties, color change temperature and phase change material of the coating are consistent, in addition, the coating also has no angle dependence. This simple spraying process avoids the complex process of traditional self-assembly, provides a solution for large-scale commercial production, and solves the bottleneck of the difficulty of large-scale application of the prior art. Moreover, the angle-independent characteristics of the coating are more suitable for anti-counterfeiting and visualization applications.

[0015] (2) The rapid structural color thermal response transition can be realized by the two-phase fusion-separation principle at the interface of H-SiO2 and EPCM. At low temperature, the structural color is generated by the high refractive index contrast between the photonic nanocolorants and air. When the temperature is higher than the superphase transition melting onset temperature, the EPCM melts and replaces the air medium, and the low refractive index contrast makes the wrapped area optically transparent, exposing the internal carbon layer of H-SiO2, and the color gradually changes to black, and the reflection peak intensity continues to decrease. When the temperature is higher than the melting point, the complete phase fusion phenomenon makes the color black. The scanning electron microscope proves the dynamic interaction of the interface, and the optical microscope proves the excellent recovery performance of the coating.

[0016] (3) Based on the interface fusion-separation mechanism of EPCM and H-SiO2 photonic nanocolorants, the coating realizes precise thermal response in a narrow physiological temperature window of 33-37℃. When the temperature is higher than 33℃, the structural color gradually fades; when it reaches 37℃, it completely changes to black; when it cools below 33℃, the structural color can completely recover. This high sensitivity, fast response and strong reversibility far exceed the performance of traditional thermochromic materials, and is especially suitable for scenarios that require body temperature sensing color change.

[0017] (4) Based on the mechanism that the HPTC system has a fast and specific response to the human body temperature range, it is sprayed onto a flexible substrate to make a dynamic anti-counterfeiting label paper, which is then integrated into a commercial packaging system through a ready-made adhesive. The color changes within 5 seconds of finger contact and recovers within 5 seconds of moving away. No additional tools are needed, and product authentication can be quickly completed through tactile interaction, combining convenience with existing packaging compatibility.

[0018] (5) Based on the reversible thermochromic behavior and adjustable transition temperature of the HPTC system, by selecting different carbon chain length organic fatty acids, a programmable thermochromic transition temperature HPTC is constructed. The introduction of eutectic / pure phase change materials does not affect the structural color of HPTC, and the phase change materials can be uniformly dispersed in the H-SiO2 layer in the form of flaky crystals at room temperature. A flexible temperature indicator integrating multiple different switching thresholds is prepared, which can be applied to personalized health monitoring and drinking water temperature warning.

[0019] (6) Based on the excellent durability of the sandwich structure coating, the HPTC has excellent color fastness after frequent rubbing, acid and alkali soaking, and multiple water washing treatments. At the same time, it shows wide adaptability to various substrates such as metal, paper, plastic, and glass.

[0020] The technical solutions of the present application will be further described in detail below with the help of drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is the preparation flow chart of the HPTC of the present application; Figure 2 Scanning electron microscope (SEM) images of a cross-section and a magnified portion of the sandwich-layered structure of HPTC; Figure 3 The graph shows the crystallization and thermal properties of HPTC. Figure 4 This diagram illustrates the angle-independent nature and color uniformity of HPTC, where (a) the structural colors of blue, green, and magenta HPTC at different viewing angles from 0° to 60°; (b) the reflected wavelengths of blue, green, and magenta HPTC at different viewing angles from 0° to 60°; and (c) 4×4cm. 2 Optical photograph of green HPTC on a glass substrate; (d) Reflectance spectrum; Figure 5 This diagram illustrates the reversible thermochromic mechanism of HPTC and shows SEM images under different conditions. (a) is a schematic diagram of the transformation of the H-SiO2-EPCM layer from interfacial phase separation to phase fusion induced by electron beam heating in scanning electron microscopy (SEM); (b) is an SEM image of the H-SiO2-EPCM layer under low operating voltage; (c) is an SEM image of the H-SiO2-EPCM layer under high operating voltage; and (d) is a static optical microscope image of the structural color evolution of the green sample during the color recovery process. Figure 6 Images showing the color changes of blue, green, and magenta HPTC at different temperatures, where (a) and (b) are schematic diagrams of the color changes of blue, green, and magenta HPTC with temperature during the heating and cooling processes, respectively. Figure 7 Images show the application of HPTC in the field of anti-counterfeiting, including (a) green HPTC used as an anti-counterfeiting label on commercial packaging; and (b) the time-resolved reflectance spectrum of the green HPTC area after being touched by a finger during heating and cooling processes. Figure 8 The design concept diagram and physical diagram of HPTCs for temperature monitoring applications include: (a) a schematic diagram and working principle of an integrated temperature indicator tag based on HPTCs; (b) a schematic diagram, infrared thermogram and photograph of a temperature indicator tag applied to a simulated high fever forehead; and (c) an infrared thermogram and photograph of a temperature indicator tag applied to a cup containing hot water of different temperatures. Figure 9 Optical photographs and corresponding reflection spectra of HPTC are shown, where (a) is an optical photograph of green HPTC after different tests; and (b) is a comparison of the corresponding reflection spectra before and after different tests. Detailed Implementation

[0022] To better understand the above technical solutions, a detailed description of the solutions will be provided below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0023] The terminology used in the embodiments of this invention is for the purpose of describing particular embodiments only and is not intended to limit the invention. The singular forms “a,” “the,” and “the” as used in the embodiments of this invention and the appended claims are also intended to include the plural forms, and “multiple” generally includes at least two unless the context clearly indicates otherwise.

[0024] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that an article or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the article or device that includes said element.

[0025] The present invention provides a reversible thermochromic coating, which has a sandwich structure and includes, from bottom to top, an adhesive layer, an H-SiO2-EPCM functional layer and a protective layer; the H-SiO2-EPCM functional layer is composed of H-SiO2 photonic nano pigment and organic phase change material.

[0026] To further optimize the above technical solution, the H-SiO2 photonic nanopigment is a silica nanosphere with a hollow structure. The inner wall of the shell of the silica nanosphere has an amorphous carbon layer, and the surface of the shell has a rough and disordered superstructure with a particle size range of 200-500 nm.

[0027] To further optimize the above technical solution, the organic phase change material includes a material composed of a mixture of organic fatty acids lauric acid and palmitic acid, and also includes pure components of organic fatty acids lauric acid, palmitic acid, myristic acid and stearic acid.

[0028] To further optimize the above technical solution, the adhesive layer is formed by drying an aqueous solution of one of the following polymers: waterborne acrylic resin, waterborne polyurethane, polyacrylic resin, epoxy resin, or polyethylene, with a mass fraction ranging from 40-60 wt%. The protective layer is formed by drying an aqueous solution of one of the following polymers: waterborne acrylic resin, waterborne polyurethane, polyacrylic resin, epoxy resin, or polyethylene, with a mass fraction ranging from 2 to 5 wt%.

[0029] To further optimize the above technical solution, the thickness of the adhesive layer is 15 μm, the thickness of the H-SiO2-EPCM functional layer is 40 μm, and the thickness of the protective layer is 5 μm.

[0030] A method for preparing a reversible thermochromic coating as described above is provided, comprising the following steps: (1) Preparation of organic phase change materials: Mix organic fatty acids lauric acid and palmitic acid, heat to 50°C and stir magnetically at 500 rpm for 30 minutes, and cool to room temperature to obtain organic phase change materials; or directly select one of the pure components organic fatty acids lauric acid, palmitic acid, myristic acid and stearic acid, and weigh out its pure reagent for later use. (2) Preparation of H-SiO2-EPCM ethanol solution: H-SiO2 photonic nano pigment and organic phase change material are dispersed in ethanol and ultrasonically treated at 40°C for 3 hours; wherein, the mass fraction of H-SiO2 photonic nano pigment is 30wt% and the mass fraction of organic phase change material is 70wt%; wherein, H-SiO2 photonic nano pigment is a silica nanosphere with a hollow structure, the inner wall of the shell of the silica nanosphere has an amorphous carbon layer, the surface of the shell has a rough and disordered superstructure, and its particle size ranges from 200 to 500 nm; (3) Spraying water-based acrylic adhesive layer: Spray 40-60wt% water-based polymer aqueous solution onto the substrate, and after drying, form an adhesive layer with a thickness of 15μm; (4) Spraying H-SiO2-EPCM functional layer: The H-SiO2-EPCM ethanol solution prepared in step (3) is sprayed onto the adhesive layer at 50°C, and after drying, a functional layer with a thickness of 40 μm is formed; (5) Spraying a water-based acrylic protective layer: Spraying 2-5 wt% water-based acrylic emulsion aqueous solution onto the functional layer, and after drying, forming a protective layer with a thickness of 5 μm, to obtain the programmable thermochromic coating.

[0031] To further optimize the above technical solution, the organic phase change material in step (2) is the material made of organic fatty acid lauric acid and palmitic acid in step (1), or a pure component material among organic fatty acid lauric acid, palmitic acid, myristic acid and stearic acid.

[0032] To further optimize the above technical solution, in steps (3), (4), and (5), a commercial spray gun is used in the spraying process, with a nozzle diameter of 0.5 mm, a pressure of 30 psi, and a working distance of 5 cm.

[0033] The process for preparing H-SiO2 photonic nanoparticles is as follows: polystyrene (PS) seed particles are synthesized by soap-free emulsion polymerization, and then PS@SiO2 nanospheres are prepared using a modified Stobel method. PS@E-SiO2 nanospheres are calcined to obtain H-SiO2 nanospheres. After pH adjustment, stirring, and centrifugation, H-SiO2 photonic nanoparticles with good sphericity and monodispersity are obtained, with a particle size range of 200-500 nm. The preparation process of the polystyrene seed particles is as follows: 25 g of styrene is added dropwise to 250 mL of deionized solution containing 3.75 g of polyvinylpyrrolidone and 0.65 g of 2,2'-azobis(2-methylpropanediamine) dihydrochloride, and stirred at 70 °C for 24 hours. The preparation process of the PS@SiO2 nanospheres is as follows: polystyrene seeds are dispersed in a mixed solution of 350 mL ethanol and 20 mL water, 20 mL ammonia is added and stirred for 1 hour, then 20 mL tetraethyl orthosilicate is added and stirred at room temperature for 8 hours. The shell of PS@SiO2 is etched with 50 mL sodium hydroxide solution with a concentration of 0.01 g / mL at room temperature for 30 minutes. Then, three centrifugal washing cycles are performed and the nanospheres are dried at 60 °C to obtain PS@E-SiO2 nanospheres. The processing conditions for obtaining H-SiO2 nanospheres by calcining PS@E-SiO2 nanospheres were as follows: PS@E-SiO2 nanospheres were calcined in a muffle furnace at 500℃ for 4 hours to obtain H-SiO2 nanospheres. The processing conditions for obtaining H-SiO2 photonic nanopigment after pH adjustment, stirring, and centrifugation of H-SiO2 nanospheres were as follows: 1g of H-SiO2 nanospheres were dispersed in 45mL of deionized water, the pH of the mixture was adjusted to 12 with ammonia water, and after ultrasonic treatment to ensure uniform dispersion, the mixture was stirred at 95℃ for 10 hours, and then centrifuged three times to obtain H-SiO2 photonic nanopigment.

[0034] To provide a clearer and more detailed description of the reversible thermochromic coating and its preparation method provided by the embodiments of the present invention, the following description will be based on specific embodiments.

[0035] Example 1 A reversible thermochromic coating is prepared by means of the following steps: Preparation of organic phase change material: Lauric acid and palmitic acid were mixed at a mass ratio of 8:2, heated to 50°C and magnetically stirred at 500 rpm for 30 minutes, and then cooled to room temperature to obtain EPCM.

[0036] Preparation of H-SiO2-EPCM ethanol solution: H-SiO2 photonic nanoparticles and EPCM were dispersed in ethanol at a mass ratio of 8:2 and ultrasonically treated at 40℃ for 3 hours. The particle size of the H-SiO2 photonic nanoparticles was 344 nm.

[0037] Spraying water-based acrylic adhesive layer: Using a commercial spray gun (nozzle diameter 0.5 mm, pressure 30 psi, working distance 5 cm), a 40 wt% aqueous solution of water-based acrylic emulsion is sprayed onto the glass substrate, and after drying, an adhesive layer with a thickness of 15 μm is formed.

[0038] Spraying H-SiO2-EPCM functional layer: The H-SiO2-EPCM ethanol solution prepared above is sprayed onto the adhesive layer at 50°C using the above spray gun, and after drying, a functional layer with a thickness of 40μm is formed.

[0039] Spraying a water-based acrylic protective layer: Using the above-mentioned spray gun, a 5wt% aqueous solution of water-based acrylic emulsion is sprayed onto the functional layer. After drying, a protective layer with a thickness of 5μm is formed, thus obtaining the programmable thermochromic coating.

[0040] Example 2 A reversible thermochromic coating is prepared in a manner that is basically the same as in Example 1, except that the particle size of the prepared H-SiO2 photonic nanopigment is 406 nm, resulting in a green thermochromic coating.

[0041] Example 3 A reversible thermochromic coating is prepared in a manner that is basically the same as in Example 1, except that the particle size of the prepared H-SiO2 photonic nanopigment is 284 nm, resulting in a purplish-red thermochromic coating.

[0042] Example 4 A reversible thermochromic coating is prepared by the following method: The organic phase change material uses a mixture of lauric acid and palmitic acid. The two organic fatty acids are mixed in a specific ratio, heated to 50°C, and magnetically stirred at 500 rpm for 30 minutes. After cooling to room temperature, it is ready for use. To prepare the H-SiO2-EPCM ethanol solution, 30 wt% H-SiO2 photonic nanoparticle pigment and 70 wt% organic phase change material are dispersed in ethanol and ultrasonically treated at 40°C for 3 hours. Next, an adhesive layer is prepared using a 55 wt% aqueous acrylic resin solution, sprayed onto an aluminum foil substrate using a commercial spray gun. After drying, an adhesive layer with a thickness of 15 μm is formed.

[0043] After the adhesive layer has completely dried, the prepared H-SiO2-EPCM ethanol solution is sprayed onto the surface of the adhesive layer using the same spray gun at 50°C. After drying, a 40 μm thick H-SiO2-EPCM functional layer is formed. Finally, a protective layer is prepared by taking a 3 wt% aqueous solution of aqueous acrylic resin and spraying it onto the functional layer using the same spray gun. After drying, a 5 μm thick protective layer is formed, ultimately yielding a reversible thermochromic coating.

[0044] Example 5 The organic phase change material also uses a mixture of lauric acid and palmitic acid, with the preparation method and parameters remaining unchanged. The preparation ratio and processing conditions of the H-SiO2-EPCM ethanol solution are also kept the same. A 55wt% aqueous acrylic resin solution was used as the adhesive layer, sprayed onto the glass substrate using a commercial spray gun, and dried to form a 15μm thick adhesive layer. The spraying and drying process for the functional layer was the same as in Example 4, forming a 40μm thick H-SiO2-EPCM functional layer. A 2wt% aqueous polyurethane solution was used as the protective layer, sprayed onto the surface of the functional layer using the same spray gun, and dried to form a 5μm thick protective layer, thus obtaining the target thermochromic coating.

[0045] Example 6 The preparation of H-SiO2 photonic nanopigments, organic phase change materials, and H-SiO2-EPCM ethanol solutions were all carried out according to the steps and parameters of Example 4. The adhesive layer was a 55 wt% aqueous acrylic resin solution, sprayed onto a stainless steel substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The preparation process of the functional layer was the same as in Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. The protective layer was a 4 wt% aqueous polyacrylic resin solution, sprayed onto the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, thus completing the preparation of the programmable thermochromic coating.

[0046] Example 7 The preparation steps of the H-SiO2 photonic nanopigment, the formulation of the organic phase change material, and the treatment of the H-SiO2-EPCM ethanol solution were all consistent with those in Example 4. A 55 wt% aqueous acrylic resin solution was used as the adhesive layer, sprayed onto an aluminum foil substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The functional layer was sprayed and dried according to the conditions of Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. A 5 wt% epoxy resin aqueous solution was used as the protective layer, sprayed onto the surface of the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, resulting in a reversible thermochromic coating.

[0047] Example 8 The preparation process of the H-SiO2 photonic nanopigment, organic phase change material, and H-SiO2-EPCM ethanol solution was exactly the same as in Example 4. The adhesive layer was a 55 wt% aqueous acrylic resin solution, sprayed onto the glass substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The spraying and drying conditions for the functional layer were the same as in Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. The protective layer was a 2.5 wt% aqueous polyethylene solution, sprayed onto the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, thus completing the preparation of the thermochromic coating.

[0048] Example 9 The preparation of H-SiO2 photonic nanopigments and organic phase change materials, as well as the treatment of the H-SiO2-EPCM ethanol solution, all followed the steps and parameters of Example 4. The adhesive layer was a 40 wt% aqueous acrylic resin solution, sprayed onto a stainless steel substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The functional layer was prepared according to the conditions of Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. The protective layer was a 5 wt% aqueous acrylic resin solution, sprayed onto the surface of the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, thus obtaining the target programmable thermochromic coating.

[0049] Example 10 The preparation processes for the H-SiO2 photonic nanopigment, organic phase change material, and H-SiO2-EPCM ethanol solution were the same as in Example 4. The adhesive layer was a 40 wt% aqueous acrylic resin solution, sprayed onto an aluminum foil substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The preparation conditions for the functional layer were the same as in Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. The protective layer was a 3 wt% aqueous polyurethane solution, sprayed onto the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, thus completing the preparation of the thermochromic coating.

[0050] Example 11 The preparation of H-SiO2 photonic nanopigments and organic phase change materials, as well as the treatment of the H-SiO2-EPCM ethanol solution, were consistent with those in Example 4. A 45 wt% aqueous polyurethane solution was used as the adhesive layer, sprayed onto the glass substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The functional layer was sprayed and dried according to the conditions of Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. A 2 wt% aqueous acrylic resin solution was used as the protective layer, sprayed onto the surface of the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, resulting in a reversible thermochromic coating.

[0051] Example 12 The preparation process of the H-SiO2 photonic nanopigment, organic phase change material, and H-SiO2-EPCM ethanol solution was exactly the same as in Example 4. A 45 wt% aqueous polyurethane solution was used as the adhesive layer, sprayed onto a stainless steel substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The spraying and drying conditions for the functional layer were the same as in Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. A 4 wt% aqueous polyurethane solution was used as the protective layer, sprayed onto the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, thus completing the preparation of the thermochromic coating.

[0052] Example 13 The preparation of H-SiO2 photonic nanopigments and organic phase change materials, as well as the treatment of the H-SiO2-EPCM ethanol solution, all followed the steps and parameters of Example 4. The adhesive layer was a 45 wt% aqueous polyurethane solution, sprayed onto an aluminum foil substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The functional layer was prepared according to the conditions of Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. The protective layer was a 5 wt% aqueous polyacrylic acid resin solution, sprayed onto the surface of the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, thus obtaining the target programmable thermochromic coating.

[0053] Example 14 The preparation of H-SiO2 photonic nanopigments and organic phase change materials, as well as the treatment of the H-SiO2-EPCM ethanol solution, all followed the steps and parameters of Example 4. The adhesive layer was a 40 wt% aqueous polyurethane solution, sprayed onto an aluminum foil substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The functional layer was prepared according to the conditions of Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. The protective layer was a 2 wt% aqueous polyacrylic acid resin solution, sprayed onto the surface of the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, resulting in the target programmable thermochromic coating.

[0054] Example 15 The preparation of H-SiO2 photonic nanopigments and organic phase change materials, as well as the treatment of the H-SiO2-EPCM ethanol solution, all followed the steps and parameters of Example 4. The adhesive layer was a 60 wt% aqueous polyurethane solution, sprayed onto an aluminum foil substrate using a commercial spray gun, and dried to form a 15 μm thick adhesive layer. The functional layer was prepared according to the conditions of Example 4, forming a 40 μm thick H-SiO2-EPCM functional layer. The protective layer was a 5 wt% aqueous polyacrylic acid resin solution, sprayed onto the surface of the functional layer using the same spray gun, and dried to form a 5 μm thick protective layer, thus obtaining the target programmable thermochromic coating.

[0055] Example 16 A reversible thermochromic coating is prepared by the following method: The organic phase change material is made by mixing lauric acid and palmitic acid in a mass ratio of 1:1. The mixture is placed in a three-necked flask and placed in a constant temperature water bath at 50°C. It is then magnetically stirred at a rate of 500 rpm for 30 minutes to ensure that the two fatty acids are completely melted and mixed evenly. After heating is stopped, the mixture is allowed to cool naturally to room temperature to obtain a stable organic phase change material.

[0056] When preparing the H-SiO2-EPCM ethanol solution, the H-SiO2 photonic nano pigment and the organic phase change material were dispersed together in anhydrous ethanol at a mass fraction of 30% and 70%, respectively. The solution was then ultrasonically treated in an ultrasonic water bath at 40°C for 3 hours to ensure that the pigment and the phase change material formed a stable system with uniform dispersion in ethanol.

[0057] The adhesive layer used a 60wt% aqueous solution of polyacrylic acid resin. A cleaned copper sheet was selected as the substrate. The aqueous solution was uniformly sprayed onto the copper sheet surface using a commercial spray gun. After spraying, the substrate was placed in a 65℃ forced-air drying oven for 2 hours to form an adhesive layer with a precise thickness of 15μm. After the adhesive layer was completely dry, the H-SiO2-EPCM functional layer was prepared: the prepared H-SiO2-EPCM ethanol solution was heated to 50℃ and kept at a stable temperature. It was then sprayed onto the adhesive layer surface using the same spray gun and the same spraying parameters. During spraying, the spraying rate was controlled at 2mL / min to ensure uniform coating coverage. After spraying, the sample was allowed to air dry at room temperature for 3 hours, and then placed in a 50℃ vacuum drying oven for 1 hour to remove residual ethanol, ultimately forming a functional layer with a thickness of 40μm.

[0058] Finally, a protective layer was prepared by using a 2.2 wt% epoxy resin aqueous solution. After stirring it evenly, the sample was uniformly sprayed onto the surface of the functional layer using the same commercial spray gun and spraying parameters. After spraying, the sample was placed in a 70°C forced-air drying oven for 1.5 hours to allow the epoxy resin to fully cure and form a protective layer with a thickness of 5 μm (thickness verified by a thickness gauge). This completes the preparation of a reversible thermochromic coating. Example 17 The organic phase change material used is pure myristic acid. Myristic acid is placed in a beaker and heated in a 55°C constant temperature water bath until completely melted without stirring. It is then allowed to cool naturally to room temperature before use. The H-SiO2-EPCM ethanol solution is prepared with a ratio of 30wt% H-SiO2 photonic nano-pigment and 70wt% organic phase change material. Molten myristic acid and H-SiO2 photonic nano-pigment are added together to anhydrous ethanol and ultrasonically treated at 40°C for 3 hours to form a uniformly dispersed solution. The adhesive layer uses a 48wt% polyethylene aqueous solution. A ceramic sheet is selected as the substrate. The polyethylene aqueous solution is sprayed onto the surface of the ceramic sheet using a commercial spray gun. After spraying, the sheet is dried in a 60°C forced-air drying oven for 2.5 hours to form an adhesive layer with a thickness of 15μm.

[0059] During the preparation of the functional layer, an H-SiO2-EPCM ethanol solution was maintained at 50°C and sprayed onto the adhesive layer at a spraying rate of 2 mL / min. After drying at room temperature for 3 hours, it was then vacuum dried at 50°C for 1 hour to form a 40 μm thick functional layer. For the protective layer, a 3.8 wt% polyacrylic acid resin aqueous solution was used. After thorough stirring, it was sprayed onto the surface of the functional layer using the same spray gun. Following spraying, it was placed in a 65°C forced-air drying oven and dried for 1.5 hours to form a 5 μm thick protective layer, ultimately yielding a programmable thermochromic coating. Example 18 The organic phase change material was prepared by mixing lauric acid and stearic acid in a mass ratio of 3:2. The two fatty acids were placed in a three-necked flask, heated in a constant temperature water bath at 50°C, and magnetically stirred at 500 rpm for 30 minutes. The mixture was then cooled to room temperature for later use. The H-SiO2-EPCM ethanol solution was prepared with 30 wt% pigment and 70 wt% phase change material, and sonicated at 40°C for 3 hours.

[0060] The adhesive layer uses a 52wt% epoxy resin aqueous solution on an aluminum plate as the substrate. It is sprayed with a commercial spray gun and dried at 65℃ for 2 hours to form a 15μm thick adhesive layer. During the functional layer spraying, the H-SiO2-EPCM ethanol solution is kept at 50℃, and after spraying, it is dried at room temperature for 3 hours and then vacuum dried at 50℃ for 1 hour to a thickness of 40μm.

[0061] The protective layer is made of 4.5 wt% aqueous polyurethane solution. After spraying, it is dried at 70°C for 1.2 hours to form a 5 μm thick protective layer, thus completing the coating preparation. Example 19 The organic phase change material was prepared by mixing palmitic acid and stearic acid at a mass ratio of 2:1, stirring at 500 rpm for 30 minutes in a constant temperature water bath at 50°C, and then cooling for later use. The H-SiO2-EPCM ethanol solution was prepared with 30 wt% pigment and 70 wt% phase change material, and ultrasonicated at 40°C for 3 hours.

[0062] The adhesive layer was prepared using a 43wt% aqueous polyurethane solution on a glass substrate. After being sprayed with a commercial spray gun, it was dried at 60°C for 2 hours to form a 15μm thick adhesive layer. The functional layer was prepared by spraying an H-SiO2-EPCM ethanol solution at 50°C, drying at room temperature for 3 hours, and then vacuum drying for 1 hour to achieve a thickness of 40μm.

[0063] The protective layer is made of 2.7wt% polyethylene aqueous solution. After spraying, it is dried at 65℃ for 2 hours to form a 5μm thick protective layer, thus obtaining the target thermochromic coating. Example 20 The organic phase change material is made of pure stearic acid, which is heated to melt and then cooled for later use. The H-SiO2-EPCM ethanol solution is prepared with 30wt% pigment and 70wt% stearic acid, and sonicated at 40℃ for 3 hours.

[0064] The adhesive layer is made of 57wt% aqueous acrylic resin solution, with a stainless steel sheet as the substrate. After being sprayed with a commercial spray gun, it is dried at 65°C for 1.8 hours to form a 15μm thick adhesive layer. The functional layer is sprayed with H-SiO2-EPCM ethanol solution at 50°C, and the thickness after drying is 40μm.

[0065] The protective layer is made of 3.3 wt% epoxy resin aqueous solution. After spraying, it is dried at 70°C for 1.5 hours to form a 5 μm thick protective layer, thus completing the preparation of the programmable thermochromic coating.

[0066] The performance of the thermochromic coatings obtained in Examples 1-3 above was tested.

[0067] Cross-sectional scanning electron microscopy revealed that HPTCs exhibited a sandwich structure with a total thickness of 60 μm. Figure 2 As shown in the magnified SEM image of the thermochromic layer, EPCM is distributed in the form of plate-like crystals around a large number of randomly arranged H-SiO2 nanospheres, indicating that EPCM and photonic nanopigment are in a phase-separated state at room temperature. This phase separation phenomenon is crucial as it directly affects the thermochromic properties of HPTC. Furthermore, the cross-section of the top WA protective layer shows that WA has penetrated into the surface voids of the thermochromic layer, bonding the photonic nanopigment and EPCM together. Therefore, the WA protective layer prevents the H-SiO2 nanospheres and EPCM from delaminating under normal friction. This non-self-assembly-free spraying method provides a pathway for manufacturing.

[0068] Tests have shown that the thermochromic coating has the following characteristics: like Figure 5 As shown, by utilizing the fusion and separation characteristics of the H-SiO2 and EPCM interface, the constructed HPTC exhibits reversible thermochromic properties: at temperatures below T... onset At this temperature, the solid-state EPCM separates from the photonic nanoparticles, which are surrounded by air. The high refractive index contrast results in structural colors. When the temperature exceeds T... onset The EPCM melt-encapsulation and low refractive index contrast make the encapsulated area appear distinctly black, with a decreased reflection peak higher than T. m It is black, dominated by carbon. For example... Figure 5 (b)- Figure 5 As shown in (c), SEM electron beam heating revealed dynamic changes at the interface: phase separation at low voltage, melting and encapsulation at high voltage, and separation upon cooling. Microscopic monitoring showed complete and consistent color recovery, verifying the aforementioned mechanism. Furthermore, the color recovery process was monitored in real-time using a microscope camera. Figure 5 As shown in (d), the results show that the micron-scale EPCM region reappears and is evenly distributed on the HPTC surface, ensuring the integrity and consistency of the overall structural color.

[0069] like Figure 6 As shown, HPTC achieves rapid and reversible thermochromism: its narrow thermal response window of 33-37℃ (ΔT=4℃) demonstrates ultra-high sensitivity, making it suitable for body temperature-triggered intelligent systems, such as... Figure 6 (a)- Figure 6 As shown in (b), the intensity of the characteristic reflection peak changes with temperature, and the transition is completed within 4 seconds, indicating a fast response speed.

[0070] The programmable thermochromic coatings obtained in Examples 1-3 above are applied to anti-counterfeiting and temperature monitoring fields, as follows: Applications in anti-counterfeiting: HPTC is sprayed onto flexible substrates such as paper to create dynamic anti-counterfeiting labels, which are then integrated into commercial packaging systems using conventional adhesives, demonstrating compatibility. Figure 7 (a) Finger contact (body temperature above T) onset It changes color within 5 seconds and returns to normal within 5 seconds after being removed. Figure 7 (b) Quick authentication without the need for tools.

[0071] Applications in temperature monitoring: HPTC possesses reversible thermochromic properties and an adjustable transition temperature, allowing it to be developed into a flexible temperature indicator with multi-threshold sensing. By selecting organic fatty acids with different carbon chain lengths (such as lauric acid, myristic acid, palmitic acid, and stearic acid), a series of HPTCs with transition temperatures ranging from 44 to 70°C can be constructed. Based on this programmable design, temperature indicators integrating six switching thresholds (Tm = 37°C, 38°C, 39°C, 44°C, 63°C, and 70°C) can be fabricated. Figure 8 (a) The introduction of different phase change materials does not affect the structural color of HPTC; at room temperature, the phase change materials are uniformly dispersed as plate-like crystals in the H-SiO2 layer. This indicator can be applied to personalized health monitoring: as a wearable skin patch, it provides continuous visual alerts through color changes at 37-39℃ (critical heating threshold). Figure 8 (b) Infrared thermal imaging verifies its performance, providing parents with a non-invasive, real-time tool for initial fever screening and continuous monitoring (not a replacement for traditional thermometers). It can also be applied to drinking water temperature detection, providing visual feedback based on tiered thresholds in drinking water temperature alerts: 43℃ (nearly suitable for drinking), 63℃ and 70℃ (high risk). A noticeable color change occurs upon contact with hot water, visually indicating the transition from "suitable for drinking" to "too hot." Figure 8 (c) Improve drinking water safety.

[0072] The technical feasibility of customized anti-counterfeiting labels and temperature indicators is based on the structural design of the HPTC system. Due to the design of the WA protective layer, the HPTC system exhibits excellent stability of its structural color under mechanical wear and chemical treatment. Durability tests were conducted, and the results are as follows... Figure 9(a) HPTC can withstand 50 cycles of physical friction, 30 minutes of water washing, 12 hours of immersion in water, and 12 hours of soaking in hydrochloric acid solution (pH=2) and ammonia solution (pH=12); before and after the test, the structural color and reflectance spectrum of the sample showed almost no change. Figure 9 (b) indicates that it has excellent color stability, mechanical robustness and chemical resistance, ensuring long service life and reliability as an anti-counterfeiting label and temperature indicator.

[0073] The programmable thermochromic coatings obtained in Examples 4-20, from the perspective of adhesive layer preparation, regardless of whether waterborne acrylic resin, waterborne polyurethane, polyacrylic resin, epoxy resin, or polyethylene is used as the polymer raw material, and regardless of whether its concentration is controlled at the lower limit of 40wt%, the intermediate equilibrium range of 45-55wt%, or increased to the upper limit of 60wt%, the resulting adhesive layer can accurately achieve the designed thickness of 15μm and can tightly adhere to different types of substrates such as aluminum foil, glass, stainless steel, copper sheet, and ceramics. At the same time, it forms a stable interface bond with the upper H-SiO2-EPCM functional layer without defects such as delamination, peeling, or cracking. Even if a 40wt% concentration adhesive layer is prepared using the more cost-effective polyacrylic resin, or a 60wt% concentration adhesive layer is prepared using the more adhesive waterborne polyurethane, the final adhesive layer can still meet the core functions of carrying the functional layer and connecting the substrate, laying the foundation for the overall structural stability of the coating.

[0074] In terms of the preparation of the protective layer, it also exhibits great flexibility: whether selecting waterborne acrylic resin with excellent transparency, waterborne polyurethane with outstanding water resistance, cost-effective polyacrylic resin, epoxy resin with excellent weather resistance, or polyethylene suitable for special scenarios as raw materials, whether setting its concentration to a critical value of 2wt% to ensure light transmittance, a conventional value of 3-4wt% to balance the protective effect and lightweight, or adjusting it to the upper limit of 5wt% to enhance the abrasion resistance, the prepared 5μm thick protective layer can completely cover the surface of the H-SiO2-EPCM functional layer. It will not cause the film layer to be discontinuous and lose its protective function due to too low a concentration, nor will it block the light response of the functional layer due to too high a concentration. It can always provide effective physical protection for the functional layer, while being compatible with the thermochromic properties of the coating.

[0075] Therefore, this invention employs the aforementioned reversible thermochromic coating and its preparation method. By sequentially spraying a high-concentration aqueous acrylic acid, an ethanol solution of H-SiO2-eutectic phase change material, and a low-concentration aqueous acrylic acid onto a substrate, a sandwich-structured H-SiO2-EPCM thermochromic coating is obtained. In this coating, the phase change material exists as plate-like crystals separated from the H-SiO2 nanospheres, and the top protective layer can penetrate the surface pores, ensuring excellent colorfastness. The coating's crystallinity and color-changing temperature are consistent with the phase change material. Furthermore, the coating exhibits angle-independent properties. This simple spraying process avoids the complex process of traditional self-assembly, providing a solution for large-scale commercial production and overcoming the bottleneck of existing technologies' difficulty in large-scale application. Moreover, the angle-independent nature of the coating makes it more suitable for anti-counterfeiting and visualization applications. Rapid structural color thermal response transformation can be achieved through the two-phase fusion-separation principle at the H-SiO2 and EPCM interface. At low temperatures, during interfacial phase separation, the high refractive index contrast between the photonic nanopigment and air generates structural color. After the temperature exceeds the phase transition melting initiation temperature, the EPCM melts and encapsulates the air medium. The low refractive index contrast makes the encapsulated area optically transparent, exposing the internal carbon layer of H-SiO2. The color gradually turns black, and the reflection peak intensity continuously decreases. Complete phase fusion above the melting point results in a black color. Scanning electron microscopy demonstrates the dynamic interaction of the interface, while optical microscopy demonstrates the coating's excellent recovery performance. Based on the interfacial fusion-separation mechanism of EPCM and H-SiO2 photonic nanopigment, the coating achieves precise thermal response within a narrow physiological temperature window of 33-37℃: when the temperature is above 33℃, the structural color gradually fades; at 37℃, it completely turns black; and when cooled below 33℃, the structural color is completely recovered. This high sensitivity, rapid response, and strong reversibility far exceed the performance of traditional thermochromic materials, making it particularly suitable for scenarios requiring body temperature-sensing color changes.

[0076] The HPTC system exhibits a rapid and specific response to human body temperature range, seamless integration with existing packaging processes, and strong commercial viability, demonstrating significant potential in anti-counterfeiting technology. Spraying it onto a flexible substrate to create dynamic anti-counterfeiting labels, which can then be integrated into commercial packaging systems using readily available adhesives, allows for a noticeable color change upon finger contact within just 5 seconds, with complete recovery within 5 seconds of finger removal. This physiologically triggered mechanism requires no additional tools, enabling rapid product authentication through intuitive tactile interaction, combining convenience with compatibility with existing packaging technologies. Leveraging the reversible thermochromic behavior and adjustable transition temperature of the HPTC system, flexible temperature indicators with multi-threshold sensing capabilities can be developed. By selecting organic fatty acids with different carbon chain lengths, a series of HPTCs with programmable thermochromic transition temperatures can be constructed, leading to the fabrication of temperature indicators integrating six different switching thresholds. Furthermore, the introduction of different eutectic or pure phase change materials does not affect the structural color of the HPTC, and the phase change material can be uniformly dispersed in the H-SiO2 layer in a plate-like crystalline form at room temperature. This indicator can be applied to personalized health monitoring and drinking water temperature warnings. The design of the water-based acrylic adhesive layer, H-SiO2-EPCM functional layer, and water-based acrylic protective layer significantly improves the mechanical properties of the coating. The protective layer prevents the H-SiO2 nanospheres and EPCM from delaminating under friction. After 50 cycles of physical friction, 30 minutes of water washing, 12 hours of immersion in water, and soaking in acidic and alkaline environments (pH=2 hydrochloric acid, pH=12 ammonia water), the structural color and reflectance spectrum remain almost unchanged. Simultaneously, the coating can stably adhere to various rigid and flexible substrates such as metal, paper, fabric, plastic, and glass, maintaining pattern integrity even after repeated rubbing without delamination or peeling, thus solving the problems of traditional coatings' strong dependence on substrates and poor mechanical stability.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A reversible thermochromic coating, characterized in that, The coating has a sandwich structure, consisting of an adhesive layer, an H-SiO2-EPCM functional layer, and a protective layer from bottom to top; the H-SiO2-EPCM functional layer is composed of H-SiO2 photonic nano pigments and organic phase change materials.

2. The reversible thermochromic coating according to claim 1, characterized in that, The H-SiO2 photonic nanopigment is a silica nanosphere with a hollow structure. The inner wall of the shell of the silica nanosphere has an amorphous carbon layer, and the surface of the shell has a rough and disordered superstructure with a particle size range of 200-500 nm.

3. The reversible thermochromic coating according to claim 1, characterized in that, The organic phase change material includes a material composed of a mixture of organic fatty acids lauric acid and palmitic acid, and also includes pure components of organic fatty acids lauric acid, palmitic acid, myristic acid and stearic acid.

4. The reversible thermochromic coating according to claim 1, characterized in that, The adhesive layer is formed by drying an aqueous solution of one of the following polymers: waterborne acrylic resin, waterborne polyurethane, polyacrylic resin, epoxy resin, or polyethylene, with a mass fraction ranging from 40-60 wt%. The protective layer is formed by drying an aqueous solution of one of the following polymers: waterborne acrylic resin, waterborne polyurethane, polyacrylic resin, epoxy resin, or polyethylene, with a mass fraction ranging from 2 to 5 wt%.

5. The reversible thermochromic coating according to claim 1, characterized in that, The adhesive layer has a thickness of 15 μm, the H-SiO2-EPCM functional layer has a thickness of 40 μm, and the protective layer has a thickness of 5 μm.

6. A method for preparing a reversible thermochromic coating as described in any one of claims 1-5, characterized in that, Includes the following steps: (1) Preparation of organic phase change materials: Mix organic fatty acids lauric acid and palmitic acid, heat to 50°C and stir magnetically at 500 rpm for 30 minutes, and cool to room temperature to obtain organic phase change materials; or directly select one of the pure components organic fatty acids lauric acid, palmitic acid, myristic acid and stearic acid, and weigh out its pure reagent for later use. (2) Preparation of H-SiO2-EPCM ethanol solution: H-SiO2 photonic nano pigment and organic phase change material are dispersed in ethanol and ultrasonically treated at 40°C for 3 hours; wherein, the mass fraction of H-SiO2 photonic nano pigment is 30wt% and the mass fraction of organic phase change material is 70wt%; wherein, H-SiO2 photonic nano pigment is a silica nanosphere with a hollow structure, the inner wall of the shell of the silica nanosphere has an amorphous carbon layer, the surface of the shell has a rough and disordered superstructure, and its particle size ranges from 200 to 500 nm; (3) Spraying water-based acrylic adhesive layer: Spray 40-60wt% water-based polymer aqueous solution onto the substrate, and after drying, form an adhesive layer with a thickness of 15μm; (4) Spraying H-SiO2-EPCM functional layer: The H-SiO2-EPCM ethanol solution prepared in step (3) is sprayed onto the adhesive layer at 50°C, and after drying, a functional layer with a thickness of 40 μm is formed; (5) Spraying a water-based acrylic protective layer: Spraying 2-5 wt% water-based acrylic emulsion aqueous solution onto the functional layer, and after drying, forming a protective layer with a thickness of 5 μm, to obtain the programmable thermochromic coating.

7. The method for preparing a reversible thermochromic coating according to claim 6, characterized in that, The organic phase change material in step (2) is the material made from a mixture of organic fatty acids lauric acid and palmitic acid in step (1), or a pure component material among organic fatty acids lauric acid, palmitic acid, myristic acid and stearic acid.

8. The method for preparing a reversible thermochromic coating according to claim 6, characterized in that, In steps (3), (4), and (5), a commercial spray gun with a nozzle diameter of 0.5 mm, a pressure of 30 psi, and a working distance of 5 cm is used in the spraying process.

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