Preparation method of photo-thermal response type thermochromic microcapsule

Abstract

The invention discloses a preparation method of a photo-thermal response type thermochromic microcapsule, and relates to the technical field of thermochromic materials. In recent years, a phase change material, a thermochromic material and a photothermal conversion material show good application potential in the low-temperature deicing direction, but a single-function material is often difficult to meet the requirements of deicing efficiency, response speed and stability at the same time. The preparation method comprises the following steps: preparing a core material composite solution; dropwise adding into a diluted SMA alkaline solution, and emulsifying to obtain an emulsion; preparing an MUF prepolymer, adding the MUF prepolymer into the emulsion to obtain a mixture, diluting and adjusting the pH value to 4.0-4.5 to obtain a reaction solution; carrying out acid catalysis condensation polymerization on the reaction liquid, adjusting the pH value to 6.8-7.25, and stirring for 10-20 minutes to obtain a microcapsule suspension; and washing, filtering and drying the microcapsule suspension to constant weight to obtain the photo-thermal response type thermochromic microcapsule which has the characteristics of photo-thermal response, thermochromic indication and phase change heat storage.

Description

Technical Field

[0001] This invention relates to the field of functional microcapsule materials technology, specifically to a method for preparing a photothermal responsive thermochromic microcapsule that integrates photothermal conversion, thermochromic properties, and phase change heat storage functions. This method can be applied to building envelopes, functional coatings, and temperature control and protection materials in low-temperature environments. Background Technology

[0002] In Northeast, Northwest my country, and the Qinghai-Tibet Plateau, icing and snow accumulation on photovoltaic modules, bridge steel structures, and building exteriors are common occurrences during winter and transitional seasons. This can lead to reduced load-bearing capacity, increased surface adhesion, and decreased operational efficiency, ultimately impacting infrastructure safety and energy utilization. Current de-icing / anti-icing methods include electric heating, chemical de-icing agent spraying, and mechanical removal. However, these methods generally suffer from high energy consumption, corrosiveness to substrates, high construction and maintenance costs, and difficulty in achieving refined and intelligent management, failing to meet the demands for green, low-carbon, and long-term stable operation. Therefore, developing surface protection materials that combine environmental friendliness, sustainability, and multifunctional synergy has become a research hotspot in the field of anti-icing and thermal management of cold-region structural components.

[0003] At the materials level, phase change energy storage materials, thermochromic materials, and photothermal conversion materials show application potential in the thermal response regulation of low-temperature environments. Phase change materials can buffer temperature changes by absorbing or releasing latent heat in their phase change temperature range, thereby reducing temperature fluctuations and delaying thermal shock during the freezing-thawing process. Thermochromic systems can undergo reversible color changes with temperature, which can regulate light absorption to a certain extent and provide a visual temperature indication. Photothermal materials can absorb solar radiation and quickly convert it into heat energy, promoting the surface temperature of components to cross 0°C and enter the critical freeze-thaw window for melting or inhibiting refreezing. However, existing research focuses on improving single functions, making it difficult to achieve the synergistic integration of "rapid heating under light, visual temperature indication, and latent heat buffering to delay refreezing" while ensuring structural stability and cycle durability. In addition, single-function systems are prone to problems such as leakage of phase change core materials, color decay, or unstable response under actual working conditions such as multiple freeze-thaw cycles and alternating light and cooling, which limits their long-term application in coatings and composite structures. Summary of the Invention

[0004] In view of the fact that existing phase change energy storage materials, thermochromic materials and photothermal materials often have single functions, make it difficult to balance structural stability and cycle durability, and are prone to problems such as core material leakage, color decay or unstable response in coating or composite applications, this invention aims to provide a method for preparing photothermally responsive thermochromic microcapsules. By controlling the process conditions, the method achieves the synergistic effect of photothermal heating, thermochromic visualization and phase change heat storage, thereby improving the stability and application adaptability of the material under alternating freeze-thaw and light conditions.

[0005] To achieve the above objectives, the present invention provides a method for preparing photothermal responsive thermochromic microcapsules, comprising the following steps: (1) Preparation of core material composite solution; (2) Prepare an alkaline solution of styrene-maleic anhydride copolymer and dilute it to obtain a diluted alkaline solution of styrene-maleic anhydride copolymer; (3) The core material composite solution is added dropwise to the diluted alkaline solution of styrene-maleic anhydride copolymer, deionized water is added, and emulsification is carried out under mechanical stirring to obtain an emulsion; (4) Prepare melamine-urea-formaldehyde prepolymer. Add melamine-urea-formaldehyde prepolymer to emulsion at a rate of 3.0-10.0 mL / min to obtain a mixture. Then add deionized water to obtain a diluted mixture. Then add an acid regulator to adjust the pH of the diluted mixture to 4.0-4.5 to obtain a reaction solution. (5) The reaction solution undergoes acid-catalyzed polycondensation under mechanical stirring to form a melamine-urea-formaldehyde shell coating the core material. After the acid-catalyzed polycondensation reaction is completed, an alkaline regulator is added to adjust the pH to 6.8–7.25, and the mixture is stirred for 10–20 min to obtain a microcapsule suspension. (6) The microcapsule suspension was washed, filtered and dried to constant weight to obtain photothermal responsive thermochromic microcapsules.

[0006] Furthermore, in the preparation process of the core material composite solution, crystal violet lactone and bisphenol A are mixed at a mass ratio of 1:1 to 1:5 to obtain a colorimetric reagent mixture; Heat 8–12 g of nonanoic acid to 60–65 °C, add the colorimetric reagent mixture, and then add 2 mL of isopropanol. Stir at a constant temperature of 800–1000 r / min for 50–70 min at 65–75 °C to ensure uniform mixing and obtain the core material composite solution.

[0007] Furthermore, the mass ratio of crystal violet lactone to bisphenol A is 1:3.

[0008] Further, in the process of preparing an alkaline solution of styrene-maleic anhydride copolymer and diluting it to obtain a diluted alkaline solution of styrene-maleic anhydride copolymer, Styrene-maleic anhydride copolymer was mixed with 3M sodium hydroxide and stirred for 30 minutes at a temperature of 70°C. After stirring, deionized water was added every 30 minutes for a total of 6 times. After the reaction was completed, an acidic regulator was added dropwise to adjust the pH to 10, resulting in a 2wt% alkaline solution of styrene-maleic anhydride copolymer. Deionized water was added to a 2wt% alkaline solution of styrene-maleic anhydride copolymer, with a mass ratio of deionized water to styrene-maleic anhydride copolymer alkaline solution of 5:1. The solution was stirred at 70℃ for 30 min to obtain a diluted alkaline solution of styrene-maleic anhydride copolymer.

[0009] Furthermore, the core material composite solution was added dropwise to the diluted styrene-maleic anhydride copolymer emulsion system, deionized water was added, and emulsification was carried out under mechanical stirring conditions. The emulsification temperature was 65–75℃, the mechanical stirring rate was 800–1000 r / min, and the emulsification time was 50–70 min.

[0010] Furthermore, the preparation method of melamine-urea-formaldehyde prepolymer is as follows: urea, melamine, formaldehyde aqueous solution and deionized water are mixed, nano titanium dioxide is added and mechanically stirred for 10 min under 70℃ water bath conditions, an alkaline regulator is added to adjust the pH to 10.0±0.1, and the reaction is continued to be stirred at 70℃ for 1 h to obtain melamine-urea-formaldehyde prepolymer; The formaldehyde aqueous solution has a mass fraction of 37%, the solid mass ratio of urea to melamine is 1.625:1, and the mass ratio of 37wt% formaldehyde aqueous solution to deionized water is 1.85:1; the amount of urea used is 1.82–2.73g, and the amount of deionized water used is 4–6mL.

[0011] Furthermore, the acid-catalyzed polycondensation reaction is carried out at a temperature of 80–95℃, a mechanical stirring rate of 800–1500 r / min, and a reaction time of 90–160 min.

[0012] Furthermore, the nano-titanium dioxide has a particle size of 10–200 nm and is added in an amount of 0.1–5 wt% of the melamine-urea-formaldehyde prepolymer.

[0013] The present invention also proposes a photothermal responsive thermochromic microcapsule, comprising a core material and a shell encapsulating the core material; The core material is made from the following raw materials in parts by weight: 8–12 g of nonanoic acid, and the mass ratio of crystal violet lactone to bisphenol A is 1:1–1:5. The shell is made from the following raw materials in parts by weight: the solid mass ratio of urea to melamine and formaldehyde is 1.625:1:2.445; Nano-titanium dioxide is dispersed in the shell, and the amount of nano-titanium dioxide added is 0.1–5 wt% of the shell mass.

[0014] Furthermore, the particle size of the photothermal responsive thermochromic microcapsules is 5–50 μm.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The present invention combines SMA emulsification with MUF prepolymer in situ condensation at the emulsion interface to form a shell, which can form a complete microcapsule shell under acid catalysis, which is beneficial to suppress leakage of phase change core material and improve structural stability.

[0016] (2) This invention controls the polycondensation and crosslinking process by controlling key process parameters such as the addition rate of MUF prepolymer, pH window, stirring intensity, and reaction temperature, which is beneficial for obtaining microcapsule products with good dispersibility and high repeatability. By diluting to reduce the viscosity of the system and suppressing heterogeneous polycondensation triggered by local over-acidity, the risk of demulsification and local gelation can be reduced, thereby improving the continuity and batch repeatability of shell formation. By establishing a three-stage pH control method of emulsion stability alkaline window, interfacial polycondensation acidic window, reaction termination and storage stability neutral window, the synergistic control of droplet stability, in-situ interfacial shell formation, and post-reaction system stability is achieved, thereby enhancing the controllability of the shell formation process and improving the system's tolerance to process fluctuations.

[0017] (3) The present invention encapsulates the thermochromic color-changing system and the phase change core material together, and combines photothermal heating conditions to realize the visual indication of temperature changes and latent heat buffering, which helps to improve the temperature regulation and protective adaptability of the material under working conditions such as freeze-thaw and alternating light.

[0018] (4) The preparation method provided by the present invention has a clear process route and strong operability. The obtained microcapsules can be used in functional coatings, composite material interlayers or related temperature control / protection applications, and have good application and promotion value.

[0019] It should be noted that thermochromic color development systems may experience a certain degree of color drift or appearance change under continuous strong light or ultraviolet irradiation. In practical applications, the weather resistance and long-term stability of microcapsules can be further improved by introducing ultraviolet absorbers, hindered amine light stabilizers and / or surface anti-ultraviolet protective layers into the microcapsules or their composite coating systems. Attached Figure Description

[0020] Figure 1 The image shows the morphology of the ultra-depth-of-field microsystem of the prepared thermochromic phase change microcapsules. Figure 2 Comparison of Fourier transform infrared (FT-IR) spectra of microcapsules, core materials, and MUF shell materials; Figure 3 Differential scanning calorimetry (DSC) heat flux-temperature curve for microcapsules; Figure 4 Thermogravimetric analysis (TGA) mass loss-temperature curve of microcapsules; Figure 5 The color difference ΔE – temperature change curve for microcapsules with different CVL and BPA mass ratios; Figure 6 Comparison of appearance colors of microcapsules with different CVL and BPA mass ratios before and after heating; Figure 7 The mass change curves of microcapsules with different CVL to BPA mass ratios during 10 thermal cycles are shown. Figure 8 A bar chart showing the mass loss rate of microcapsules under different CVL to BPA mass ratios; Figure 9 The graph shows the surface temperature of microcapsules under 10 min of light irradiation under different CVL to BPA mass ratios. Figure 10 The temperature rise response curves of microcapsules under 60 min of light irradiation are shown for different CVL to BPA mass ratios. Figure 11 The figure shows the Δg–t curves of the ice melting mass change of composite systems with different microcapsule contents under light conditions. Figure 12 Comparison of appearance changes of composite systems with different microcapsule contents during the light-induced ice melting process. Detailed Implementation

[0021] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the specific embodiments of this invention will be further described below with reference to the accompanying drawings. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of this invention; various changes or substitutions can be made by those skilled in the art without departing from the spirit and scope defined by the claims, and all such changes or substitutions should fall within the scope of protection of this invention.

[0022] In existing technologies, phase change energy storage materials, thermochromic materials, and photothermal conversion materials can respectively achieve latent heat buffering, temperature visualization, and light-induced heating. However, single-function materials or simple composite systems generally suffer from problems such as incomplete coating, core material leakage, color decay, and insufficient cycle stability. They are difficult to simultaneously ensure response speed and structural stability under low-temperature fluctuation conditions, thus limiting their application in functional coatings and composite structures.

[0023] To address the aforementioned problems, this invention provides a method for preparing photothermal-responsive thermochromic microcapsules. The method uses nonanoic acid as the phase change core material and crystal violet lactone / bisphenol A as the thermochromic color development system. A melamine-urea-formaldehyde (MUF) prepolymer is used for in-situ acid-catalyzed condensation polymerization at the emulsion interface to form a shell, achieving synergistic encapsulation of the core material and the color development system. By controlling the ratio, emulsification, and shell-forming reaction conditions, a phase change microcapsule material with complete encapsulation, stable structure, and photothermal response and thermochromic indication functions is obtained.

[0024] Example 1: Preparation process of photothermal responsive thermochromic microcapsules See Figure 1 – Figure 12 This embodiment provides a method for preparing photothermal-responsive thermochromic microcapsules. The main reagents used include nonanoic acid (NA), crystal violet lactone (CVL), bisphenol A (BPA), isopropanol, styrene-maleic anhydride copolymer (SMA), urea, melamine, formaldehyde aqueous solution (37% by mass), and nano-titanium dioxide (…). The reagents included triethanolamine (TEA), an acidity regulator (citric acid in this example), an alkalinity regulator (sodium hydroxide in this example), and deionized water. The main instruments included a super depth-of-field microscope system, a Fourier transform infrared spectrometer (FT-IR), a differential scanning calorimeter (DSC), a thermogravimetric analyzer (TGA), and a colorimeter.

[0025] (1) Preparation of core material composite solution. CVL and BPA were mixed at mass ratios of 0:0, 1:2, 1:3, and 1:5 to obtain a colorimetric reagent mixture. 8–12 g of nonanoic acid was heated to 60–65 °C and then the colorimetric reagent mixture was added, along with 2 mL of isopropanol as a co-solvent. The mixture was stirred at 800–1000 r / min at 65–75 °C for 50–70 min to ensure uniform mixing and obtain the core material composite solution.

[0026] (2) Preparation of an alkaline solution of styrene-maleic anhydride copolymer. Mix 9.6 g of SMA powder with 60 mL of 3M NaOH and stir for 30 min at a stirring temperature of 70 °C. After stirring, add 60 mL of deionized water every 30 min for a total of 360 mL in 6 additions to reduce the viscosity of the system and promote the complete dissolution of SMA. After the reaction, the pH of the system is usually high. Then, 70-75 mL of 10 wt% citric acid solution is added dropwise to neutralize the excess alkali and bring the pH of the system back to 10. Here, pH=10 is the alkaline window required for emulsion stability. The amount of citric acid solution used is adjusted according to the initial pH fluctuation. In this example, the amount used is 70-75 mL, which yields an approximately 2 wt% alkaline solution of styrene-maleic anhydride copolymer (SMA alkaline solution). 40 mL of 2 wt% SMA alkaline solution was mixed with 200 mL of deionized water and stirred at 70 °C for 30 min to obtain a diluted SMA alkaline solution. The dilution was used to reduce the viscosity of the aqueous phase and improve the emulsion stability. It is preferable to keep the effective concentration of SMA in the aqueous phase in the emulsion stage within the range of 0.2–0.6 wt% to obtain an emulsion with stable particle size and less prone to demulsification. The degree of dilution was determined to meet the stability requirements of the emulsification process.

[0027] The acid regulator is at least one of citric acid, acetic acid, and lactic acid, and the dropping rate is 1–2 mL / min.

[0028] The water dilution process can be carried out in batches or continuously. The number of water additions and the amount of water added at one time are not limited. They can be adjusted according to the viscosity of the system, the degree of SMA dissolution, and the emulsification stability, so that the aqueous phase is within the viscosity and concentration window that allows for stable emulsification.

[0029] (3) Emulsification. The core material composite solution is slowly added dropwise (continuously without forming a stable liquid column) to the diluted SMA alkaline solution, and 50 mL of deionized water is added. The mixture is mechanically stirred at 800–1000 r / min for 50–70 min at 65–75 °C to obtain a stable emulsion. Water is added to adjust the total volume of the aqueous phase during the emulsification stage. Preferably, the ratio of the total volume of the aqueous phase to the volume of the core material composite solution during the emulsification stage is 8:1–20:1 to obtain an emulsion with stable particle size and that is not easily demulsified. (4) Preparation of melamine-urea-formaldehyde (MUF) prepolymer.

[0030] Urea, melamine, formaldehyde aqueous solution and deionized water were mixed, nano titanium dioxide was added and mechanically stirred for 10 min under 70℃ water bath conditions, and an alkaline regulator was added to adjust the pH to 10.0±0.1. The reaction was continued for 1 h while maintaining 70℃ to obtain melamine-urea-formaldehyde prepolymer. The formaldehyde aqueous solution has a mass fraction of 37%, the urea to melamine solid mass ratio is 1.625:1, and the 37wt% formaldehyde aqueous solution to deionized water mass ratio is 1.85:1 (based on the mass of solution and water). When measuring the 37wt% formaldehyde aqueous solution and / or deionized water by volume, the volume is converted to mass based on the density at room temperature before proportioning. The amount of urea used is 1.82–2.73 g, and the amount of deionized water used is 4–6 mL.

[0031] Specifically, the base formula uses 2.275g urea, 1.4g melamine, 9.25mL formaldehyde aqueous solution (mass fraction of 37%, i.e., 3.4225g formaldehyde), and 5.0mL deionized water as the base formula, and adds nano titanium dioxide ( 0.1g was used as an inorganic nano-dispersible component, and mechanically stirred for 10 min at 70℃ to ensure uniform dispersion, thereby forming a composite with the MUF shell to form a nano-dispersible component. The thermal response-enhanced shell was then formed; subsequently, 7.0 mL of triethanolamine (TEA) was used as an alkaline regulator to adjust the pH of the solution to 10.0 ± 0.1, and the reaction was continued at 70 °C for 1 h with stirring to obtain a transparent and viscous MUF prepolymer.

[0032] The alkalinity regulator is at least one of sodium hydroxide, potassium hydroxide, sodium carbonate, and triethanolamine, and the dropping rate is 1–2 mL / min.

[0033] The dosage of each component in the baseline formulation is simultaneously scaled down within the range of 0.8–1.2 times.

[0034] The nano-titanium dioxide has a particle size of 10–200 nm and is added in an amount of 0.1–5 wt% based on the MUF prepolymer, preferably 0.3–0.8 wt%.

[0035] (5) In-situ polycondensation and neutralization. The MUF prepolymer was added to the emulsion at a rate of 3.0–10.0 mL / min using a peristaltic pump for mixing. Then, deionized water was added to dilute the mixture of MUF prepolymer and emulsion. The preferred amount added was 0.8–2.0 times the volume of the mixture to reduce the viscosity of the mixture and reduce the local over-acidity during the addition of the acid regulator, thereby inhibiting heterogeneous polycondensation and local gelation, and improving the continuity and batch repeatability of shell formation.

[0036] Subsequently, a citric acid solution (25 wt%) was added dropwise at a rate of 1–2 mL / min using a peristaltic pump to adjust the pH of the diluted mixture to 4.0–4.5 and maintain it at a relatively stable level. Maintaining the pH at 4.0–4.5 was used to trigger acid-catalyzed condensation polymerization of the MUF prepolymer at the droplet interface to form a continuous shell. After the reaction, the pH was adjusted back to 6.8–7.25 to achieve in-situ condensation polymerization of the shell and to inhibit further condensation polymerization after the reaction, thereby improving the storage stability of the microcapsule suspension.

[0037] After the pH reaches the specified range and remains relatively stable, an acid-catalyzed polycondensation reaction is carried out for 90–160 min under mechanical stirring at 80–95℃ and 800–1500 r / min. This allows the MUF prepolymer to undergo in-situ cross-linking and solidification at the emulsion interface, forming a MUF shell coating the core material, resulting in a microcapsule suspension. After the polycondensation reaction is complete, an alkaline regulator is added dropwise using a peristaltic pump to adjust the pH of the microcapsule suspension to 6.8–7.25, and stirring continues for 10–20 min. This process inhibits further polycondensation and improves the storage and use stability of the microcapsule suspension.

[0038] Preferably, the acidic and alkaline pH adjusters are added dropwise using a peristaltic pump at a rate controlled at 1–2 mL / min to improve the controllability and repeatability of the pH adjustment process.

[0039] (6) Post-processing. The adjusted microcapsule suspension was washed with deionized water and filtered, and then dried at 20–30℃ to constant weight to obtain photothermal responsive thermochromic microcapsules. The constant weight criterion was that the interval between two consecutive weighings should not be less than 2 hours and the mass difference should not exceed 0.3 mg.

[0040] A photothermal-responsive thermochromic microcapsule was prepared using the above method. The microcapsule comprises a core-shell structure, wherein: the core material includes nonanoic acid (a phase change material) and a thermochromic color-developing system, the thermochromic color-developing system including crystal violet lactone and bisphenol A; the shell is a melamine-urea-formaldehyde resin shell, and nano-titanium dioxide is dispersed in the shell as an inorganic nano-dispersed component. The particle size of the microcapsule is 5–50 μm, preferably 10–30 μm.

[0041] Example 2: Characterization and Performance Testing of Photothermal Responsive Thermochromic Microcapsules The specific steps for preparing photothermal responsive thermochromic microcapsules are as follows: (1) CVL and BPA were mixed at mass ratios of 0:0, 1:2, 1:3 and 1:5 to obtain multiple color developer mixtures. 10g of nonanoic acid was heated to 60–65℃ and then each color developer mixture was added. 2mL of isopropanol was added as a co-solvent. The mixture was stirred at 70℃ and 300r / m for 60min to obtain the core material composite solution.

[0042] (2) Preparation of styrene-maleic anhydride copolymer alkaline solution. Mix 9.6g of SMA powder with 60mL of 3M NaOH and stir for 30min at 70℃. After stirring, add 60mL of deionized water every 30min for a total of 360mL in 6 additions. Then add 70-75 mL of 10 wt% citric acid to adjust the pH to 10, and obtain a 2 wt% alkaline solution of styrene-maleic anhydride copolymer (SMA alkaline solution). Use 40 mL of 2 wt% SMA alkaline solution, add 200 mL of deionized water, stir at 70 °C for 30 min to obtain diluted SMA alkaline solution; (2) The core material composite solution was slowly added dropwise to the diluted SMA alkaline solution, 50 mL of deionized water was added, and the mixture was mechanically stirred at 800 r / min for 60 min at 70 °C to obtain a stable emulsion.

[0043] (3) Using 2.275g of urea, 1.4g of melamine, 9.25mL of formaldehyde aqueous solution (mass fraction of 37%) and 5.0mL of deionized water as the base formula, nano-titanium dioxide was added. 0.1g of the solution was mechanically stirred for 10 min at 70℃ to ensure uniform dispersion. Then, 7.0 mL of triethanolamine (TEA) was used as an alkaline regulator to adjust the pH of the solution to 10.0±0.1. The reaction was continued at 70℃ for 1 h to obtain the MUF prepolymer.

[0044] (4) Use a peristaltic pump to add the MUF prepolymer to the emulsion at a rate of 5.0 mL / min to ensure thorough mixing of the system; then add 400 mL of deionized water to dilute the mixture of MUF prepolymer and emulsion.

[0045] Subsequently, a citric acid solution with a concentration of 25 wt% was added dropwise at a rate of 1–2 mL / min using a peristaltic pump, and the pH of the diluted mixture was adjusted to 4.1–4.3. An acid-catalyzed polycondensation reaction was then carried out for 120 min under mechanical stirring at 80 °C and 1500 rpm, allowing the MUF prepolymer to undergo in-situ cross-linking and solidification at the emulsion interface, forming a MUF shell coating the core material, resulting in a microcapsule suspension.

[0046] After the polycondensation reaction was completed, NaOH was added dropwise at a rate of 1–2 mL / min using a peristaltic pump, and the pH of the microcapsule suspension was adjusted to 6.8–7.25. Stirring was continued for 10 min.

[0047] (5) The adjusted microcapsule suspension was washed with deionized water and filtered, and then dried at 30°C to constant weight for 12 hours to obtain photothermal responsive thermochromic microcapsules.

[0048] The sources of the above reagents are shown in Table 1.

[0049] Table 1. Reagents used in the experiment

[0050] This embodiment characterizes and tests the color development performance, cycle stability, photothermal response performance, and light-induced ice-melting performance of the composite coating of the microcapsules prepared above, as detailed below.

[0051] (1) Colorimetric performance test. Microcapsules were uniformly spread on the substrate surface, and the temperature was gradually increased in increments of 10℃ within the range of 0–70℃. The L*, a*, and b* values ​​at each temperature point were measured using a colorimeter, and the color difference ΔE was calculated according to CIELAB standards with 0℃ as a reference. Each temperature point was measured in parallel three times and the average value was taken to characterize the thermochromic response characteristics of the microcapsules. During the test, the same substrate and the same measurement background (such as white background) were used for each group of samples, and the spread area and spread quality (or thickness) of the samples were kept consistent. The measuring aperture and measurement position of the colorimeter were fixed, and the measurement was carried out under constant ambient light conditions.

[0052] (2) Cyclic stability test. The sample was alternately heated between 0℃ and 50℃ for at least 60 min in each temperature zone, for a total of 10 cycles. The ΔE and sample mass change Δg after each cycle were recorded to characterize the reversibility of microcapsule color development and encapsulation stability.

[0053] (3) Photothermal response test. The samples were continuously irradiated for 60 min at room temperature using a xenon lamp providing 1 sun of illumination (approximately 1000 W / m²). The sample surface temperature T was recorded at 0, 10, 30, and 60 min, and the corresponding ΔE and mass change Δg were simultaneously measured to characterize the temperature rise behavior and mass stability of the microcapsules under light-driven conditions. During the test, the spreading area and spreading mass (or thickness) of each group of samples remained consistent, and the temperature measurement points were fixed. The distance between the xenon lamp and the sample was fixed and consistent across groups, and the irradiation angle was kept perpendicular. The ambient temperature and airflow conditions were kept consistent and recorded.

[0054] (4) Photoluminescence melting test of composite coatings. Four groups of samples were prepared using glass slides as substrates: blank glass, pure water-based resin coating, 5% mass fraction microcapsule composite coating, and 10% mass fraction microcapsule composite coating. Each group of samples was pre-cooled at -10℃ for 20 min and then transferred to an environment of approximately 20℃. An ice block with a mass of (0.55±0.05) g was placed on the sample surface. The mass change of melting ice Δg within 0–20 min was recorded under vertical irradiation by a 1-sun xenon lamp (n=3, weighing error ≤0.005g), and the photoluminescence melting effect of different coatings was compared by the Δg–time curve. During the test, the ice block was placed in a fixed position (e.g., placed in the center of the sample), the coating preparation process of each group was consistent, and the coating thickness was kept consistent; the distance between the xenon lamp and the sample was fixed and consistent for each group, and the irradiation angle was kept vertical; the weighing time interval was consistent with the operation procedure to reduce human error.

[0055] Example 3: Performance Test Results 1. Analysis of microcapsule morphology and chemical structure To characterize the particle size, surface morphology, and encapsulation integrity of the prepared thermochromic phase change microcapsules, this embodiment employs a super-depth-of-field microscopy system and Fourier transform infrared spectroscopy (FT-IR) for morphological and chemical structure characterization. The results are as follows: Figure 1 – Figure 2 As shown. The microcapsule particle size was obtained by acquiring particle images using a super depth-of-field microscope system, and the equivalent diameter of the particles was statistically determined based on the image scale. At least 50 microcapsules were randomly selected for measurement.

[0056] like Figure 1 As shown, the dried microcapsules are generally nearly spherical or ellipsoidal in shape, with clear particle edges and a relatively dense and smooth surface, without any obvious cracking or collapse. Based on the scale, the characteristic particle size is mainly concentrated between approximately 10–30 μm. A small number of particles exhibit slight agglomeration, but the overall dispersion is good. This indicates that the in-situ encapsulation process of this invention can obtain microcapsules with intact structures and relatively uniform particle size, which is beneficial for their dispersion and stable application in subsequent coatings or composite systems.

[0057] To further verify the core material coating effect and analyze the core-shell interface compatibility, FT-IR tests were performed on the core material mixture, MUF wall material, and composite microcapsules. The spectra are shown below. Figure 2 As shown (4000–500) Core material samples were found at approximately 1730. A distinct C=O stretching vibration absorption peak appears at approximately 2850°C. C–H stretching vibration absorption peaks appear nearby; MUF wall materials show absorption peaks at approximately 3300. A relatively broad –OH / –NH stretching vibration absorption band is observed nearby. Compared to the core material, the intensity of the aforementioned core material characteristic peaks in the microcapsule sample is weakened and shows a certain shift. At the same time, the –OH / –NH absorption band overlaps with the wall material, indicating that the core material characteristic peaks are affected by the shell shielding effect, the core material is effectively encapsulated, and there may be hydrogen bonds and other interactions between the core and shell, which is beneficial to improving interfacial compatibility and structural stability.

[0058] 2. Study on the thermal properties and phase transition behavior of microcapsules To characterize the thermal response and thermal stability of the microcapsules, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used in this embodiment. The results are as follows: Figure 3 – Figure 4 As shown.

[0059] like Figure 3 As shown, the microcapsules exhibit a single and clear endothermic peak during heating, with a peak temperature of approximately 13.4℃, indicating a typical solid-liquid phase transition process. Combined with the DSC curve morphology, it can be seen that the system exhibits a significant phase transition heat buffering characteristic in the approximately 10–25℃ range, providing latent heat regulation for temperature fluctuation conditions. It should be noted that the DSC test was conducted at 5℃. The heating rate, and the weak endothermic tails at both ends of the curve may be related to the heat capacity of the shell and the heat transfer hysteresis, and are not equivalent to the main contribution region of the phase change.

[0060] like Figure 4 As shown, the TGA curves of the microcapsules exhibit a phased weight loss characteristic: the first stage is from room temperature to approximately 113℃, with relatively small mass loss, mainly related to the evaporation of adsorbed water and residual solvent; the second stage is from approximately 113–183℃, corresponding to the pyrolysis process of the core material and low molecular weight components; the third stage is the slow weight loss region from approximately 183–396℃ and thereafter, corresponding to the further decomposition and carbonization of the shell and organic matrix. The system still retains a certain residual mass after heating to 800℃, indicating that the microcapsules have good thermal stability under ambient to medium-temperature light-induced thermal management conditions.

[0061] 3. Colorimetric performance analysis To compare the effects of different CVL to BPA mass ratios on the thermochromic behavior of microcapsules, the color difference ΔE of samples with different ratios was measured in the range of 0–70℃. The results are as follows: Figure 5 – Figure 6 As shown.

[0062] like Figure 5 As shown, except for the blank group (0:0), the ΔE of the other ratio samples increased with increasing temperature during the heating process, indicating that the system has a thermochromic response; the ΔE of the blank group remained low throughout and can be regarded as background color difference. In contrast, the ΔE of the 1:2 and 1:3 ratio samples increased more slowly in the 20–40℃ range, and further increased in the higher temperature range, with a maximum ΔE of about 7–8, showing a relatively smooth response trend; the ΔE of the 1:5 ratio sample was generally higher, reaching about 15 in the high temperature range, but with some fluctuations, reflecting the trend of decreased response consistency under high chromogenic reagent ratios.

[0063] like Figure 6 As shown, there are significant differences in appearance color before and after heating for samples with different ratios: the 0:0 group showed almost no significant hue change; the 1:2 and 1:3 groups showed uniform color change and relatively fine powder morphology; although the 1:5 group showed a deeper color, flocculation or agglomeration was observed in some areas. Based on the combined ΔE–temperature response curve and appearance comparison results, it can be seen that the 1:3 ratio achieves a good balance between color intensity and response smoothness, and can be considered one of the preferred embodiments of this invention.

[0064] 4. Cyclic stability analysis To evaluate the encapsulation stability of microcapsules with different CVL to BPA mass ratios under thermal cycling conditions, this embodiment conducted 10 heating-cooling cycles between 0°C and 50°C (each temperature zone was maintained for at least 60 minutes). The mass change Δg was calculated based on the mass of the 0th cycle. The results are as follows: Figure 7 – Figure 8 As shown.

[0065] like Figure 7 As shown, all samples with different ratios experienced a certain degree of mass reduction during cycling, but the magnitude of the reduction varied: the Δg change of the 1:3 ratio sample was relatively small (e.g., the cumulative amount after 10 cycles was approximately -2 mg), and the curve was relatively flat; the mass reduction of the 1:2 ratio sample was slightly larger (e.g., approximately -3.3 mg); the mass reduction of the 1:5 ratio sample was more significant in the later stages of cycling (e.g., the cumulative amount was approximately -7.3 mg); and the mass reduction of the blank group (0:0) was relatively larger (e.g., the cumulative amount was approximately -12.4 mg), indicating that the samples had a weaker ability to maintain mass under cycling conditions without the introduction of a colorimetric system.

[0066] like Figure 8As shown, the mass loss rate of the samples varied after 10 cycles. The 1:3 ratio sample had a lower mass loss rate (e.g., approximately 0.10%), the 1:2 ratio sample was at an intermediate level (e.g., approximately 0.20%), while the 1:5 and 0:0 ratio samples had relatively higher mass loss rates (e.g., approximately 0.63% and 0.82%, respectively). Figure 7 and Figure 8 The results show that the 1:3 ratio sample exhibits better overall characteristics in terms of cycle stability and can be used as one of the preferred embodiments of the present invention.

[0067] 5. Photothermal response performance analysis To evaluate the effect of the colorimetric system ratio on the photothermal behavior of microcapsules, this embodiment continuously irradiated microcapsule samples with different ratios for 60 min under a light intensity of 1 sun (approximately 1000 W / m²), and recorded the temperature-time curves. The results are as follows: Figure 9 – Figure 10 As shown, the results of parallel tests in each group were consistent (e.g., the difference within each group at the same time point was on the order of 0.2–0.4℃), indicating that the test had good repeatability.

[0068] like Figure 9 As shown, all samples in each group rapidly heated up within the first 10 minutes of illumination. Compared to the blank group (0:0), the samples containing the CVL / BPA colorimetric system heated up much faster, reaching temperatures on the order of approximately 50°C (e.g., approximately 50.2–51.6°C) after 10 minutes, while the blank group had relatively lower temperatures under the same conditions (e.g., approximately 41.5°C). This indicates that the colorimetric system is beneficial for enhancing visible light absorption and increasing the initial heating rate.

[0069] like Figure 10 As shown, the temperature evolution of the samples differed between the 10–60 min groups: the 1:2 and 1:5 ratio samples reached relatively high temperatures in the early and middle stages before experiencing a slight drop; the 1:3 ratio sample maintained a relatively stable temperature (e.g., around 52–54 °C) with minimal fluctuations during the 30–60 min period; the blank group heated up more slowly. Combining the aforementioned cycle stability and colorimetric response results, it can be concluded that the 1:3 ratio sample exhibits better overall characteristics in terms of light-induced heating efficiency, temperature stability, and quality retention.

[0070] 6. Test of the light-induced ice-melting performance of microcapsule composite coating To evaluate the effect of microcapsule dosage on the light-induced ice-melting effect of the composite coating, this embodiment selected microcapsules with a 1:3 ratio as filler to prepare coatings with different dosages. Four groups of samples were prepared using glass slides as substrates: blank glass, pure water-based resin coating, and composite coatings with 5% and 10% mass fraction microcapsule coatings. After pre-cooling the samples at -10℃ for 20 min, they were transferred to an environment at approximately 20℃. An ice block with a mass of (0.55±0.05) g was placed on the surface, and the change in ice-melting mass Δg (n=3) was recorded within 0–20 min under vertical irradiation with a 1-sun xenon lamp. The results are as follows: Figure 11 – Figure 12 As shown.

[0071] like Figure 11 As shown, the Δg of each sample group increased with prolonged illumination time, indicating that the ice continued to melt. Under the conditions of this embodiment, the Δg of the 10% mass fraction microcapsule composite coating was higher at 20 min (e.g., approximately 0.47 g), while the Δg of the blank glass, pure resin coating, and 5% mass fraction microcapsule composite coating was relatively lower at the same time point (e.g., approximately 0.39–0.41 g), showing that higher microcapsule dosage can enhance the light-induced ice-melting effect.

[0072] like Figure 12 As shown, the appearance comparison of different groups of samples at 0 min and 20 min is as follows. Figure 11 The changes in ice melting amount were consistent: the 10% mass fraction microcapsule composite coating had less residual ice and a more obvious surface water film after 20 minutes, indicating that a higher dosage of microcapsules can improve the light-induced ice melting effect of the coating. This result shows that when the microcapsules of this invention are used as functional fillers in the coating system, they can enhance the heating and thermal buffering effects under light conditions, thereby improving the ice melting process and helping to reduce the risk of re-icing.

[0073] In summary, this invention achieves synergistic encapsulation of a phase change core material and a thermochromic color-developing system through SMA emulsification and MUF prepolymer in-situ acid-catalyzed polycondensation at the emulsion interface. The resulting microcapsules exhibit reversible thermochromic response and phase change heat buffering characteristics, and demonstrate good structural stability and response consistency under thermal cycling and light irradiation conditions. Furthermore, when these microcapsules are used as functional fillers in composite coatings, they can enhance the heating and de-icing processes under light irradiation, indicating their potential value in temperature control and surface protection (anti-icing / de-icing and de-icing assistance) applications in low-temperature environments.

[0074] The embodiments described above are merely illustrative of the technical solutions of the present invention and do not constitute a limitation on the scope of protection of the present invention. For those skilled in the art, various modifications, equivalent substitutions, or improvements can be made to the present invention without departing from the spirit and scope defined by the claims, and all such modifications, substitutions, or improvements should fall within the scope of protection of the present invention. The scope of protection of the present invention is determined by the appended claims.

Claims

1. A method for preparing photothermally responsive thermochromic microcapsules, characterized in that, Includes the following steps: (1) Preparation of core material composite solution; (2) Prepare an alkaline solution of styrene-maleic anhydride copolymer and dilute it to obtain a diluted alkaline solution of styrene-maleic anhydride copolymer; (3) The core material composite solution is added dropwise to the diluted alkaline solution of styrene-maleic anhydride copolymer, deionized water is added, and emulsification is carried out under mechanical stirring to obtain an emulsion; (4) Prepare melamine-urea-formaldehyde prepolymer. Add melamine-urea-formaldehyde prepolymer to emulsion at a rate of 3.0-10.0 mL / min to obtain a mixture. Then add deionized water to obtain a diluted mixture. Then add an acid regulator to adjust the pH of the diluted mixture to 4.0-4.5 to obtain a reaction solution. (5) The reaction solution undergoes acid-catalyzed polycondensation under mechanical stirring to form a melamine-urea-formaldehyde shell coating the core material. After the acid-catalyzed polycondensation reaction is completed, an alkaline regulator is added to adjust the pH to 6.8–7.25, and the mixture is stirred for 10–20 min to obtain a microcapsule suspension. (6) The microcapsule suspension was washed, filtered and dried to constant weight to obtain photothermal responsive thermochromic microcapsules.

2. The method for preparing a photothermal responsive thermochromic microcapsule according to claim 1, characterized in that: In the preparation of the core material composite solution, crystal violet lactone and bisphenol A are mixed at a mass ratio of 1:1 to 1:5 to obtain a colorimetric reagent mixture; Heat 8–12 g of nonanoic acid to 60–65 °C, add the colorimetric reagent mixture, and then add 2 mL of isopropanol. Stir at a constant temperature of 800–1000 r / min for 50–70 min at 65–75 °C to ensure uniform mixing and obtain the core material composite solution.

3. The method for preparing a photothermal responsive thermochromic microcapsule according to claim 2, characterized in that: The mass ratio of crystal violet lactone to bisphenol A is 1:

3.

4. The method for preparing a photothermal responsive thermochromic microcapsule according to claim 3, characterized in that: In the process of preparing an alkaline solution of styrene-maleic anhydride copolymer and diluting it to obtain a diluted alkaline solution of styrene-maleic anhydride copolymer, Styrene-maleic anhydride copolymer was mixed with 3M sodium hydroxide and stirred for 30 minutes at a temperature of 70°C. After stirring, deionized water was added every 30 minutes for a total of 6 times. After the reaction was completed, an acidic regulator was added dropwise to bring the pH back to 10, resulting in a 2 wt% alkaline solution of styrene-maleic anhydride copolymer. Deionized water was added to a 2wt% alkaline solution of styrene-maleic anhydride copolymer, with a mass ratio of deionized water to styrene-maleic anhydride copolymer alkaline solution of 5:

1. The solution was stirred at 70℃ for 30 min to obtain a diluted alkaline solution of styrene-maleic anhydride copolymer.

5. The method for preparing a photothermal responsive thermochromic microcapsule according to claim 4, characterized in that: The core material composite solution was added dropwise to the diluted styrene-maleic anhydride copolymer emulsion system, deionized water was added, and emulsification was carried out under mechanical stirring conditions. The emulsification temperature was 65–75℃, the mechanical stirring rate was 800–1000 r / min, and the emulsification time was 50–70 min.

6. The method for preparing a photothermal responsive thermochromic microcapsule according to claim 5, characterized in that, The preparation method of melamine-urea-formaldehyde prepolymer is as follows: urea, melamine, formaldehyde aqueous solution and deionized water are mixed, nano titanium dioxide is added and mechanically stirred for 10 min under 70℃ water bath conditions, an alkaline regulator is added to adjust the pH to 10.0±0.1, and the reaction is continued to be stirred for 1 h at 70℃ to obtain melamine-urea-formaldehyde prepolymer; The mass ratio of urea to melamine solids was 1.625:1, and the mass ratio of 37wt% formaldehyde aqueous solution to deionized water was 1.85:

1. The amount of urea used was 1.82–2.73 g, and the amount of deionized water used was 4–6 mL.

7. The method for preparing a photothermal responsive thermochromic microcapsule according to claim 6, characterized in that: The acid-catalyzed polycondensation reaction was carried out at a temperature of 80–95℃, a mechanical stirring rate of 800–1500 r / min, and a reaction time of 90–160 min.

8. The method for preparing a photothermal responsive thermochromic microcapsule according to claim 7, characterized in that: The nano-titanium dioxide has a particle size of 10–200 nm and is added in an amount of 0.1–5 wt% based on the mass of the melamine-urea-formaldehyde prepolymer.

9. A photothermal-responsive thermochromic microcapsule, characterized in that, Includes the core material and the shell layer surrounding the core material; The core material is made from the following raw materials in parts by weight: 8–12 g of nonanoic acid, and the mass ratio of crystal violet lactone to bisphenol A is 1:1–1:

5. The shell is made from the following raw materials in parts by weight: the solid mass ratio of urea to melamine and formaldehyde is 1.625:1:2.445; Nano-titanium dioxide is dispersed in the shell, and the amount of nano-titanium dioxide added is 0.1–5 wt% of the shell mass.

10. A photothermal responsive thermochromic microcapsule according to claim 9, characterized in that: The particle size of the photothermal responsive thermochromic microcapsules is 5–50 μm.

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