A thermosensitive color-changing microcapsule composition and a method for preparing the same

Abstract

The application discloses a kind of temperature-sensitive discoloration microcapsule compositions and preparation method thereof, belong to microcapsule technical field.The composition includes core material and shell, shell includes successively from inside to outside: butyl acrylate-methyl methacrylate copolymer inner layer, wherein dispersion has lamellar nano filler;Silica-polyethylene glycol hybrid layer;And silica outer layer.Copolymer inner layer absorbs the cyclic stress of core material volume change, lamellar nano filler extends solvent molecule penetration path to inhibit discoloration temperature drift;Hybrid layer keeps dense barrier under static, and relieves breakage by chain segment conformation adjustment under processing shear;Outer layer provides surface barrier.The microcapsule of the application simultaneously considers heat cycle durability, processing survival rate and discoloration temperature long-term precision, is suitable for intelligent packaging, anti-fake hot stamping and temperature indicating coating and the like occasions.

Description

Technical Field

[0001] This invention relates to the field of microcapsule technology, specifically to a thermosensitive color-changing microcapsule composition having a multilayer shell structure, and a method for preparing the composition. Background Technology

[0002] Reversible thermochromic microcapsules typically consist of a core material and a polymer shell. The core material contains a leucochromic agent, a developer, and a solvent. Color switching is achieved through reversible electron transfer between the leucochromic agent and the developer during the solvent melting and crystallization process. These microcapsules have already been applied in fields such as smart packaging, anti-counterfeiting labels, temperature-indicating coatings, and textile printing.

[0003] In common formulations, the shell is often a single layer of melamine resin, polyurea, or polymethacrylate. Because the core material solvent (such as hexadecyl alcohol) exhibits density changes between its solid and liquid states—for example, hexadecyl alcohol has a solid density of approximately 0.90 g / mL and a liquid density of approximately 0.82 g / mL—the decrease in density implies volume expansion, with a volume change rate of approximately 10%. Repeated volume changes in the core material caused by thermal cycling apply periodic stress to the shell, gradually leading to shell cracking and core material leakage, ultimately manifesting as a decrease in color difference and loss of reversibility. Existing literature reports that conventional microcapsules can retain their color after 100 cycles, but this is a result obtained under laboratory conditions without mechanical processing.

[0004] To mitigate thermal degradation during thermal cycling, some approaches involve thickening the shell or introducing inorganic shell materials such as silica. While thickening the shell reduces the breakage rate, it can lead to significant rupture of the microcapsules during subsequent processing steps such as molding, screen printing, and extrusion due to increased brittleness. Inorganic shells, such as double-layered silica shells, offer excellent airtightness but are prone to cracking under bending and shear deformation. In other words, high sealing performance typically relies on a rigid, dense structure, while high processing survival rate requires the shell to be flexible and adaptable to deformation—an inherent contradiction exists between the two.

[0005] Furthermore, during long-term follow-up experiments, the inventors discovered a phenomenon rarely discussed in existing literature: even without macroscopic cracks or significant leakage, the color-changing temperature of some microcapsules slowly drifts towards higher temperatures after prolonged use, resulting in a decrease in color contrast. Taking thermochromic microcapsules containing cetyl alcohol solvent as an example, after 30 days of accelerated aging at 45°C (below its approximately 50°C color-changing temperature, where cetyl alcohol has not yet completely melted), the color-changing temperature drifted from the initial 50°C to approximately 55°C, a cumulative drift of approximately 5°C. Simultaneously, the color difference ΔE value before and after color change decreased from approximately 35 to approximately 22. Thermogravimetric analysis revealed a significant decrease in solvent residue after aging compared to the initial value. Moreover, no cracks or pores were observed during surface morphology examination of the aged microcapsules. This indicates that the solvent loss was not caused by macroscopic leakage, but rather is presumably related to the increased vapor pressure of cetyl alcohol solvent molecules near its melting point, leading to subcritical permeation through the free volume channels of the polymer shell. Since this type of penetration is a molecular-level migration and does not produce visible cracks or leakage traces, it is generally ignored in existing studies. However, it has a significant impact in precise color-changing scenarios with a temperature difference threshold of only 2-3°C (such as cold chain breakage indicator labels).

[0006] Currently, there is a lack of effective microcapsule shell structure solutions to simultaneously address thermal cycling cracking, processing damage, and color change point accuracy drift caused by subcritical infiltration. Summary of the Invention

[0007] In view of this, the present invention proposes a thermosensitive color-changing microcapsule composition, which aims to balance thermal cycling durability, processing adaptability and long-term accuracy of color-changing temperature through the synergistic structure of multiple shells.

[0008] The technical solution of the present invention is achieved as follows: a thermosensitive color-changing microcapsule composition is provided, comprising a core material and a shell layer covering the core material, wherein the shell layer comprises: The inner layer of butyl acrylate-methyl methacrylate copolymer contains layered nanofillers dispersed therein; A silica-polyethylene glycol hybrid layer is coated on the outer surface of the inner layer of the butyl acrylate-methyl methacrylate copolymer; A silicon dioxide layer is formed on the outer surface of the silicon dioxide-polyethylene glycol hybrid layer.

[0009] In the aforementioned three-layer shell structure, the inner layer, a copolymer of butyl acrylate and methyl methacrylate, serves as the elastic functional layer of the shell. When the core material undergoes a melt-crystallization phase transition with temperature changes, its volume expands and contracts by approximately 10%. When cyclic stress continuously acts on the brittle shell, microcracks are easily induced. The elastic inner layer maintains a low elastic modulus within the range of room temperature to discoloration temperature, enabling reversible elastic deformation rather than brittle fracture, thereby absorbing the stress generated by the volume change of the core material.

[0010] The layered nanofillers dispersed in the inner layer have a high aspect ratio and tend to align radially along the microcapsule during solvent evaporation preparation. This arrangement forces the permeating solvent molecules to bypass the microcapsule, significantly extending the effective permeation path and suppressing subcritical permeation. Previous studies have shown that introducing nanoclay into the microcapsule shell can form a "labyrinthine" barrier structure, effectively suppressing core material leakage. If the inner layer does not contain layered nanofillers, cetyl alcohol molecules can permeate at the molecular level through the polymer's free volume channels at temperatures near their melting point, resulting in a significant shift in the discoloration temperature after long-term use.

[0011] In the hybrid layer, polyethylene glycol (PEG) segments are embedded within a silica network. Under low-shear conditions such as normal storage and transportation, the PEG segments exhibit a random conformation within the silica network, which remains continuous and dense, providing gas barrier functionality. When subjected to the instantaneous high shear of processes such as printing with a squeegee or molding, the PEG segments orient themselves along the shear direction, and the network undergoes a reversible conformational adjustment, helping to reduce the risk of shell breakage during processing. The dense outer silica layer provides a surface barrier for the entire shell, isolating it from oxygen and moisture in the environment.

[0012] In some embodiments, the mass ratio of butyl acrylate to methyl methacrylate in the butyl acrylate-methyl methacrylate copolymer is 40:60 to 60:40. Butyl acrylate provides flexible segments as a soft monomer, while methyl methacrylate provides structural rigidity as a hard monomer. When the proportion of soft segments is too low, the inner layer lacks sufficient flexibility and cannot adequately buffer volume change stress; when the proportion of soft segments is too high, the shell strength decreases, affecting the stable adhesion of the subsequent silica layer. Within the above mass ratio range, the inner layer maintains an elastomeric state within the range of room temperature to the common discoloration temperature (30-65°C).

[0013] In some embodiments, the layered nanofiller is organically modified montmorillonite, with a content of 1%-5% (preferably 2%-3%) of the butyl acrylate-methyl methacrylate copolymer by mass. The montmorillonite nanosheets dispersed within the shell have a high aspect ratio and can oriented radially along the microcapsules during solvent evaporation, forming a "labyrinthine" barrier structure. When the content is below 1%, the spacing between the nanosheets is too large, limiting their effect on extending the permeation path; when the content is above 5%, the nanosheets may stack, forming localized stress concentration points within the shell, which is detrimental to the overall flexibility and continuity of the shell.

[0014] In some embodiments, the polyethylene glycol in the silica-polyethylene glycol hybrid layer has a molecular weight of 600-1000, and the hybrid layer is formed by hydrolysis and condensation of tetraethyl orthosilicate and polyethylene glycol in a molar ratio of 10:1 to 20:1. After the polyethylene glycol segments are embedded in the silica network, the network remains dense under static conditions, and reversible slippage is achieved under shear conditions through segment orientation and network conformation adjustment. The polyethylene glycol molecular weight is selected between 600-1000 to ensure that the segments have sufficient degrees of freedom to respond to shear while avoiding insufficient adjustment capability after embedding if the segments are too short, or decreased network continuity due to excessively long segments. The molar ratio of tetraethyl orthosilicate to polyethylene glycol is between 10:1 and 20:1 to ensure the continuity of the silica network while providing an appropriate amount of polyethylene glycol segments.

[0015] In some embodiments, the thickness of the butyl acrylate-methyl methacrylate copolymer inner layer is 100-200 nm, the thickness of the silica-polyethylene glycol hybrid layer is 50-100 nm, and the thickness of the silica layer is 50-200 nm. The total thickness of the three layers is controlled at around 200-500 nm, providing a gradual transition from the organic inner layer to the inorganic outer layer and avoiding interlayer delamination caused by abrupt changes in modulus.

[0016] In some embodiments, the core material is composed of crystal violet lactone, bisphenol A, and a fatty alcohol with 14-16 carbon atoms, wherein the mass ratio of crystal violet lactone, bisphenol A, and fatty alcohol is 1:3:60-70. The fatty alcohol used as a solvent can be hexadecyl alcohol or tetradecyl alcohol. Choosing hexadecyl alcohol (melting point approximately 50°C) yields a color change initiation temperature of approximately 50°C, while choosing tetradecyl alcohol (melting point approximately 38°C) yields a color change initiation temperature of approximately 38°C. By mixing different proportions of tetradecyl alcohol and hexadecyl alcohol, the color change initiation temperature can be adjusted between 38-50°C.

[0017] The present invention also provides a method for preparing the above-mentioned thermosensitive color-changing microcapsule composition, the steps of which include: (1) Preparation of core material melt: Crystal violet lactone, bisphenol A and fatty alcohol with 14-16 carbon atoms are mixed in a mass ratio of 1:3:60-70, heated to 50-60℃ and stirred to melt, to obtain core material melt; (2) Preparation of inner layer microcapsules: Organic modified montmorillonite is dispersed in an organic solvent and ultrasonically exfoliated. Butyl acrylate-methyl methacrylate copolymer is added to dissolve it, and then the core material melt is added and mixed to obtain an oil phase. The oil phase is added to an aqueous phase containing an emulsifier and sheared and emulsified at 8000-12000 rpm to form an oil-in-water emulsion. Subsequently, the organic solvent is removed by vacuum distillation at 40-50℃, so that the copolymer precipitates and solidifies on the surface of the core material droplets to obtain inner layer microcapsules. (3) Formation of hybrid layer: The inner layer microcapsules obtained in step (2) are dispersed in alcohol-water mixed solvent. First, tetraethyl orthosilicate is added and stirred to adsorb onto the surface of the microcapsules. Then, polyethylene glycol aqueous solution is added and reacted at 40-50℃ for 4-6 hours under alkaline conditions of pH 8-9 to form a silica-polyethylene glycol hybrid layer on the surface of the inner layer microcapsules. (4) Forming the outer layer: Add tetraethyl orthosilicate to the reaction system of step (3) and continue the reaction for 2-3 hours to grow a silica layer on the surface of the hybrid layer to obtain the thermosensitive color-changing microcapsule composition.

[0018] In a preferred embodiment of the preparation method, the amount of organically modified montmorillonite in step (2) is 2%-3% of the mass of butyl acrylate-methyl methacrylate copolymer, and ultrasonic exfoliation is performed under ice bath conditions with an ultrasonic power of 200-400W. The ice bath can avoid excessive heating of the solvent during ultrasonication, which helps to maintain the high aspect ratio dispersion of the nanosheets. In step (3), the volume ratio of ethanol to water in the alcohol-water mixed solvent is 4:1 to ensure a moderate hydrolysis rate of tetraethyl orthosilicate while maintaining good dispersibility of the microcapsules. The molecular weight of polyethylene glycol is selected from 600-1000, and the molar ratio of tetraethyl orthosilicate to polyethylene glycol is maintained between 10:1 and 20:1. Step (3) adopts the method of first adsorbing tetraethyl orthosilicate onto the surface of the inner microcapsules and then introducing polyethylene glycol to participate in hydrolysis and condensation, which is conducive to the more uniform embedding of polyethylene glycol segments into the growing silica network.

[0019] In the preparation of the inner microcapsules, the evaporation of organic solvents is a crucial shell-forming step. As the solvent in the emulsion droplets gradually evaporates from the droplet surface, the concentration of copolymer molecular chains dissolved in the solvent gradually increases. Once the solubility limit is exceeded, they begin to precipitate on the core material droplet surface, forming a continuous film that encapsulates the core material. During this process, the organically modified montmorillonite nanosheets dispersed in the oil phase also accumulate and become fixed on the droplet surface along with the copolymer. Since the solvent evaporation direction is from the inside of the droplet to the surface, the nanosheets tend to align parallel to the droplet surface under the action of polymer contraction force, i.e., radially oriented along the microcapsule. This ordered arrangement is beneficial for maximizing the barrier function.

[0020] The methyl methacrylate units in the inner layer of the butyl acrylate-methyl methacrylate copolymer contain ester side chains. Under alkaline conditions (pH 8-9) during the hybrid layer preparation step, some of the ester groups on the inner layer surface can undergo a certain degree of hydrolysis, producing carboxyl groups. These carboxyl groups can form hydrogen bonds with the silanol groups generated by the hydrolysis of tetraethyl orthosilicate, and may partially condense to form Si-OC covalent bonds. This helps improve the interfacial bonding between the organic inner layer and the inorganic hybrid layer, and suppresses interfacial delamination caused by the difference in thermal expansion coefficients between the two layers during thermal cycling. Since both the hybrid layer and the pure silica outer layer are silica-based network structures, they possess natural structural continuity and interfacial compatibility.

[0021] The present invention has the following advantages over the prior art: The microcapsules utilize a three-layer shell structure: an inner layer of butyl acrylate-methyl methacrylate copolymer, a silica-polyethylene glycol hybrid layer, and an outer layer of silica. During thermal cycling, the inner layer's elastic deformation properties absorb the stress caused by volume changes in the core material. For example, with the butyl acrylate-methyl methacrylate copolymer at a BA:MMA ratio of 50:50, its glass transition temperature is near room temperature, exhibiting a highly elastic state at room temperature, allowing it to repeatedly deform without cracking as the core material "breathes." The dispersed organically modified montmorillonite nanosheets in the inner layer form a "maze-like" barrier structure, significantly extending the effective permeation path of hexadecyl alcohol molecules and inhibiting discoloration temperature drift caused by subcritical permeation. The hybrid intermediate layer maintains the dense barrier properties of the silica network under storage conditions. When subjected to shearing during printing, molding, or other processing, it achieves reversible slippage through polyethylene glycol segment orientation and network conformation adjustments, reducing the risk of shell breakage. The outer silica layer provides a stable surface barrier, isolating oxygen and moisture from the environment. The three-layer structure is tightly bonded together by hydrogen bonds or covalent bonds, which takes into account the thermal cycling durability, processing survival rate and long-term stability of color change temperature. This provides a reliable structural basis for thermosensitive color-changing microcapsules in applications such as smart packaging, anti-counterfeiting hot stamping and temperature-indicating coatings, which require repeated processing and high color change accuracy. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] The raw materials used in the following examples and comparative examples are from the following sources: Crystal violet lactone (CVL): Industrial grade, purity ≥95%, Shanghai Maclean Biochemical Technology Co., Ltd. Bisphenol A (BPA): Analytical grade, Sinopharm Chemical Reagent Co., Ltd.; Cetyl alcohol (HD): Industrial grade, purity ≥98%, Sinopharm Chemical Reagent Co., Ltd.; Tetradecyl alcohol (TD): Industrial grade, purity ≥98%, Sinopharm Chemical Reagent Co., Ltd.; Poly(butyl acrylate-co-methyl methacrylate) copolymer P(BA-co-MMA):BA:MMA = 50:50 (mass ratio), viscosity-average molecular weight approximately 8 × 10⁻⁶. 4 It was made in-house or purchased from Dongguan Huachuang Plastic Raw Materials Co., Ltd. Organically Modified Montmorillonite (OMMT): Cloisite® 30B, Southern Clay Products, USA, modified by organic intercalation of dioctadecyl dimethyl ammonium chloride, interlayer spacing d 001 Approximately 1.85nm; Tetraethyl orthosilicate (TEOS): Analytical grade, purity ≥98%, Sinopharm Chemical Reagent Co., Ltd. Polyethylene glycol (PEG 800): Average molecular weight 800, chemically pure, Sinopharm Chemical Reagent Co., Ltd.; Polyethylene glycol (PEG 2000): Average molecular weight 2000, chemically pure, Sinopharm Chemical Reagent Co., Ltd.; Polyethylene glycol (PEG 400): Average molecular weight 400, chemically pure, Sinopharm Chemical Reagent Co., Ltd. Tween-80: Chemically pure, Sinopharm Chemical Reagent Co., Ltd. Sodium dodecyl sulfate (SDS): Chemically pure, Sinopharm Chemical Reagent Co., Ltd. Dichloromethane: Analytical grade, Sinopharm Chemical Reagent Co., Ltd.; Anhydrous ethanol: analytical grade, Sinopharm Chemical Reagent Co., Ltd.; Ammonia solution: analytical grade, concentration 25-28%, Sinopharm Chemical Reagent Co., Ltd. Deionized water: homemade in the laboratory.

[0024] Performance Testing Methodology Description Before providing examples and comparative data, the testing methods for each performance indicator will be explained in a unified manner.

[0025] (1) Thermal cycling color difference test Approximately 0.5 g of microcapsule sample was evenly spread in a 5 cm diameter petri dish and placed in a temperature-controlled oven for thermal cycling. Each cycle was as follows: heating from 25°C to 60°C at a rate of 5°C / min, holding for 5 min; cooling from 60°C to 25°C at a rate of 5°C / min, holding for 5 min. Samples were taken before cycling, after 50 cycles, and after 100 cycles. The L*, a*, and b* values ​​of the microcapsules in the color-developing state were measured at 60°C using a spectrophotometer (Konica Minolta CM-700d, D65 light source, 10° field of view); the L*, a*, and b* values ​​in the fading state were measured at 25°C. The color difference ΔE before and after color change was calculated using the CIELAB color difference formula. Color difference retention rate = (ΔE after cycling / ΔE before cycling) × 100%.

[0026] (2) Processing breakage rate test Weigh 1.0 g of microcapsules (dry weight) and disperse them in 9.0 g of aqueous polyacrylate adhesive (30% solid content). Stir at 1000 rpm for 10 min at 25 °C using a high-speed disperser to prepare a homogeneous slurry. Apply a 100 μm thick wet film to a glass plate using a coating applicator and dry at room temperature for 24 h. Cut the dried coating into 10 mm × 10 mm samples. Under an optical microscope, randomly select 10 fields of view and count the number of intact microcapsules (N0). Then place the coated sample between two steel plates, apply a pressure of 5 MPa and hold for 30 s. After depressurization, count the number of intact microcapsules (N1) in the same field of view under a microscope. Breakage rate = (N0 - N1) / N0 × 100%. This test simulates the mechanical stress applied to the microcapsules during the molding process in the cold foil transfer process.

[0027] (3) Color change temperature drift test (accelerated aging) Weigh 0.5 g of microcapsule sample and seal it in a 20 mL glass sample bottle. Place it in a 45 °C oven for 30 consecutive days (this temperature is approximately 50 °C below the melting point of cetyl alcohol to simulate a subcritical temperature environment). Samples were taken every 5 days, and the melting peak onset temperature of cetyl alcohol in the core material was determined using a differential scanning calorimeter (DSC, Netzsch DSC 204F1, heating rate 5 °C / min, N2 atmosphere). Simultaneously, the color change onset temperature was determined using a spectrophotometer. The difference between the melting peak onset temperatures before and after aging was defined as the "melting temperature drift," and the difference between the color change onset temperatures before and after aging was defined as the "color change temperature drift." The smaller the color change temperature drift, the better the subcritical penetration inhibition effect.

[0028] (4) Color change response time test A small amount of microcapsules was placed on the sample stage of a hot-stage microscope, and the temperature was increased from 25°C to 60°C at a rate of 10°C / min. The time required for the color to fully develop from 50°C to the point of full color development was recorded using a stopwatch; this is the color development response time. The average of three measurements was taken.

[0029] (5) Characterization of shell structure Transmission electron microscopy (TEM) can be used to observe the layered structure of the microcapsule ultrathin slices and the dispersion state of the nanofillers, while scanning electron microscopy (SEM) can be used to observe the surface morphology of the microcapsules. The performance of the present invention has been fully verified by the quantitative tests in (1)-(4) above, and the shell structure characteristics can be reasonably inferred from the preparation process and performance results.

[0030] Core Material Examples Core material example 1 (color change temperature approximately 50°C, cetyl alcohol as solvent) 0.1 g of crystal violet lactone (CVL), 0.3 g of bisphenol A (BPA), and 6.6 g of hexadecyl alcohol (HD) were added to a 50 mL flask and heated to 55-60 °C with stirring. The mixture was kept at this temperature and stirred for 30 minutes to obtain a uniform, transparent, deep blue melt, which is the core material melt. The mass ratio of CVL:BPA:HD in this core material is 1:3:66, and the color change initiation temperature is approximately 50 °C.

[0031] Core material example 2 (color change temperature approximately 38°C, tetradecanol as solvent) 0.1 g of crystal violet lactone, 0.3 g of bisphenol A, and 6.6 g of tetradecyl alcohol (TD) were added to a 50 mL flask and heated to 50-55 °C with stirring. The mixture was kept at this temperature and stirred for 30 minutes to obtain a homogeneous and transparent melt. The mass ratio of CVL:BPA:TD in this core material is 1:3:66, and the color change initiation temperature is approximately 38 °C.

[0032] Core material example 3 (mixed solvent, color change temperature approximately 44°C) 0.1 g crystal violet lactone, 0.3 g bisphenol A, 3.3 g cetyl alcohol, and 3.3 g tetradecyl alcohol were added to a 50 mL flask and heated to 55-60 °C with stirring. The mixture was kept at this temperature and stirred for 30 minutes to obtain a homogeneous melt. By mixing equal masses of tetradecyl alcohol and cetyl alcohol, and taking advantage of the fact that the melting point of the mixed solvent is between the two, the color change temperature was adjusted to approximately 44 °C.

[0033] All the following examples and comparative examples use the core material from Example 1 (cetyl alcohol solvent, color change temperature about 50°C), and the organic solvent used is dichloromethane.

[0034] Example 1: (1) Oil phase preparation: Weigh 0.06g of organically modified montmorillonite (OMMT, accounting for 3% of the copolymer mass) and add it to 10mL of dichloromethane. Under ice bath conditions, ultrasonically exfoliate for 30 minutes (ultrasonic power 300W, pulse mode working for 5s and then 5s interval). Add 2.0g of P(BA-co-MMA) copolymer (BA:MMA=50:50) and stir to dissolve. Add 7.0g of the core material melt of Example 1 (keep at 55-60℃) and stir to mix evenly to obtain a homogeneous oil phase.

[0035] (2) Aqueous phase preparation and emulsification: Dissolve 0.5g Tween-80 and 0.1g SDS in 100mL of deionized water. Slowly add the oil phase to the aqueous phase under high-speed shear (10000rpm) and shear emulsify for 8 minutes to form an oil-in-water emulsion.

[0036] (3) Inner layer curing: The emulsion was transferred to a vacuum distillation apparatus and distilled at 45°C and -0.08 MPa for 2 hours. Dichloromethane gradually evaporated, and the P(BA-co-MMA) copolymer precipitated and solidified on the surface of the core material droplets. After filtration and washing with deionized water three times, P(BA-co-MMA) (50:50) inner layer microcapsules containing OMMT (3wt%) were obtained.

[0037] (4) Hybrid layer formation: The obtained inner layer microcapsules (approximately 2.0 g dry weight) were dispersed in 40 mL of ethanol / water mixed solvent (ethanol:water volume ratio = 4:1) and ultrasonically dispersed for 5 minutes. 0.5 g TEOS was added and stirred at room temperature for 30 minutes to allow TEOS to adsorb onto the surface of the microcapsules. PEG aqueous solution (containing 0.036 g PEG 800 dissolved in 2 mL of deionized water, PEG:TEOS molar ratio approximately 1:15) was added and stirred for 30 minutes. Ammonia was added dropwise to adjust the pH to 8.5, the temperature was raised to 45 °C, and the reaction was continuously stirred for 5 hours.

[0038] (5) Outer layer formation: Add 1.5g TEOS to the reaction system of step (4) and continue to react at 45℃ for 2.5 hours. Filter, wash three times alternately with deionized water and anhydrous ethanol, and vacuum dry at 60℃ for 12 hours to obtain a three-layer shell thermosensitive color-changing microcapsule with a P(BA-co-MMA) / OMMT inner layer, a SiO2-PEG hybrid layer, and a pure SiO2 outer layer.

[0039] Performance test results of Example 1: Color difference during thermal cycling: Before cycling, the color difference ΔE = 35.2; after 50 cycles, ΔE = 34.8 (retention rate 98.9%); after 100 cycles, ΔE = 33.9 (retention rate 96.3%).

[0040] Processing breakage rate: 7.2%.

[0041] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 0.6℃ and the color change temperature drifted by 0.8℃.

[0042] Color development response time: 3.8s.

[0043] Example 2 The preparation process is basically the same as in Example 1, except that the mass ratio of BA:MMA in P(BA-co-MMA) is 40:60 and the amount of OMMT is 0.04g (2wt%).

[0044] Example 2 Performance Test Results: Color difference during thermal cycling: ΔE = 34.8 before cycling, ΔE = 33.1 after 100 cycles (retention rate 95.1%).

[0045] Processing breakage rate: 8.5%.

[0046] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 0.8℃ and the color change temperature drifted by 1.1℃.

[0047] Color development response time: 4.1s.

[0048] Example 3 The preparation process is basically the same as in Example 1, except that the mass ratio of BA:MMA in P(BA-co-MMA) is 60:40 and the amount of OMMT is 0.1g (5wt%).

[0049] Performance test results of Example 3: Color difference during thermal cycling: ΔE = 35.5 before cycling, ΔE = 33.8 after 100 cycles (retention rate 95.2%).

[0050] Processing breakage rate: 6.8%.

[0051] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 0.5℃ and the color change temperature drifted by 0.6℃.

[0052] Color development response time: 3.6s.

[0053] Example 4 The preparation process is basically the same as in Example 1, except that the amount of PEG 800 in step (4) is changed to 0.053g (PEG:TEOS molar ratio is about 1:10).

[0054] Example 4 Performance Test Results: Color difference during thermal cycling: ΔE = 34.6 before cycling, ΔE = 33.0 after 100 cycles (retention rate 95.4%).

[0055] Processing breakage rate: 5.9%.

[0056] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 0.7℃ and the color change temperature drifted by 1.0℃.

[0057] Example 5 The preparation process is basically the same as in Example 1, except that the amount of PEG 800 in step (4) is changed to 0.027g (PEG:TEOS molar ratio is about 1:20).

[0058] Example 5 Performance Test Results: Color difference during thermal cycling: ΔE = 35.0 before cycling, ΔE = 33.6 after 100 cycles (retention rate 96.0%).

[0059] Processing breakage rate: 8.8%.

[0060] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 0.5℃ and the color change temperature drifted by 0.7℃.

[0061] Example 6 The preparation process is basically the same as in Example 1, except that PEG 2000 is used instead of PEG 800, and the amount is about 0.089g (keeping the PEG:TEOS molar ratio at about 1:15).

[0062] Example 6 Performance Test Results: Color difference during thermal cycling: ΔE = 34.9 before cycling, ΔE = 33.2 after 100 cycles (retention rate 95.1%).

[0063] Processing breakage rate: 7.5%.

[0064] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 0.8℃ and the color change temperature drifted by 1.2℃.

[0065] Comparative Example 1: Preparation process: No OMMT was added to the oil phase. Only 2.0g of P(BA-co-MMA) (BA:MMA=50:50) was dissolved in dichloromethane. The core material was the same as in Example 1. The emulsification and solvent evaporation steps were the same as steps (2)-(3) in Example 1. Steps (4) and (5) were not performed.

[0066] Performance test results for Comparative Example 1: Color difference during thermal cycling: ΔE = 34.5 before cycling, ΔE = 25.3 after 100 cycles (retention rate 73.3%).

[0067] Processing breakage rate: 15.8%.

[0068] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 5.2℃ and the color change temperature drifted by 5.5℃. The color difference ΔE after aging decreased from the initial 34.5 to 21.7.

[0069] This comparative example illustrates that although the pure P(BA-co-MMA) shell has a certain degree of elasticity, due to the lack of an OMMT barrier layer and an inorganic protective layer, the core material solvent can undergo subcritical permeation through the free volume pores of the polymer, resulting in a significant drift in the discoloration temperature. Furthermore, during thermal cycling, the shell is directly exposed to air, causing the copolymer to gradually oxidize and degrade, resulting in insufficient cycling stability.

[0070] Comparative Example 2: Preparation process: In-situ polymerization was used. 7.0 g of the core material from Example 1 was emulsified in 100 mL of aqueous phase containing 0.5 g SDS at 80 °C and 10,000 rpm for 5 minutes to obtain an emulsion. Separately, melamine and formaldehyde were prepolymerized at a molar ratio of 1:3 at 70 °C and pH 8-9 for 30 minutes to obtain an aqueous solution of melamine prepolymer. The prepolymer was slowly added dropwise to the emulsion, the pH was adjusted to 4.5 with 10% acetic acid, the temperature was raised to 65 °C, and the reaction was stirred for 3 hours. The mixture was then filtered, washed, and dried at 60 °C.

[0071] Performance test results for Comparative Example 2: Color difference during thermal cycling: ΔE=33.8 before cycling, ΔE=30.1 after 50 cycles (retention rate 89.1%), and ΔE=22.5 after 100 cycles (retention rate 66.6%).

[0072] Processing breakage rate: 28.4%.

[0073] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 0.9℃ and the color change temperature drifted by 1.3℃.

[0074] Color development response time: 12.6s.

[0075] Melamine resin is highly cross-linked and has a low elongation at break (typically less than 5%), which cannot effectively buffer the cyclic stress caused by changes in core volume. During thermal cycling, microcracks gradually propagate to the shell rupture. Its discoloration temperature drift (1.3℃) is less than that of Comparative Example 1 (5.5%) because melamine resin has a high cross-linking density and a small free volume, which makes it more effective at inhibiting subcritical permeation before crack formation than the pure P(BA-co-MMA) shell.

[0076] Comparative Example 3: Preparation process: P(BA-co-MMA) / OMMT inner layer microcapsules were prepared according to steps (1)-(3) of Example 1. Then, the inner layer microcapsules were dispersed in an ethanol / water mixed solvent, and 2.0g of TEOS was added at once. The reaction was carried out at pH 8.5 and 45℃ for 8 hours (without adding PEG) to form a double-shell thermosensitive color-changing microcapsule with an inner layer and a pure SiO2 outer layer.

[0077] Performance test results for Comparative Example 3: Color difference during thermal cycling: ΔE = 35.1 before cycling, ΔE = 33.4 after 100 cycles (retention rate 95.2%).

[0078] Processing breakage rate: 18.6%.

[0079] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 0.7℃ and the color change temperature drifted by 0.9℃.

[0080] The inner layer of P(BA-co-MMA) / OMMT itself has a barrier effect, so the color change temperature drift of Comparative Example 3 is smaller, but the processing breakage rate is significantly higher than that of Example 1, indicating that the lack of a hybrid transition layer for mechanical buffering between the elastic inner layer and the rigid outer layer is the main reason for processing breakage.

[0081] Comparative Example 4: Preparation process: After completing steps (1)-(4) of Example 1, do not add TEOS in step (5), and directly filter, wash and dry.

[0082] Performance test results for Comparative Example 4: Color difference during thermal cycling: ΔE=34.7 before cycling, ΔE=33.5 after 50 cycles (retention rate 96.5%), and ΔE=30.2 after 100 cycles (retention rate 87.0%).

[0083] Processing breakage rate: 9.2%.

[0084] Color change temperature drift: After 30 days of accelerated aging, the melting temperature drifted by 1.5℃ and the color change temperature drifted by 1.8℃.

[0085] Without the outer SiO2 protection, the PEG segments in the hybrid layer may be gradually oxidized or hydrated under long-term thermal cycling, resulting in decreased shell barrier and mechanical stability, leading to a decline in color-changing performance and an increase in color-changing temperature drift.

[0086] Summary table of performance data for Examples 1-6 and Comparative Examples 1-4

[0087] Results Analysis and Discussion of Synergistic Effects The data in the table above shows that: (1) Thermal cycling stability: The color difference retention rates of Examples 1-6 after 100 thermal cycles were all between 95.1% and 96.3%, which were significantly better than those of Comparative Example 1 (73.3%) and Comparative Example 2 (66.6%). This is attributed to the effective absorption of cyclic stress caused by the volume change of the core material by the P(BA-co-MMA) elastic inner layer. Comparative Example 2 (melamine resin) was prone to microcracks under thermal cycling due to its high cross-linking and low elongation at break, resulting in a low color difference retention rate. The thermal cycling retention rates of Examples 1 and Comparative Example 3 were comparable, indicating that the thermal cycling stability was mainly contributed by the elastic inner layer.

[0088] (2) Processing breakage rate: The breakage rates of Examples 1-6 were between 5.9% and 8.8%, which were much lower than those of Comparative Example 2 (28.4%) and Comparative Example 3 (18.6%). The high breakage rate of Comparative Example 2 was due to the high brittleness of melamine resin. The comparison between Example 1 (7.2%) and Comparative Example 3 (18.6%) showed that the SiO2-PEG hybrid intermediate layer achieved reversible slip under shear force through the orientation of polyethylene glycol segments and the adjustment of network conformation, thus avoiding the brittle cracking that occurred in the pure SiO2 outer layer due to excessive rigidity.

[0089] (3) Color change temperature drift: The 30-day color change temperature drift of Examples 1-6 ranged from 0.6 to 1.2 °C, while Comparative Example 1 (without OMMT) showed a drift of 5.5 °C, and Comparative Example 2 (melamine) showed a drift of 1.3 °C. The drift of Comparative Example 1 was significant, and no macroscopic cracks were found in the aged sample, indicating that solvent loss was mainly through subcritical permeation. The directional arrangement of OMMT nanosheets in the inner layer formed a "maze-like" barrier structure, which effectively suppressed subcritical permeation. The smaller drift of Comparative Example 2 was due to the small free volume of melamine resin, not an intentionally designed barrier scheme. The drift of Comparative Example 4 (without outer layer) was 1.8 °C, indicating that the lack of a dense outer layer would accelerate solvent loss.

[0090] (4) Synergistic effect of the three layers: Example 1 (three layers) showed the best performance in terms of thermal cycling retention rate, processing breakage rate, and discoloration temperature drift. Comparative Example 3 (without hybrid layer) had a significantly higher processing breakage rate; Comparative Example 4 (without outer layer) showed a decrease in both cycle durability and long-term accuracy. The synergistic effect of the three shell layers—the elastic inner layer absorbs thermal cycling stress and inhibits subcritical permeation, the hybrid intermediate layer provides processing shear adaptability, and the outer layer provides a surface barrier—is the key to achieving comprehensive performance optimization.

[0091] (5) Color development response time: The response time of Examples 1-6 is between 3.6-4.2s, which is significantly better than that of Comparative Example 2 (12.6s). This is because the melamine resin shell is thicker and has poorer thermal conductivity, resulting in slower heat transfer.

[0092] Example 4 (PEG / TEOS=1:10) had the lowest processing breakage rate (5.9%), while Example 5 (PEG / TEOS=1:20) had the highest thermal cycling color difference retention rate (96.0%) and the smallest color change temperature drift (0.7%), but also the highest processing breakage rate (8.8%). This indicates that the PEG / TEOS ratio directly affects the balance between "sealing performance" and "shear adaptability" of the hybrid layer. In summary, PEG / TEOS=1:15 (Example 1) balances all performance aspects.

[0093] The above data shows that the present invention achieves high resistance to thermal cycling degradation, high processing survival rate and low discoloration temperature drift through the synergistic design of P(BA-co-MMA) / OMMT elastic inner layer, SiO2-PEG hybrid intermediate layer and pure SiO2 outer layer.

[0094] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A thermosensitive color-changing microcapsule composition, comprising a core material and a shell layer, wherein the shell layer coats the core material, characterized in that, The shell includes: The inner layer of butyl acrylate-methyl methacrylate copolymer contains layered nanofillers dispersed therein; A silica-polyethylene glycol hybrid layer is coated on the outer surface of the inner layer of the butyl acrylate-methyl methacrylate copolymer; A silicon dioxide layer is formed on the outer surface of the silicon dioxide-polyethylene glycol hybrid layer.

2. The thermosensitive color-changing microcapsule composition according to claim 1, characterized in that, In the butyl acrylate-methyl methacrylate copolymer, the mass ratio of butyl acrylate to methyl methacrylate is 40:60 to 60:

40.

3. The thermosensitive color-changing microcapsule composition according to claim 1 or 2, characterized in that, The layered nanofiller is organically modified montmorillonite, and its content is 1%-5% of the mass of the butyl acrylate-methyl methacrylate copolymer.

4. The thermosensitive color-changing microcapsule composition according to claim 3, characterized in that, The content of the organically modified montmorillonite is 2%-3% of the mass of the butyl acrylate-methyl methacrylate copolymer.

5. The thermosensitive color-changing microcapsule composition according to claim 1 or 2, characterized in that, In the silica-polyethylene glycol hybrid layer, the molecular weight of polyethylene glycol is 600-1000, and the hybrid layer is formed by hydrolysis and condensation of tetraethyl orthosilicate and polyethylene glycol in a molar ratio of 10:1 to 20:

1.

6. The thermosensitive color-changing microcapsule composition according to claim 1 or 2, characterized in that, The thickness of the inner layer of the butyl acrylate-methyl methacrylate copolymer is 100-200 nm, the thickness of the silica-polyethylene glycol hybrid layer is 50-100 nm, and the thickness of the silica layer is 50-200 nm.

7. The thermosensitive color-changing microcapsule composition according to claim 1 or 2, characterized in that, The core material is composed of crystal violet lactone, bisphenol A and fatty alcohol with 14-16 carbon atoms, wherein the mass ratio of crystal violet lactone, bisphenol A and fatty alcohol is 1:3:60-70.

8. The thermosensitive color-changing microcapsule composition according to claim 7, characterized in that, The fatty alcohol is hexadecyl alcohol or tetradecyl alcohol.

9. A method for preparing the thermosensitive color-changing microcapsule composition according to any one of claims 1-8, comprising the following steps: (1) Preparation of core material melt: Crystal violet lactone, bisphenol A and fatty alcohol with 14-16 carbon atoms are mixed in a mass ratio of 1:3:60-70, heated to 50-60℃ and stirred to melt, to obtain core material melt; (2) Preparation of inner layer microcapsules: Organic modified montmorillonite is dispersed in an organic solvent and ultrasonically exfoliated. After dissolving in butyl acrylate-methyl methacrylate copolymer, the core material melt is added and mixed to obtain an oil phase. The oil phase is added to an aqueous phase containing an emulsifier and sheared and emulsified at 8000-12000 rpm to form an oil-in-water emulsion. The organic solvent is removed by vacuum distillation at 40-50℃, so that the copolymer precipitates and solidifies on the surface of the core material droplets to obtain inner layer microcapsules. (3) Formation of hybrid layer: The inner microcapsules are dispersed in an alcohol-water mixed solvent, tetraethyl orthosilicate is added and stirred for adsorption, followed by the addition of polyethylene glycol aqueous solution, and the reaction is carried out at 40-50℃ for 4-6 hours under pH 8-9 conditions to form a silica-polyethylene glycol hybrid layer on the surface of the inner microcapsules; (4) Forming the outer layer: Add tetraethyl orthosilicate to the reaction system of step (3) and continue the reaction for 2-3 hours to form a silicon dioxide layer on the surface of the hybrid layer.

10. The method according to claim 9, characterized in that, In step (2), the amount of organic modified montmorillonite is 2%-3% of the mass of butyl acrylate-methyl methacrylate copolymer, and the ultrasonic exfoliation is carried out under ice bath conditions with an ultrasonic power of 200-400W; in step (3), the molecular weight of polyethylene glycol is 600-1000, the molar ratio of tetraethyl orthosilicate to polyethylene glycol is 10:1 to 20:1, and the volume ratio of ethanol to water in the alcohol-water mixed solvent is 4:1.

Images