Thermoplastic polyurethane (TPU) film capable of changing color as well as preparation method and application thereof

Abstract

The invention discloses a color-changeable TPU (thermoplastic polyurethane) film as well as a preparation method and application thereof, and belongs to the technical field of intelligent high polymer materials. The TPU film comprises a thermoplastic polyurethane (TPU) matrix, a color change response system, an interface stabilizer and a functional additive. The color-changing response system comprises at least two different types of color-changing materials, so that multiple environmental stimulus response color changing, such as temperature and light dual response, is realized. The film has the characteristics of sensitive color change response, high color contrast, good color change reversibility, excellent fatigue resistance, high mechanical property retention rate and the like. Through formula and process optimization, the technical problems that a single color-changing material is insensitive in response, low in color-changing contrast ratio, poor in durability and poor in compatibility with a TPU matrix are solved. The TPU film prepared by the invention can be widely applied to the fields of intelligent wearing, safety anti-counterfeiting, dynamic decoration, sensor indication and the like.

Description

Technical Field

[0001] This application relates to the field of polymer materials technology, and in particular to a color-changing TPU film, its preparation method, and its application. Background Technology

[0002] Thermoplastic polyurethane (TPU) is widely used in films, pipes, cable sheaths, and various elastic products due to its excellent elasticity, abrasion resistance, oil resistance, and processing properties. With the development of smart materials, endowing traditional materials with "sensing" and "response" functions has become a research hotspot. Color-changing materials, as a typical smart material, can undergo reversible color changes in response to external stimuli, and have enormous application potential in fields such as displays, sensing, decoration, and anti-counterfeiting.

[0003] Currently, in the field of polymer materials technology, to prepare films with color-changing functions, the industry typically combines color-changing materials with polymer matrices. Specifically, the main methods for preparing color-changing films by combining color-changing materials with polymer matrices (such as TPU) are blending and coating. The blending method involves directly mixing the color-changing material with the polymer matrix, so that the color-changing material is uniformly dispersed in the matrix; the coating method involves making a coating of the color-changing material and applying it to the surface of the polymer matrix.

[0004] However, existing technologies have many shortcomings. Most color-changing TPU films respond to only one stimulus (such as temperature), resulting in limited functionality. Furthermore, color-changing materials (especially organic dyes) are prone to decomposition or deactivation under the high temperature and high shear conditions of polymer melting processing, leading to a rapid decline in color-changing performance. Moreover, inorganic color-changing materials or microcapsules have poor compatibility with the TPU matrix, easily agglomerating, which in turn reduces the film's mechanical properties, reduces transparency, and causes uneven color changes. In addition, existing color-changing TPU films also suffer from insufficient color contrast and durability; the color change is not obvious, and the color-changing ability decreases significantly after multiple cycles. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a color-changing TPU film, its preparation method, and its applications. This film achieves high contrast, high durability, and multi-environmental responsiveness through an innovative multi-color-response system design, a dedicated interface stabilizer, and a mild processing technology, while maintaining the excellent mechanical properties of TPU.

[0006] To achieve the above objectives, the technical solution of the present invention is as follows:

[0007] A color-changing TPU film, comprising the following components by weight percentage:

[0008] Thermoplastic polyurethane (TPU) resin: 75-92%;

[0009] Color-changing response system: 5-20%, containing at least two different types of environmental stimulus-responsive color-changing materials;

[0010] Interface stabilizer: 1-3%;

[0011] Functional additives: 1-5%.

[0012] By adopting the above technical solution, TPU film can achieve color change in response to multiple environmental stimuli, solving the problems of insensitive response, low color contrast, poor durability, and poor compatibility with TPU matrix of single color-changing materials. It has the characteristics of sensitive color change response, high color contrast, good color reversibility, excellent fatigue resistance, and high mechanical property retention rate, and can be widely used in smart wearables, security anti-counterfeiting, dynamic decoration, sensor indication and other fields.

[0013] Preferably, the color-changing response system comprises a thermochromic material and a photochromic material in a weight ratio of (1:3) to (3:1); the thermochromic material is an encapsulated thermochromic microcapsule with a color-changing temperature range of 10-50℃; and the photochromic material is a spiropyran or spiroxazine compound.

[0014] By adopting the above technical solutions, the color-changing response system uses thermochromic materials and photochromic materials, with a weight ratio of (1:3) to (3:1), enabling the TPU film to achieve dual temperature and light response; the thermochromic material uses encapsulated thermochromic microcapsules with a color-changing temperature range of 10-50℃, which can achieve thermochromic changes within a certain temperature range; the photochromic material uses spiropyran or spiroxazine compounds, which can achieve photochromic changes.

[0015] Preferably, the wall material of the thermochromic microcapsule is melamine resin or gelatin-gum arabic composite wall material, and the core material is a mixture of crystal violet lactone, fluorane dyes and fatty acids.

[0016] By adopting the above technical solution, the color-changing response system in the TPU film includes thermochromic materials and photochromic materials. The thermochromic material adopts encapsulated thermochromic microcapsules, with melamine resin or gelatin-gum arabic composite wall material and crystal violet lactone, fluorane dyes and fatty acid mixture as core material. It can achieve temperature-light dual-response color change and improve the dispersion stability and interfacial bonding of the color-changing material in the TPU matrix. This makes the TPU film have the characteristics of sensitive color change response, high color contrast, good color change reversibility, excellent fatigue resistance and high mechanical property retention rate. It solves the problems of insensitive response, low color change contrast, poor durability and poor compatibility with TPU matrix of single color-changing materials.

[0017] Preferably, the color-changing response system comprises an electrochromic material and a wet-chromic material; the electrochromic material is polyaniline or WO3 nanoparticles; and the wet-chromic material is a complex of CoCl2 and an organic ligand.

[0018] By adopting the above technical solution, TPU film can achieve dual environmental response color-changing function of electrical stimulation and humidity stimulation. Among them, polyaniline or WO3 nanoparticles, as electrochromic materials, can change the color of the film under electrical stimulation, and the complex of CoCl2 and organic ligands, as a hygrochromic material, can change the color of the film under humidity stimulation.

[0019] Preferably, the interface stabilizer is a polyurethane-type superdispersant or a silane coupling agent, used to improve the dispersion stability and interfacial bonding of the color-changing material in the TPU matrix.

[0020] By adopting the above technical solution and using polyurethane-type superdispersants or silane coupling agents as interface stabilizers, the dispersion stability and interfacial bonding force of the color-changing material in the TPU matrix can be improved, so that the color-changing material is uniformly and stably dispersed in the TPU matrix, thereby improving the color development efficiency and contrast. This avoids problems such as the reduction of color development points per unit area, the decrease in contrast, and the agglomeration of color-changing materials leading to stress concentration points that accelerate material fatigue and failure. In this way, the color-changing contrast, cycle durability, and mechanical properties of the TPU film are improved.

[0021] Preferably, the functional additives include at least two of the following: ultraviolet light absorbers, antioxidants, anti-hydrolysis agents, antistatic agents, and reinforcing fibers.

[0022] By adopting the above technical solution, the functional additives in the TPU film include at least two of the following: ultraviolet light absorbers, antioxidants, anti-hydrolysis agents, antistatic agents, and reinforcing fibers. These additives can delay the aging of the polymer matrix and discoloration materials caused by light, oxygen, and heat, and improve the weather resistance, mechanical strength, and fatigue resistance of the film. Adding reinforcing fibers can also improve tensile strength, meeting the application requirements that require high strength.

[0023] Preferably, the thickness of the TPU film is 0.05-0.3mm; its color change contrast (ΔE*ab) under specific stimuli is ≥15, its response time is ≤30 seconds, and its color change cycle durability is ≥5000 times.

[0024] By adopting the above technical solution, the thickness of the TPU film is controlled at 0.05-0.3mm, so that the color change contrast of the film under specific stimuli reaches ≥15, which improves the obviousness of the color change; the response time is ≤30 seconds, realizing rapid color change response; and the color change cycle durability is ≥5000 times, ensuring that the TPU film has excellent fatigue resistance and long-term stability.

[0025] A method for preparing a color-changing TPU film includes the following steps:

[0026] a) The TPU resin, color-changing response system, interface stabilizer and functional additives are premixed at low temperature under a protective atmosphere;

[0027] b) The premixed material is melt-blended and extruded into granules at a low temperature of 150-180℃ using a twin-screw extruder to avoid deactivation of the discoloration material;

[0028] c) Cast the granules into a film using a single-screw extruder or calender the film using a calender, with the forming temperature controlled at 140-170℃;

[0029] d) Cool and rewind the formed film to avoid prolonged exposure to strong light or high temperature environments.

[0030] By adopting the above technical solutions, low-temperature premixing under a protective atmosphere can avoid the oxidation of raw materials. The twin-screw extruder can melt-blend and extrude granulation at a low temperature of 150-180℃ to avoid the deactivation of the color-changing material. Casting the granules into a film using a single-screw extruder or calendering them into a film using a calender and controlling the forming temperature at 140-170℃ can ensure the forming quality of the film. Cooling and winding the formed film and avoiding prolonged exposure to strong light or high temperature environments can maintain the activity of the color-changing material. Finally, a smart TPU film with high color-changing contrast, fast response, excellent cycle durability and good mechanical properties is prepared.

[0031] Preferably, the temperature of the low-temperature premixing in step a) is 10-30°C, the twin-screw extruder in step b) adopts a low-shear screw combination, and the material residence time is controlled at 1-2 minutes.

[0032] By adopting the above technical solutions, the low-temperature premixing temperature is controlled at 10-30℃, which can better protect the activity of the color-changing material during the premixing stage. The twin-screw extruder uses a low-shear screw combination and controls the material residence time to 1-2 minutes, which can prevent the color-changing material from decomposing or deactivating due to high temperature and high shear during melt blending, extrusion granulation, and ensure that the TPU film has high color contrast, fast response, excellent cycle durability and good mechanical properties.

[0033] Applications of a color-changing TPU film in smart temperature-indicating clothing, photochromic automotive films, ambient humidity indicator labels, or dynamic anti-counterfeiting packaging materials.

[0034] By adopting the above technical solutions, the color-changing TPU film can be used to realize temperature display function in smart temperature-display clothing, to realize photochromic effect in photochromic automotive film, to realize humidity indication function in environmental humidity indicator labels, and to realize dynamic anti-counterfeiting function in dynamic anti-counterfeiting packaging materials. Moreover, the TPU film has the characteristics of sensitive color change response, high color contrast, good color change reversibility, excellent fatigue resistance and high mechanical property retention rate, which can meet the needs of these application scenarios.

[0035] In summary, this application includes at least one of the following beneficial technical effects:

[0036] 1. The color-changing response system contains at least two different types of environmental stimulus-responsive color-changing materials, which can achieve multiple environmental stimulus-responsive color changes, solving the problem of single response of existing color-changing TPU films;

[0037] 2. The processing technology of low-temperature premixing, low-temperature melt blending extrusion granulation and molding avoids the decomposition or deactivation of color-changing materials under high temperature and high shear environment, and solves the problem of rapid degradation of color-changing performance of existing color-changing TPU films;

[0038] 3. Interface stabilizers can improve the dispersion stability and interfacial bonding of color-changing materials in TPU matrix, solving the problems of poor compatibility and easy aggregation of inorganic color-changing materials or microcapsules with TPU matrix;

[0039] 4. The TPU film has a color change contrast (ΔE*ab) ≥15 under specific stimuli, a response time ≤30 seconds, and a color change cycle durability ≥5000 times, which solves the problem of insufficient color change contrast and durability of existing color-changing TPU films. Detailed Implementation

[0040] The present invention will be described in detail below through specific embodiments and comparative examples.

[0041] The color-changing TPU film provided in this application includes thermoplastic polyurethane (TPU) resin, a color-changing responsive system, an interface stabilizer, and functional additives. By weight percentage, the thermoplastic polyurethane (TPU) resin accounts for 75-92%, the color-changing responsive system for 5-20%, the interface stabilizer for 1-3%, and the functional additives for 1-5%. This combination of proportions enables the TPU film to possess color-changing functionality while maintaining its basic performance. The TPU resin, as the matrix, provides the film's basic physical properties, such as elasticity and abrasion resistance. The color-changing responsive system imparts the color-changing ability, the interface stabilizer improves the compatibility between the color-changing material and the matrix, and the functional additives further enhance the overall performance of the film.

[0042] Specifically, thermoplastic polyurethane (TPU) resin is a material with excellent elasticity, abrasion resistance, oil resistance, and processing performance. Various grades are available on the market; for example, in this embodiment, a TPU resin with a Shore hardness of 90A can be selected, which has good flexibility and strength. Of course, other TPU resins with different Shore hardnesses can also be selected according to actual needs, as long as they meet the performance requirements of the membrane.

[0043] The color-changing response system comprises at least two different types of environmentally stimuli-responsive color-changing materials. For example, the system may contain thermochromic and photochromic materials in a weight ratio of (1:3) to (3:1). The thermochromic material is an encapsulated thermochromic microcapsule with a color-changing temperature range of 10-50℃. The wall material of this microcapsule can be melamine resin or a gelatin-gum arabic composite wall material, and the core material is a mixture of crystal violet lactone, fluorane dyes, and fatty acids. Melamine resin wall material has good stability and chemical resistance, effectively protecting the core material; gelatin-gum arabic composite wall material has good film-forming properties and biocompatibility. When the thermochromic microcapsule reaches a specific temperature, the dye in the core material undergoes a structural change, resulting in a color change. The photochromic material is a spiropyran or spiroxazine compound; when exposed to light of a specific wavelength, its molecular structure changes, leading to a color change.

[0044] Color-changing response systems can also include electrochromic and hygrochromic materials. Electrochromic materials are polyaniline or WO3 nanoparticles; when a certain voltage is applied, the oxidized or reduced state of the material changes, thus achieving a color change. Polyaniline has good conductivity and color-changing properties, while WO3 nanoparticles have high color-changing efficiency and stability. Hygrochromic materials are complexes of CoCl2 and organic ligands; when the ambient humidity changes, the structure of the complex changes, leading to a color change.

[0045] The interface stabilizer is a polyurethane-type superdispersant or a silane coupling agent, used to improve the dispersion stability and interfacial bonding of the color-changing material in the TPU matrix. The polyurethane-type superdispersant has excellent dispersing properties, enabling the color-changing material to be uniformly dispersed in the TPU matrix and preventing agglomeration; the silane coupling agent can form chemical bonds between the color-changing material and the TPU matrix, enhancing the interfacial bonding between the two.

[0046] Functional additives include at least two of the following: UV absorbers, antioxidants, anti-hydrolysis agents, antistatic agents, and reinforcing fibers. UV absorbers absorb ultraviolet light, preventing the TPU film from aging due to UV exposure; antioxidants inhibit the oxidation reaction of the TPU film, extending its service life; anti-hydrolysis agents improve the hydrolysis resistance of the TPU film; antistatic agents reduce static electricity accumulation on the surface of the TPU film; and reinforcing fibers improve the mechanical strength of the TPU film. For example, UV absorber UV-360 and antioxidant 1010 can be selected as functional additives, as they can effectively delay the aging of the polymer matrix and discoloration materials caused by light and oxygen.

[0047] The TPU film has a thickness of 0.05-0.3mm, a color change contrast (ΔE*ab) ≥15 under specific stimuli, a response time ≤30 seconds, and a color-changing cycle durability ≥5000 cycles. These properties give the TPU film excellent color-changing effects and stability in practical applications.

[0048] TPU film achieves multi-environmental stimulus-responsive color change through a combination of various color-changing materials, overcoming the limitation of single-response color-changing TPU films in existing technologies. Simultaneously, the use of interface stabilizers improves the compatibility and dispersion stability of the color-changing materials with the TPU matrix, preventing aggregation and performance degradation of the color-changing materials and ensuring the film's color contrast and durability. The addition of functional additives further enhances the film's overall performance, enabling it to maintain excellent performance under various environmental conditions. This TPU film can be widely used in smart wearables, security anti-counterfeiting, dynamic decoration, sensor indication, and other fields, providing a high-performance basic material solution for these applications.

[0049] The following are specific implementation examples:

[0050] I. Experimental Materials

[0051] TPU resin: grade WHT-1190, Shore hardness 90A.

[0052] Color-changing material:

[0053] Thermochromic microcapsules (TCM-1): Blue ⇌ Colorless, color change point 28℃, melamine resin wall material.

[0054] Thermochromic microcapsules (TCM-2): Red ⇌ Yellow, color change point 45℃, gelatin-gum arabic wall material.

[0055] Photochromic powder (PCM-1): Spiropyran type, colorless to blue, UV excited.

[0056] Photochromic powder (PCM-2): Spirooxazine, colorless ⇌ purple, UV excited.

[0057] Wet-chromatic material (HCM): CoCl2 complex, blue ⇌ pink.

[0058] Interface stabilizers: polyurethane-type superdispersant (Disper-1), silane coupling agent KH-550.

[0059] Functional additives: UV absorber UV-360, antioxidant 1010, anti-hydrolysis agent C, chopped aramid fiber (0.5mm).

[0060] II. Preparation Process

[0061] Premixing: Mix TPU resin, color-changing material, interface stabilizer and other additives in a high-speed mixer at room temperature (25°C) for 5 minutes, with nitrogen gas purging during the mixing process.

[0062] Melt granulation: A co-rotating twin-screw extruder is used, with a set temperature range of 150-165℃. A low-shear screw element combination is employed, with a screw speed of 100 rpm and vacuum degassing. The material residence time is approximately 1.5 minutes.

[0063] Film formation: The granules are added to a single-screw extruder, the barrel temperature is set to 145-160℃, the T-die temperature is 155℃, and the melt is cast and cooled by three rollers (roller temperature 20℃) to form a film with a thickness of about 0.15mm.

[0064] Post-processing: Wrap the wound film in a black PE bag and store it away from light.

[0065] III. Experimental formulation (by weight percentage)

[0066] The following table shows the detailed formulation design for each embodiment and comparative example.

[0067] Table 1: Formulation composition of the examples (wt%) Components / Case Studies TPU resin TCM-1 (28℃) TCM-2 (45℃) PCM-1 (Photochromic Blue) PCM-2 (Photoviolet) HCM (wet-induced) Interface stabilizers Functional Additive Pack Reinforcing fibers Example 1 88 5 0 5 0 0 1 1 0 Example 2 83 10 0 5 0 0 1 1 0 Example 3 86 8 0 0 4 0 1 1 0 Example 4 81 7 7 0 0 0 2 2 1 Example 5 85 0 12 0 0 0 1 2 0 Example 6 82 6 0 0 0 8 2 2 0 Example 7 79 5 5 5 0 0 2 3 1 Example 8 84 9 0 0 5 0 1 1 0 Example 9 80 6 6 0 0 0 2 2 4 Example 10 77 8 0 8 0 0 2 3 2

[0068] Table 2: Comparative Formulation Composition (wt%) Components / Case Studies TPU resin TCM-1 PCM-1 Interface stabilizers Functional Additive Pack Common blue dye Processing technology description Comparative Example 1 93 5 0 0 2 0 Standard process Comparative Example 2 88 5 5 0 2 0 Standard process Comparative Example 3 88 5 5 1 1 0 High-temperature processing: Extrusion temperature 200℃ Comparative Example 4 88 0 0 1 1 10 Standard process Comparative Example 5 85 5 (Unwrapped) 5 1 3 0 Standard process Comparative Example 6 89 5 5 1 0 0 Standard process

[0069] IV. Performance Testing and Results

[0070] The following tests were performed on the film: 1) Color difference test: The L*a*b values ​​before and after a specific stimulus were measured using a colorimeter, and the color difference ΔEab was calculated. 2) Response time: The time required for ΔE*ab to reach 90% of its maximum value from the application of the stimulus was recorded. 3) Cyclic durability: A "stimulus-recovery" cycle was performed under controlled conditions until ΔE*ab decreased to 50% of its initial value, and the number of cycles was recorded. 4) Mechanical properties: Tensile strength and elongation at break (ASTM D638). 5) Dispersibility evaluation: The aggregation of the color-changing material was observed under a microscope.

[0071] Table 3: Performance Test Results Performance metrics / case studies Thermochromic ΔE*ab(28℃) Photochromic ΔE*ab(UV) Response time (seconds) Cycle durability (cycles) Tensile strength (MPa) Elongation at break (%) Dispersion Example 1 22.5 18.3 ≤20 >8000 32 450 excellent Example 2 35.1 19 ≤15 >7000 30 420 excellent Example 3 20.8 25.4(purple) ≤25 >7500 31 440 good Example 4 18.5(28℃) / 16.2(45℃) - ≤30 >6000 35 400 excellent Example 5 - / 32.0(45℃) - ≤10 >5000 33 430 excellent Example 6 - -(Wetness-induced: 20.1) ≤40 >3000 29 410 good Example 7 15.3 15 ≤20 >6000 38 350 excellent Example 8 30.5 22.8 ≤20 >7000 31 460 excellent Example 9 19.1 / 17.5 - ≤15 >5500 42 300 good Example 10 26 28.5 ≤25 >6500 40 320 excellent Comparative Example 1 8.5 - ≤10 <1000 34 480 Poor (reunion) Comparative Example 2 9.2 7.8 ≤20 <1500 32 470 Poor (reunion) Comparative Example 3 2.1 1.5 extremely slow <100 30 440 Good (deactivated) Comparative Example 4 0 0 - - 28 500 Excellent (no color change) Comparative Example 5 5.5 6.2 ≤10 <500 25 400 Difference (decomposition) Comparative Example 6 18 14.5 ≤20 <2000 30 450 Good (fades quickly)

[0072] V. Data Analysis and Discussion

[0073] 1. Synergistic effect of multiple color-changing response systems (Examples 1, 2, 7, 10):

[0074] Example 1 (5%TCM-1 + 5%PCM-1) exhibited significant thermochromic (ΔE*ab = 22.5) and photochromic (ΔE*ab = 18.3) effects simultaneously, achieving a dual temperature-light response. Its color-changing contrast was significantly higher than Comparative Example 2 (similar formulation but without interface stabilizers, ΔE*ab < 10). This demonstrates that interface stabilizers are crucial for the uniform and stable dispersion of the color-changing material within the TPU matrix, directly determining color development efficiency and contrast.

[0075] Examples 7 and 10, based on the dual-color-changing material, added reinforcing fibers and more functional additives. Although the elongation at break decreased, the tensile strength was significantly improved (38-40 MPa), and the cycle durability remained high (>6000 cycles). This indicates that the introduction of functional additives, with a slight sacrifice of some elasticity, yields higher mechanical strength and overall durability, making it suitable for more demanding applications.

[0076] 2. The core role of interface stabilizers (Comparative Example 1 vs. Comparative Examples 1 & 2):

[0077] Comparative Examples 1 and 2, containing little or no interface stabilizer, showed poor dispersibility, extremely low color-changing contrast (ΔE*ab<10), and extremely poor cycle durability (<1500 cycles). Microscopic observation confirmed severe aggregation of the color-changing microcapsules. Aggregation leads to: a) a reduction in color development points per unit area, resulting in decreased contrast; b) agglomerates becoming stress concentration points, accelerating material fatigue and failure; and c) the color-changing material becoming more susceptible to environmental degradation.

[0078] All embodiments contain 1-2% interface stabilizer, and the dispersibility is rated as "good" or "excellent", which is the structural basis for obtaining high ΔE*ab values ​​and long cycle life.

[0079] 3. The importance of low-temperature processing for activity protection (Example 1 vs. Comparative Example 3):

[0080] Comparative Example 3 used a conventional TPU processing temperature (200℃), far exceeding the tolerance limit of the color-changing materials used (especially spiropyran-based photochromic materials). This resulted in severe thermal decomposition and deactivation of the color-changing materials, almost completely losing their color-changing properties (ΔE*ab≈2), and achieving a cycle durability of less than 100 cycles. This highlights the devastating impact of high temperatures on organic color-changing materials.

[0081] All embodiments of this invention employ a low-temperature extrusion process of 150-165℃ and control the short residence time to maximize the protection of the chemical activity of the color-changing material, which is a prerequisite for achieving high-performance color change.

[0082] 4. Performance characteristics of different types of color-changing materials:

[0083] Thermochromic materials, such as Example 2 (high content TCM-1) and Example 5 (single high temperature color change), exhibit extremely high color change contrast (ΔE*ab>30) and extremely fast response speed (≤15 seconds). However, the cycle durability of Example 5 is relatively low (>5000 times), which may be related to the fatigue of the high temperature color change system itself.

[0084] Photochromic materials: As in Example 3 (using PCM-2), a unique photochromic purple effect was obtained (ΔE*ab=25.4), expanding the range of color changes.

[0085] Composite Response and Special Functions: Example 4 uses two thermochromic microcapsules with different color-changing points to achieve a step-like color change at two temperature points. Example 6 demonstrates a hygrochromic function, enabling the indication of ambient humidity.

[0086] 5. The balance between functional additives and reinforcing fibers:

[0087] Comparative Example 6, which does not contain functional additives (especially UV absorbers and antioxidants), exhibited significantly lower cycle durability (<2000 cycles) than Example 1, which had a similar composition (>8000 cycles). This demonstrates that functional additives can effectively delay the aging of polymer matrices and discoloration materials caused by light, oxygen, and heat.

[0088] Example 9, with the addition of 4% reinforcing fiber, achieved the highest tensile strength (42 MPa), but the elongation at break decreased to 300%. This provides formulation options for applications requiring high strength, such as certain industrial sensor sheaths.

[0089] 6. The essential difference between this dye and ordinary dyes (Example 1 vs. Comparative Example 4):

[0090] Comparative Example 4 used ordinary blue dye, and the film color remained unchanged (ΔE*ab=0), showing no responsiveness. This, in turn, confirms that the environmentally responsive color-changing material used in this invention is the core material for achieving intelligent functions.

[0091] in conclusion

[0092] This invention successfully prepared a smart TPU film with high color-changing contrast, rapid response, excellent cycle durability, and good mechanical properties by carefully designing a multi-response system containing at least two color-changing materials, employing key interface stabilizers, and implementing a rigorous low-temperature processing technology. This technical solution effectively solves the common problems of traditional color-changing polymer materials, such as single response, rapid performance decay, and processing difficulties, providing a high-performance basic material solution for fields such as smart wearables, adaptive camouflage, advanced anti-counterfeiting, and interactive sensing.

[0093] This application also provides a method for preparing a color-changing TPU film, including the following steps:

[0094] S1, the TPU resin, color-changing response system, interface stabilizer, and functional additives are premixed at low temperature under a protective atmosphere. Nitrogen can be used as the protective atmosphere, and the premixing temperature is 10-30℃. In this step, a high-speed mixer is used to thoroughly mix the components, and the mixing time can be controlled to approximately 5 minutes. Low-temperature premixing prevents the color-changing material from reacting or deactivating prematurely at high temperatures, and the protective atmosphere also prevents oxidation of the material.

[0095] S2 involves melting and blending the premixed material using a twin-screw extruder at a low temperature of 150-180℃, followed by extrusion granulation to prevent deactivation of the discoloring material. The twin-screw extruder employs a low-shear screw combination, with the material residence time controlled at 1-2 minutes. The low-shear screw combination reduces the shear force on the discoloring material, lowering the risk of its decomposition. Controlling the material residence time is also crucial; too short a time may lead to uneven mixing, while too long a time increases the probability of discoloration material deactivation.

[0096] S3 involves casting the granules into a film using a single-screw extruder or calendering it into a film using a calender, with the forming temperature controlled between 140-170℃. If a single-screw extruder is used for casting, the barrel temperature can be set to 145-160℃, the T-die temperature to 155℃, and the melt is cast and cooled by three rollers (roller temperature 20℃) before forming. By controlling the forming temperature, the activity of the color-changing material can be ensured, while also achieving good film quality.

[0097] S4 involves cooling and rewinding the formed film to prevent prolonged exposure to strong light or high temperatures. Cooling can be achieved using air or water to rapidly lower the film temperature, preventing deformation, discoloration, or material deactivation at high temperatures. After rewinding, the film can be wrapped in a black PE bag and stored away from light to extend its lifespan.

[0098] The implementation principle of this embodiment is as follows: The preparation method of this embodiment maximizes the protection of the activity of the color-changing material through measures such as low-temperature premixing, low-temperature melt blending, low-temperature molding, and avoiding strong light and high-temperature environments. This effectively solves the problem of easy decomposition or deactivation of the color-changing material under high temperature and high shear environment in traditional preparation methods. At the same time, by strictly controlling the process parameters of each step, the quality and performance of the TPU film are guaranteed, resulting in a TPU film with high color-changing contrast, fast response, excellent cycle durability, and good mechanical properties.

[0099] In other embodiments, in step S2, the screw speed of the twin-screw extruder is adjusted to 120 rpm, and the material residence time is controlled at 1.2 minutes. In step S3, a calender is used to calender the material into a film, and the roll temperature of the calender is set to 150°C.

[0100] The principle is as follows: by adjusting the screw speed and material residence time of the twin-screw extruder, as well as selecting the calendering film-forming method and corresponding roller temperature, the preparation process is further optimized. Appropriately increasing the screw speed can improve mixing efficiency, and shortening the material residence time can reduce the heating time of the discoloring material, thus reducing the risk of deactivation. Calendering film-forming in a calender can produce a film with more uniform thickness and better surface quality, thereby improving the overall performance of the TPU film and meeting different production needs and product quality requirements.

[0101] This application also provides an application of a color-changing TPU film in smart temperature-indicating clothing, photochromic automotive films, ambient humidity indicator labels, or dynamic anti-counterfeiting packaging materials.

[0102] In the field of smart temperature-indicating clothing, color-changing TPU films are applied to specific parts of the garment. When the body temperature changes, the thermochromic material in the TPU film changes color, thus visually displaying the body's temperature status. This type of smart temperature-indicating clothing can be applied in sports, medical, and other fields, providing people with more health information.

[0103] In the area of ​​photochromic automotive window tinting, TPU film is applied to car windows. When exposed to sunlight, the photochromic material in the film changes color, adjusting the intensity of light entering the car and thus providing sun protection and privacy. At the same time, the different color changes can also add aesthetic appeal and personalization to the car.

[0104] In terms of humidity indicator labels, the wet-chromic material in TPU film changes color accordingly when the ambient humidity changes, visually reflecting the humidity level. These labels can be used in warehouses, laboratories, and other places with high humidity requirements, allowing people to easily understand changes in ambient humidity.

[0105] In the field of dynamic anti-counterfeiting packaging materials, the multi-color-changing properties of TPU film can serve as an anti-counterfeiting measure. By setting specific color-changing modes and conditions, a specific color change can only be observed when a corresponding stimulus is met, increasing the difficulty of anti-counterfeiting for packaging materials and improving product security.

[0106] The implementation principle of this embodiment is as follows: the color-changing TPU film has the characteristic of changing color in response to multiple environmental stimuli. Utilizing these characteristics in different fields can bring new functions and advantages to these fields. In applications such as smart temperature-indicating clothing, photochromic automotive films, ambient humidity indicator labels, and dynamic anti-counterfeiting packaging materials, the color-changing function of TPU film can meet the actual needs of different scenarios, improve the practicality and added value of products, and provide new solutions for the development of related fields.

[0107] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A color-changing TPU film, characterized in that, By weight percentage, it includes the following components: Thermoplastic polyurethane (TPU) resin: 75-92%; Color-changing response system: 5-20%, containing at least two different types of environmental stimulus-responsive color-changing materials; Interface stabilizer: 1-3%; Functional additives: 1-5%.

2. The color-changing TPU film according to claim 1, characterized in that, The color-changing response system comprises a thermochromic material and a photochromic material in a weight ratio of (1:3) to (3:1); the thermochromic material is an encapsulated thermochromic microcapsule with a color-changing temperature range of 10-50℃; the photochromic material is a spiropyran or spiroxazine compound.

3. The color-changing TPU film according to claim 2, characterized in that, The wall material of the thermochromic microcapsule is melamine resin or gelatin-gum arabic composite wall material, and the core material is a mixture of crystal violet lactone, fluorane dyes and fatty acids.

4. The color-changing TPU film according to claim 1, characterized in that, The color-changing response system comprises an electrochromic material and a wet color-changing material; the electrochromic material is polyaniline or WO3 nanoparticles; the wet color-changing material is a complex of CoCl2 and an organic ligand.

5. The color-changing TPU film according to claim 1, characterized in that, The interface stabilizer is a polyurethane-type superdispersant or a silane coupling agent, used to improve the dispersion stability and interfacial bonding of the color-changing material in the TPU matrix.

6. The color-changing TPU film according to claim 1, characterized in that, The functional additives include at least two of the following: ultraviolet light absorbers, antioxidants, anti-hydrolysis agents, antistatic agents, and reinforcing fibers.

7. The color-changing TPU film according to claim 1, characterized in that, The TPU film has a thickness of 0.05-0.3mm; its color change contrast (ΔE*ab) under specific stimuli is ≥15, its response time is ≤30 seconds, and its color change cycle durability is ≥5000 times.

8. A method for preparing a color-changing TPU film as described in any one of claims 1-7, characterized in that, Includes the following steps: a) The TPU resin, color-changing response system, interface stabilizer and functional additives are premixed at low temperature under a protective atmosphere; b) The premixed material is melt-blended and extruded into granules at a low temperature of 150-180℃ using a twin-screw extruder to avoid deactivation of the discoloration material; c) Cast the granules into a film using a single-screw extruder or calender the film using a calender, with the forming temperature controlled at 140-170℃; d) Cool and rewind the formed film to avoid prolonged exposure to strong light or high temperature environments.

9. The preparation method according to claim 8, characterized in that, The temperature of the low-temperature premixing in step a) is 10-30℃, and the twin-screw extruder in step b) adopts a low-shear screw combination, and the material residence time is controlled at 1-2 minutes.

10. The application of a color-changing TPU film as described in any one of claims 1-7 in smart temperature-indicating clothing, photochromic automotive films, ambient humidity indicator labels, or dynamic anti-counterfeiting packaging materials.