Thermal response dynamic color change pearlescent pigment and preparation method thereof

Abstract

The invention relates to a thermal response multistage reversible color-changing pearlescent pigment and a preparation method thereof, and belongs to the technical field of functional color-changing materials. The pigment is based on a temperature zone response programmable control principle, three thermochromic dyes are adopted to correspond to color change temperature zones of 20 DEG C, 35 DEG C and 50 DEG C respectively, the color change process is sequentially carried out according to a set sequence by regulating and controlling the type and the wall thickness of a coating material of a microcapsule, and a staged and reversible dynamic color change system is constructed. The color-changing dye is coated with chitosan, gelatin or a cross-linked substance thereof, and has good biodegradability. The microcapsule is loaded on the surface of the pearlescent master batch and further coats the inorganic reflection shell to form a core-shell type three-layer structure, so that the color stability and the light interference effect are enhanced. The multi-temperature-zone color changing sequence is accurately controlled, the optical performance is enhanced, the environmental protection performance of the material is improved, and the method is suitable for application scenes of anti-counterfeit labels, temperature sensing indication materials and degradable packages.

Description

Technical Field

[0001] This invention relates to the field of functional pigment materials technology, specifically to a thermally responsive multi-level reversible color-changing pearlescent pigment and its preparation method. Background Technology

[0002] Thermochromic materials exhibit reversible color responses to temperature changes, possessing excellent visualization and stimulus-response capabilities, making them widely applicable in anti-counterfeiting, temperature indication, smart packaging, and heat-sensitive decoration. Commonly used thermochromic dyes include crystal violet lactate derivatives, liquid crystal dyes, and fluorescent thermosensitive dyes, which can display and fade colors within a set temperature range. However, existing thermochromic systems mostly offer single-temperature responses, with limited dimensions of color change, making it difficult to meet the needs of multi-level temperature control, complex information expression, and advanced anti-counterfeiting.

[0003] Pearl pigments are a class of functional pigments with a plate-like structure and light interference effect. They possess high brightness, good saturation, and angle dependence, and are widely used in inks, anti-counterfeiting coatings, and visual decorative materials. In recent years, some studies have attempted to combine thermochromic dyes with pearlescent materials to endow them with temperature-responsive capabilities. However, most of these studies focus on color change in a single temperature range and simple physical blending, lacking systematic design and functional synergy.

[0004] Furthermore, commonly used microcapsule wall materials are mainly polyurethane or acrylate polymers, which suffer from poor biodegradability and high environmental residue risks, limiting their application in biodegradable packaging and green inks. Therefore, there is an urgent need to develop a high-performance pearlescent pigment system with multi-temperature zone reversible color-changing function, structural optics synergy effect, and good environmental adaptability to meet the comprehensive requirements of green materials and intelligent response functions. Summary of the Invention

[0005] To address the shortcomings mentioned in the background section, the present invention aims to overcome the problems of existing thermochromic materials, such as a single response temperature range, limited structural functionality, and poor environmental adaptability, and provides a thermo-responsive, multi-level reversible color-changing pearlescent pigment and its preparation method. This pigment possesses a clear temperature response sequence, good reversibility, structural stability, and biodegradability, making it suitable for anti-counterfeiting identification, temperature-sensing indication, and green packaging applications.

[0006] To achieve the above objectives, the present invention provides a thermo-responsive, dynamically color-changing pearlescent pigment, comprising the following raw materials in parts by weight:

[0007] 50-85 parts of pearlescent masterbatch substrate, selected from natural mica, titanium dioxide, tin oxide or their composite sheet-like inorganic materials;

[0008] 2-6 portions of the first thermochromic dye microcapsule, with a color change response temperature range of 15-25℃;

[0009] 2-6 portions of the second thermochromic dye microcapsule, with a color change response temperature range of 30-40℃;

[0010] 2-6 portions of the third thermochromic dye microcapsule, with a color change response temperature range of 45-55℃;

[0011] The microcapsule wall material consists of 5-15 parts, wherein the wall material is selected from chitosan, gelatin or their cross-linked network materials; the inorganic reflective shell material consists of 1-5 parts, selected from silica, titanium dioxide or their composites; the dispersing stabilizer consists of 0.1-2.0 parts; and the deionized water consists of 100-200 parts.

[0012] Optionally, in the aforementioned thermo-responsive dynamic color-changing pearlescent pigment, three thermochromic dyes are encapsulated by the aforementioned wall material to form microcapsules, and then sequentially composited onto the surface of the pearlescent masterbatch. An inorganic shell material is then used to form a core-shell three-layer structure. The resulting pigment exhibits green, blue, and red colors when heated to 20°C, 35°C, and 50°C, respectively.

[0013] Optionally, in the thermo-responsive dynamic color-changing pearlescent pigment, the first, second, and third thermochromic dyes are respectively selected from a thermochromic complex formed by crystal violet and organic acid, a cholesteric liquid crystal dye, and a fluorescent thermosensitive dye.

[0014] Optionally, in the aforementioned thermo-responsive dynamic color-changing pearlescent pigment, the microcapsule wall material is a chitosan and gelatin composite material, compounded in a mass ratio of 1:1 to 3:1, with a crosslinking degree of 10 to 25% and a coating layer thickness of 80 to 150 nm.

[0015] Optionally, the method for preparing the thermo-responsive, dynamically color-changing pearlescent pigment and the same comprises the following steps:

[0016] S1. Dissolve three thermochromic dyes in organic solvents, add chitosan and gelatin solutions, and prepare microcapsules of the three thermochromic dyes by emulsion polymerization or interfacial polycondensation, respectively.

[0017] S2. Disperse pearlescent masterbatch in deionized water, and add three types of microcapsules in sequence to form a composite coating structure;

[0018] S3. Add an inorganic oxide precursor to the system and form an inorganic reflective shell under alkaline conditions using the sol-gel method;

[0019] S4. After centrifugation, washing and drying, the multi-stage reversible color-changing pearlescent pigment is obtained.

[0020] Optionally, in the aforementioned thermo-responsive dynamic color-changing pearlescent pigment, the microcapsule particle size is 0.5–3 μm, the reaction temperature is controlled at 40–60 °C, and the reaction time is 2–6 hours.

[0021] Optionally, in the aforementioned thermo-responsive dynamic color-changing pearlescent pigment, the pigment sequentially displays green, blue, and red as the temperature increases, and the color retention rate is 85% to 95% after 30 heating-cooling cycles.

[0022] Optionally, the thermo-responsive, dynamically changing pearlescent pigment is used in anti-counterfeiting labels, temperature-sensitive indicator inks, and biodegradable packaging films.

[0023] The beneficial effects of this invention are:

[0024] This invention constructs three thermochromic dye microcapsules with different response temperatures. Combined with precise wall thickness control and composite sequence design, it achieves controllable switching of color-changing behavior at 20℃, 35℃, and 50℃, exhibiting high temperature resolution and color reversibility. The microcapsule shell is made of chitosan and gelatin-based biopolymers, forming a stable cross-linked network, which improves encapsulation efficiency while endowing the material with good biodegradability and environmental adaptability.

[0025] Furthermore, this invention constructs an inorganic oxide reflective shell on the outer layer of the pearlescent masterbatch, enhancing light interference and structural color expression, and significantly improving the visual brightness and angle dependence of the pigment. Compared with traditional single-stage color-changing or physical blending systems, this material achieves a systematic breakthrough in thermal response accuracy, color performance, and sustainability, making it suitable for high-security anti-counterfeiting and green packaging materials. Attached Figure Description

[0026] Figure 1 The graph shows the response curves of the RGB values ​​of different samples of the present invention in the range of 10-60℃ as a function of temperature.

[0027] Figure 2 This is a graph showing the ΔE color difference variation of different samples during thermal cycling.

[0028] Figure 3 The graph shows the response time test curves of different samples of the present invention at different temperatures ranging from 10 to 60°C. Detailed Implementation

[0029] 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 some embodiments of the present invention, and not all 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.

[0030] Example 1:

[0031] For ease of explanation, the "first thermochromic dye microcapsule," "second thermochromic dye microcapsule," and "third thermochromic dye microcapsule" mentioned in this invention refer to the following compositions:

[0032] First thermochromic dye microcapsules: using a thermochromic system formed by crystal violet and lactic acid as the core material;

[0033] Second thermochromic dye microcapsules: using cholesteric liquid crystal dyes as core materials;

[0034] The third type of thermochromic dye microcapsule: uses fluorescent thermosensitive dye as the core material.

[0035] All three types of microcapsules utilize a chitosan-gelatin composite bio-based wall material, prepared via emulsion polymerization. The microcapsule particle size is controlled within the range of 1–5 μm, and the ratio of wall material to core material is 2:1. Unless otherwise stated, their preparation method is consistent with that of the first thermochromic dye microcapsule, only the type of color-changing core material is changed.

[0036] A thermochromic microcapsule uses a crystal violet-lactic acid complex as the color-changing core, and the specific preparation method is as follows:

[0037] S1. Weigh 1.0g of crystal violet dye and 0.5g of lactic acid and add them to a mixed solvent consisting of 3.0g of ethanol and 2.0g of dibutyl phthalate. Stir magnetically for 15 minutes at room temperature to ensure complete dissolution and form a homogeneous oil phase solution.

[0038] S2. Add 1.2g chitosan and 0.8g gelatin to 50.0g deionized water and stir for 40 minutes in a 50℃ water bath to form a stable bio-based polymer wall material solution. Then adjust the pH to 5.5 with 1mol / L acetic acid solution.

[0039] S3. Under rapid stirring at 800 rpm, the inner phase oil phase is slowly added dropwise to the outer phase wall material solution, and emulsified for 20 minutes to form a stable O / W emulsion; then 0.3 g glutaraldehyde is added as a crosslinking agent, stirring is maintained, and the reaction is carried out at 45°C for 2 hours to form a crosslinked microcapsule structure.

[0040] S4. After the reaction is complete, stop stirring and cool to room temperature. Centrifuge the system, remove the supernatant, wash the resulting precipitate twice with deionized water, and then vacuum dry at 60°C for 12 hours to obtain thermochromic microcapsules with a particle size of 0.5–3 μm and crystal violet-lactic acid complex as the core material.

[0041] Example 2:

[0042] A thermo-responsive, dynamically color-changing pearlescent pigment coated with SiO2 and exhibiting complete tri-color response, has the following composition:

[0043] Pearl masterbatch substrate 65.0 parts, first thermochromic dye microcapsule 3.0 parts, second thermochromic dye microcapsule 3.0 parts, third thermochromic dye microcapsule 3.0 parts, microcapsule wall material (chitosan:gelatin = 2:1) 10.0 parts, SiO2 shell material 3.0 parts, dispersant stabilizer 1.0 part, deionized water 150.0 parts.

[0044] The preparation method includes the following steps:

[0045] S1. Add 65.0g of pearlite masterbatch to 150.0g of deionized water and stir at room temperature for 30 minutes to form a homogeneous suspension;

[0046] S2. Add 3.0g of the first thermochromic microcapsule, 3.0g of the second thermochromic microcapsule, 3.0g of the third thermochromic microcapsule and 1.0g of dispersant and stabilizer in sequence, and continue stirring for 20 minutes;

[0047] S3. Add 10.0g of chitosan-gelatin composite wall material solution, and under stirring conditions, make it evenly distributed and fully coat the surface of pearlite masterbatch;

[0048] S4. Slowly add silica precursor sol under stirring conditions and react at 40°C for 2 hours to construct the inorganic shell outer layer.

[0049] S5. Stop the reaction, centrifuge and wash with pure water 3 times, then vacuum dry at 60℃ for 12 hours to obtain a three-layer structure thermo-responsive color-changing pearlescent pigment.

[0050] Comparative Example 1:

[0051] A shell-less, thermo-responsive, dynamically color-changing pearlescent pigment, with the following composition:

[0052] Pearl masterbatch substrate 65.0 parts, first thermochromic dye microcapsule 3.0 parts, second thermochromic dye microcapsule 3.0 parts, third thermochromic dye microcapsule 3.0 parts, microcapsule wall material (chitosan:gelatin = 2:1) 10.0 parts, dispersant stabilizer 1.0 part, deionized water 150.0 parts.

[0053] The preparation method includes the following steps:

[0054] S1. Add 65.0g of pearlite masterbatch to 150.0g of deionized water and stir at room temperature for 30 minutes to form a homogeneous suspension;

[0055] S2. Add 3.0g of the first thermochromic microcapsule, 3.0g of the second thermochromic microcapsule, 3.0g of the third thermochromic microcapsule and 1.0g of dispersant and stabilizer in sequence, and continue stirring for 20 minutes;

[0056] S3. Add 10.0g of chitosan-gelatin composite wall material solution, and under stirring conditions, make it evenly distributed and fully coat the surface of pearlite masterbatch;

[0057] S4. Stop the reaction, centrifuge and wash with pure water 3 times, then vacuum dry at 60℃ for 12 hours to obtain a thermochromic pearlescent pigment sample without a shell structure.

[0058] Comparative Example 2:

[0059] A thermochromic pearlescent pigment with only monochromatic response has the following composition:

[0060] Pearl masterbatch 65.0 parts, first thermochromic dye microcapsules 9.0 parts, microcapsule wall material 10.0 parts, SiO2 shell material 3.0 parts, dispersant stabilizer 1.0 part, deionized water 150.0 parts.

[0061] The preparation method includes the following steps:

[0062] S1. Add 65.0g of pearlite masterbatch to 150.0g of deionized water and stir at room temperature for 30 minutes to form a homogeneous suspension;

[0063] S2. Add 9.0g of the first thermochromic microcapsule and 1.0g of dispersant and stabilizer, and continue stirring for 20 minutes;

[0064] S3. Add 10.0g of chitosan-gelatin composite wall material solution, and under stirring conditions, make it evenly distributed and fully coat the surface of pearlite masterbatch;

[0065] S4. Slowly add silica precursor sol under stirring conditions and react at 40°C for 2 hours to construct the inorganic shell outer layer.

[0066] S5. Stop the reaction, centrifuge and wash with pure water 3 times, then vacuum dry at 60℃ for 12 hours to obtain a thermochromic pearlescent pigment sample containing only a single thermally responsive dye.

[0067] Comparative Example 3:

[0068] A non-biological polyvinyl alcohol (PVA) wall coating type thermochromic pearlescent pigment, the composition of which is as follows:

[0069] Pearl masterbatch substrate 65.0 parts, first thermochromic dye microcapsule 3.0 parts, second thermochromic dye microcapsule 3.0 parts, third thermochromic dye microcapsule 3.0 parts, PVA wall material 10.0 parts, SiO2 shell material 3.0 parts, dispersant stabilizer 1.0 part, deionized water 150.0 parts.

[0070] The preparation method includes the following steps:

[0071] S1. Add 65.0g of pearlite masterbatch to 150.0g of deionized water and stir at room temperature for 30 minutes to form a homogeneous suspension;

[0072] S2. Add 3.0g of the first thermochromic microcapsule, 3.0g of the second thermochromic microcapsule, 3.0g of the third thermochromic microcapsule and 1.0g of dispersant and stabilizer in sequence, and continue stirring for 20 minutes;

[0073] S3. Dissolve 10.0g PVA in 60℃ deionized water to form a wall material solution, and slowly add it dropwise into the system. Under stirring conditions, make it evenly distributed and fully coat the surface of the pearlite masterbatch.

[0074] S4. Slowly add silica precursor sol under stirring conditions and react at 40°C for 2 hours to construct the inorganic shell outer layer.

[0075] S5. Stop the reaction, centrifuge and wash with pure water 3 times, then vacuum dry at 60℃ for 12 hours to obtain PVA wall material coated thermochromic pearlescent pigment sample.

[0076] Performance testing:

[0077] 1. Temperature response performance test

[0078] To verify whether the pigment can exhibit clear, reversible, and phased color changes sequentially within a set temperature range, a temperature response performance test was conducted. The specific experimental steps are as follows:

[0079] The color-changing pearlescent pigment was dispersed at a mass concentration of 5 wt% in a 1:1 mixture of ethanol and water and sonicated for 20 minutes to form a uniform suspension. Then, the suspension was uniformly coated onto a PET or glass substrate using a scraper, with the coating thickness controlled at 80–120 μm. Finally, the coating was dried in a 50°C forced-air oven for 30 minutes and then left at room temperature for 24 hours before use.

[0080] The temperature response test used six set temperature points (10℃, 20℃, 30℃, 40℃, 50℃, and 60℃). The sample coating was placed in a constant temperature device and heated point by point, with each point held at the same temperature for 1 minute to ensure sufficient color change. The RGB or CIELAB values ​​were measured at each temperature point using a colorimeter or image analysis tool, with a sampling area ≥1 cm². 2 Repeat the process three times at each point and take the average value; record the response time simultaneously, i.e. the time taken from heating to color stabilization; then cool down to 10℃ to observe whether the color returns to its initial state, and record its reversibility and color change stability.

[0081] Table 1. Average RGB response values ​​of different samples at various temperatures.

[0082]

[0083] The test results show that Example 2 exhibits clear, staged thermochromic behavior within the temperature range of 10–60°C. The transition between the three color-changing temperature zones is natural, and the color change range is large, fully verifying the synergistic enhancement effect between the three-color response system and the SiO2 inorganic shell. In contrast, Comparative Example 1, lacking an inorganic shell structure, exhibits defects such as unstable color response, blurred color-changing levels, and poor thermal stability. Although Comparative Example 2 has a certain degree of thermal responsiveness, its color-changing range is limited due to containing only a single dye, and it cannot achieve multi-stage dynamic color changing.

[0084] 2. Cyclic color change stability test

[0085] To verify the durability and color retention of thermochromic pigments during long-term thermal cycling, three groups of samples—Example 2, Comparative Example 1, and Comparative Example 3—were selected and dispersed at 5 wt% in an ethanol-water (1:1) mixture. After ultrasonic dispersion, the mixture was coated onto a PET substrate to form a coating with a thickness of 80–100 μm. The samples were subjected to alternating thermal cycling between 20°C and 50°C, with each heating or cooling cycle lasting 2 minutes followed by a 1-minute hold-temperature period, repeated a total of 100 times.

[0086] Every 10 cycles, the CIELAB values ​​before (20℃) and after (50℃) color change were recorded using a colorimeter. The maximum color change depth ΔE was calculated, and a ΔE-cycle number curve was plotted to evaluate the color change retention rate and stability. Three parallel samples were set up for each sample, and the average value was analyzed. The results can determine the influence of the type of inorganic shell and wall material on the thermal response stability.

[0087] Table 2. Changes in the average ΔE color difference of different samples during thermal cycling.

[0088]

[0089] Example 2 showed minimal fluctuation in ΔE value throughout the entire 100 thermal cycles, indicating that its color change was highly stable during repeated heating and cooling, demonstrating good cycle durability. In contrast, Comparative Examples 1 and 3 showed significant deterioration in color change performance. Comparative Example 2's color difference was amplified due to the lack of SiO2 shell protection, microcapsule rupture, or dye migration. Comparative Example 3 indicated that the PVA wall material experienced thermal aging or uneven dispersion under repeated high-temperature exposure, exhibiting poorer structural stability than the chitosan-gelatin system.

[0090] 3. Response time test

[0091] To evaluate the response rate of thermochromic pigments at different temperatures, each sample was dispersed at a concentration of 5 wt% in an ethanol-water (1:1) mixture. After ultrasonic dispersion for 20 minutes, the mixture was uniformly coated onto a PET substrate using a scraping method to form a coating sample with a thickness of 80–120 μm. The coating samples were then dried at 50°C for 30 minutes and allowed to stand at room temperature for 24 hours before use. In the test, three groups of samples—Example 2, Comparative Example 1, and Comparative Example 2—were selected and placed on a constant temperature platform with temperature points set at 10, 20, 30, 40, 50, and 60°C.

[0092] After being heated at a constant temperature at each point, color changes were recorded in real time using image analysis tools. The time required from the start of constant temperature to the stabilization of the color was defined as the response time. Three parallel samples were set up for each group of samples, and the response time of each sample at six temperature points was measured and recorded. The average value was used for comparative analysis to evaluate the influence of the inorganic shell and the multicolor synergistic structure on the response speed.

[0093] Table 3. Average response time of different samples at various temperatures.

[0094]

[0095] Example 2 exhibited good thermochromic response speed and stability within the temperature range of 10–60°C, especially in the critical color-changing range of 30–50°C, indicating that the synergistic effect of the three color-changing dyes and the construction of the SiO2 shell significantly improved the thermal response efficiency. In contrast, Comparative Examples 1 and 2 showed longer response times across the entire temperature range, with more significant response lags in the low-temperature range of 10–30°C and the high-temperature range of 50–60°C, indicating that the structural design has a greater impact on heat conduction and dye excitation.

[0096] The comprehensive test results in Tables 1-3 show that Example 2, with its synergistic effect of three-order dyes and SiO2 shell structure, is significantly better than the comparative examples in terms of color change range, cyclic color change stability, and response speed.

[0097] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A thermo-responsive, dynamically color-changing pearlescent pigment, characterized in that, The ingredients comprise the following parts by weight: 50-85 parts of pearlescent masterbatch substrate, selected from natural mica, titanium dioxide, tin oxide or their composite sheet-like inorganic materials; 2-6 portions of the first thermochromic dye microcapsule, with a color change response temperature range of 15-25℃; 2-6 portions of the second thermochromic dye microcapsule, with a color change response temperature range of 30-40℃; 2-6 portions of the third thermochromic dye microcapsule, with a color change response temperature range of 45-55℃; 5-15 parts of microcapsule wall material, wherein the wall material is selected from chitosan, gelatin or their cross-linked network materials; 1-5 parts of inorganic reflective shell material, selected from silicon dioxide, titanium dioxide or their composites; Dispersant stabilizer 0.1-2.0 parts; deionized water 100-200 parts.

2. The thermo-responsive, dynamically changing pearlescent pigment according to claim 1, characterized in that, Three thermochromic dyes were encapsulated in the aforementioned wall material to form microcapsules, which were then sequentially compounded onto the surface of pearlescent masterbatch. An inorganic shell material was then used to form a core-shell three-layer structure. The resulting pigments exhibited green, blue, and red colors when heated to 20°C, 35°C, and 50°C, respectively.

3. The thermo-responsive, dynamically changing pearlescent pigment according to claim 1, characterized in that, The first, second, and third thermochromic dyes are selected from thermochromic complexes formed by crystal violet and organic acids, cholesteric liquid crystal dyes, and fluorescent thermosensitive dyes, respectively.

4. The thermo-responsive, dynamically changing pearlescent pigment according to claim 1, characterized in that, The microcapsule wall material is a composite material of chitosan and gelatin, compounded in a mass ratio of 1:1 to 3:1, with a crosslinking degree of 10 to 25% and a coating layer thickness of 80 to 150 nm.

5. A method for preparing a thermo-responsive, dynamically changing pearlescent pigment, wherein the thermo-responsive, dynamically changing pearlescent pigment is as described in any one of claims 1-4, characterized in that, The steps are as follows: S1. Dissolve three thermochromic dyes in organic solvents, add chitosan and gelatin solutions, and prepare microcapsules of the three thermochromic dyes by emulsion polymerization or interfacial polycondensation, respectively. S2. Disperse pearlescent masterbatch in deionized water, and add three types of microcapsules in sequence to form a composite coating structure; S3. Add an inorganic oxide precursor to the system and form an inorganic reflective shell under alkaline conditions using the sol-gel method; S4. After centrifugation, washing and drying, the multi-stage reversible color-changing pearlescent pigment is obtained.

6. The preparation method according to claim 6, characterized in that, The microcapsules have a particle size of 0.5–3 μm, the reaction temperature is controlled at 40–60 °C, and the reaction time is 2–6 hours.

7. The thermo-responsive, dynamically changing pearlescent pigment according to claim 2, characterized in that, The pigment sequentially changes color to green, blue, and red as the temperature rises, and retains 85% to 95% of its color after 30 heating-cooling cycles.

8. The application of the thermo-responsive dynamic color-changing pearlescent pigment as described in claim 5 in anti-counterfeiting labels, temperature-sensitive indicator inks, and biodegradable packaging films.

Images