A thermochromic in-mold transfer film, its preparation method and application

Abstract

This invention discloses a thermochromic in-mold transfer film, belonging to the field of surface decorative film technology. It comprises a base film, a release layer, a hardening layer, a thermochromic printing layer, and an adhesive layer, stacked sequentially. The thermochromic printing layer contains a pattern formed by printing ink, which contains thermochromic microcapsules uniformly dispersed therein. The wall material of the thermochromic microcapsules is melamine-formaldehyde resin or urea-formaldehyde resin, and the thermal decomposition temperature of the microcapsules is higher than 220°C. This invention, through the synergistic effect of heat-resistant wall materials, interfacial incompatible dispersion, and modulus matching, enables the transfer film to possess reversible thermochromic function, long-term durability, and high-precision patterning capability. This invention also discloses the preparation method and application of this transfer film, which can be extended to in-mold labeling and insert injection molding processes, and is suitable for automotive interiors, home appliance panels, and electronic product casings.

Description

Technical Field

[0001] This invention relates to the field of surface decorative film technology, specifically to a thermochromic in-mold transfer film, its preparation method, and its application. Background Technology

[0002] In-mold transfer (IMR) technology is a mature surface decoration solution that transfers a decorative layer printed on a carrier film to the surface of a plastic product in a single injection molding process. This allows for the creation of complex patterns, high wear resistance, high gloss, and rich textures, and it has been widely used in automotive interiors, appliance panels, and other fields. However, the decorative effects provided by existing IMR technologies and their products are essentially static, lacking intelligent responsiveness to the environment or users, making it difficult to meet the growing demand for high-end and emotionally-driven consumer products.

[0003] Integrating thermochromic materials, especially reversible thermochromic systems represented by crystal violet lactone, into IMR products is an effective way to endow products with dynamic visual interactive attributes. However, successfully applying such smart materials to the IMR field, which has extremely stringent requirements for process compatibility and durability, still faces the following challenges: Firstly, it has insufficient heat resistance. The temperature of the molten resin in the IMR process is usually as high as 200-300℃, accompanied by short-term high-temperature thermal shock to the film. Conventional thermochromic materials are very prone to permanent failure due to thermal decomposition in this process.

[0004] Secondly, durability is difficult to guarantee. As automotive interior or home appliance panels, IMR products need to withstand long-term ultraviolet radiation, temperature cycling, friction, and cleaning agent wiping. Simply added color-changing materials are prone to migration and aging, which cannot meet the industry's requirements for product warranty of several years.

[0005] Third, the integration of functions is difficult. Existing technologies struggle to precisely combine color-changing functions with the multi-layered structure of IMR films, often resulting in coarse color-changing effects and limited functional areas. For example, CN1546320A achieves gloss changes through differences in release layer roughness, but it cannot achieve active, reversible thermochromic functions, nor does it address the integration of heat-sensitive functional materials.

[0006] Fourth, poor dispersion stability. Existing solutions treat microcapsules as simple mixtures of ordinary pigments. In actual production, microcapsules are prone to agglomeration and sedimentation due to poor interfacial compatibility with the binder resin, affecting printing quality and product yield.

[0007] Fifth, hidden damage to the hardened layer. In existing technologies, the hardened layer is simply regarded as a protective layer, and the damage to the underlying functional material caused by its curing shrinkage stress is completely ignored. The applicant has discovered that under the high temperature and high pressure conditions of IMR injection molding, the stress generated by the cross-linking shrinkage of the UV-cured hardened layer is sufficient to crush the thermochromic microcapsules below, resulting in partial or complete loss of the color-changing function.

[0008] Therefore, developing an in-mold transfer film that is fully compatible with existing IMR processes, can withstand its high-temperature processing, has long-term environmental durability, and can achieve exquisite and reliable dynamic color-changing effects has become an urgent technical need. Summary of the Invention

[0009] In view of the above-mentioned defects or improvement needs of the existing technology, the present invention provides a thermochromic in-mold transfer film, its preparation method and application, which enables IMR products to simultaneously possess reversible thermochromic function, long-term durability and high-precision patterning capability.

[0010] To achieve the above objectives, according to a first aspect of the present invention, a thermochromic in-mold transfer film is provided, comprising a base film, a release layer, a hardening layer, a thermochromic printing layer, and an adhesive layer stacked sequentially. The thermochromic printing layer comprises a pattern formed by printing ink, wherein the printing ink comprises thermochromic microcapsules uniformly dispersed therein; The wall material of the thermochromic microcapsule is melamine-formaldehyde resin or urea-formaldehyde resin, and the thermal decomposition temperature of the thermochromic microcapsule is higher than 220°C.

[0011] As a further improvement of the present invention, the energy storage modulus of the hardened layer at 25°C is G'. 硬 The storage modulus of the wall material of the thermochromic microcapsule at 25°C is G'. 壁 , and G' 壁 ≥G' 硬 .

[0012] Preferably, the storage modulus G' of the hardened layer at 25°C 硬 The energy storage modulus G' of the microcapsule wall material at 25°C is 0.5~3.0 GPa. 壁 The range is 1.0~4.0 GPa; and G' 壁 / G' 硬 The ratio is 1.0 to 2.5.

[0013] During IMR injection molding, the molten resin is injected, subjecting the entire film layer to a high-temperature (200~300℃) and high-pressure environment, followed by cooling to room temperature. During this process, the cured layer experiences shrinkage stress during cooling due to the difference in thermal expansion coefficients between it and adjacent layers. This stress acts on the underlying thermochromic printing layer. Since both the cured layer and the microcapsule wall material are cross-linked network structures, their mechanical properties (storage modulus) can be controlled by the cross-linking density. Based on this, the present invention designs a matching energy storage modulus between the two. When G' 硬 >G' 壁 When the modulus of the microcapsule wall material is lower than that of the hardened layer, it cannot effectively resist external stress, causing the shell to be crushed or cracked, and the color-changing function to deteriorate; when G' 壁 ≥G' 硬 At this time, the microcapsule wall material can provide sufficient mechanical support for the core material, resist external stress, and maintain structural integrity.

[0014] As a further improvement of the present invention, the printing ink further comprises a binder resin, and the contact angle between the wall material of the thermochromic microcapsule and the binder resin is ≥90°.

[0015] This invention achieves intrinsically stable dispersion of microcapsules in ink by controlling the contact angle between the microcapsule wall material and the binder resin. When the contact angle is <90°, the interfacial compatibility between the wall material and the binder is too good, and the microcapsules tend to be wetted by the binder and agglomerate and settle. When the contact angle is ≥90°, the wall material and the binder are incompatible, and the microcapsules are independently suspended in the ink, effectively avoiding agglomeration and sedimentation. This dispersion mechanism does not rely on added dispersants and has higher process stability and batch consistency.

[0016] Furthermore, uniformly dispersed microcapsules are a prerequisite for modulus matching design to function effectively. When microcapsules are aggregated, stress concentration points form within the aggregates, and the shrinkage stress of the hardened layer is concentrated and amplified at the aggregates, even if G' is satisfied. 壁 ≥G' 硬 Even within the aggregate, microcapsules can still rupture due to excessive localized stress. Therefore, a contact angle ≥90° ensures uniform dispersion and G' 壁 ≥G' 硬 The ensured mechanical strength works in tandem to guarantee the integrity of the microcapsules throughout the entire process from ink dispersion to injection molding.

[0017] As a further improvement of the present invention, the core material of the thermochromic microcapsule comprises a leuco dye, a color developer, and a solvent, wherein the leuco dye is crystal violet lactone.

[0018] As a further improvement of the present invention, the average particle size of the thermochromic microcapsules is 1~10μm, more preferably 2~5μm. If the particle size is too large, it easily clogs the printing screen and has a fast settling rate in the ink, resulting in poor dispersion stability; if the particle size is too small, the core material coating is insufficient, the color saturation decreases, and it is prone to agglomeration during printing. This particle size range can balance printability, dispersion stability, and color-changing performance.

[0019] As a further improvement of the present invention, the core material of the thermochromic microcapsule also contains a fluorescent dye; when the thermochromic microcapsule is in a decolorized state, the fluorescent dye emits fluorescence under ultraviolet light excitation; when it is in a colored state, the fluorescence of the fluorescent dye is masked by the color of the leuco dye. The fluorescent dye is pre-treated with siloxane to prevent thermal degradation at injection molding temperatures above 220°C.

[0020] The microcapsule wall material of this invention not only protects the leuco dye system but also simultaneously protects functional materials such as fluorescent dyes. The siloxane coating layer acts as a secondary protective barrier, preventing adverse reactions between fluorescent dyes and other components of the wall or core material during high-temperature injection molding. Through the unified encapsulation of highly heat-resistant microcapsules, independent protection schemes for each functional material are eliminated, enabling the synergistic survival of multiple functional materials at the high temperatures of IMR injection molding. This provides a feasible technical path for the integration of multi-response functions.

[0021] As a further improvement of the present invention, the thermochromic printing layer is formed by printing ink through gravure printing or screen printing processes, wherein the solid content of the printing ink is 40-60%, and the thermochromic microcapsules account for 5-20% of the total mass of the printing ink.

[0022] As a further improvement of the present invention, the material of the hardening layer is a UV-curable resin or a thermosetting resin, with a pencil hardness of not less than 2H; preferably 3~4H, and / or... The base film is a polyethylene terephthalate (PET) film or a polycarbonate (PC) film with a thickness of 20-200 μm; preferably 50-100 μm, and / or, The printing ink also contains at least one of colored pigments, metallic powders, or pearlescent powders to achieve integrated printing of color-changing effects and decorative patterns.

[0023] According to a second aspect of the present invention, a method for preparing the thermochromic in-mold transfer film is provided, comprising the following steps: S1: Sequentially coat the release layer and the hardening layer onto the base film; S2: The thermochromic printing layer is formed on the hardened layer by printing ink containing thermochromic microcapsules through a printing process; S3: Apply an adhesive layer to the thermochromic printing layer.

[0024] As a further improvement of the present invention, in step S2, the curing conditions of the thermochromic printing layer are: temperature 60~90℃, time 1~5 minutes. These conditions can complete the ink drying and film formation without damaging the microcapsule structure, avoid premature evaporation or migration of the core material solvent leading to deterioration of the color-changing performance, and at the same time ensure the adhesion of the printing layer.

[0025] According to a third aspect of the present invention, an application of the thermochromic in-mold transfer film is provided in an in-mold decoration process, wherein the in-mold decoration process includes at least one of an in-mold transfer process, an in-mold labeling process, and a insert injection molding process. In application, the thermochromic in-mold transfer film is placed in the mold, and after injection molding, the thermochromic printing layer is bonded to the surface of the plastic part; the injection molding temperature is 200~300℃, and the holding pressure is 50~150MPa.

[0026] The thermochromic in-mold transfer film of the present invention is suitable for in-mold decoration processes, and is preferably used for in-mold transfer processes. In application, the IMR film is placed in an injection mold, molten resin is injected, and after cooling and molding, the peelable base film is peeled off, so that the hardened layer, thermochromic printing layer and adhesive layer are transferred and firmly bonded to the surface of the plastic part. When used for in-mold labeling or insert injection molding processes, the peelable base film is replaced with a permanent substrate with thermoformability, and the binder used for the thermochromic printing layer has the flexibility to adapt to thermal stretching, so that the printing layer does not crack or fall off during the thermoforming process.

[0027] In summary, the technical solutions conceived by this invention have the following beneficial effects compared with the prior art: (1) This invention uses melamine-formaldehyde resin or urea-formaldehyde resin as the microcapsule wall material, so that the overall thermal decomposition temperature of the microcapsule is higher than 220°C, which can withstand the high temperature short-term thermal shock of the IMR injection molding process, ensuring that the thermochromic core material retains its color-changing function completely after injection molding, thus solving the technical problem that conventional thermochromic materials will permanently fail due to thermal decomposition in the IMR process. This invention protects the core material by covering it with a high heat-resistant wall material, combined with the wear resistance, scratch resistance and environmental aging protection provided by the outer hardened layer, so that the product has excellent comprehensive durability and can meet the requirements of long-term product reliability in industries such as automotive interiors and home appliance panels.

[0028] (2) By controlling the contact angle between the microcapsule wall material and the binder, the present invention achieves intrinsically stable dispersion of microcapsules in ink by utilizing the principle of interfacial incompatibility. This is different from the traditional scheme that relies on dispersants, fundamentally avoiding the problems of microcapsule aggregation and sedimentation, and ensuring the consistency of printing quality and product yield.

[0029] (3) The present invention uses a modulus matching design between the hardened layer and the microcapsule wall material to enable the microcapsule to effectively resist the shrinkage stress in the IMR process, avoid the shell being crushed or cracked, and ensure the structural integrity of the microcapsule and the long-term stability of its color-changing performance.

[0030] (4) This invention enables thermochromic microcapsules to be directly dispersed as functional pigments in printing inks and formed in one step by gravure or screen printing, realizing the integrated printing of color-changing function and decorative pattern, and supporting high-precision local pattern design. At the same time, the microcapsule platform can encapsulate multiple functional materials simultaneously, realizing the synergistic integration of multiple functions such as thermochromic and fluorescent anti-counterfeiting, and each functional unit can withstand the high temperature of IMR injection molding, without the need to design independent protection schemes for each functional material.

[0031] (5) The technical solution of the present invention is closely based on the existing IMR process design. It is only necessary to replace the ordinary ink with the thermochromic ink of the present invention in the printing process of the existing IMR film production line to realize the functional upgrade from static decoration to intelligent dynamic color change without changing the existing film structure or modifying the production line. At the same time, it can be extended to other in-mold decoration processes such as IML and INS by adjusting the performance of the substrate and binder. It has good industrial applicability and promotion value. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the structure of the thermochromic in-mold transfer film according to an embodiment of the present invention.

[0033] In all the accompanying drawings, the same reference numerals denote the same technical features, specifically: 110, base film; 120, release layer; 130, hardening layer; 140, thermochromic printing layer; 150, adhesive layer. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments and comparative examples. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention. Unless otherwise specified, the equipment and raw materials used in the following embodiments and comparative examples are conventionally selected in the art.

[0035] Example 1 This embodiment provides an IMR film for automotive air conditioning vent trim strips with localized thermochromic properties. Its preparation method is as follows: (1) Preparation of thermochromic microcapsules The wall material is melamine-formaldehyde resin; by weight, the core material consists of 1 part crystal violet lactone (leuco dye), 3 parts bisphenol A (color developer), and 20 parts tetradecyl alcohol (solvent).

[0036] Thermochromic microcapsules were prepared by in-situ polymerization. The components of the core material were mixed and heated to melt, and an aqueous solution of melamine-formaldehyde prepolymer was added. The pH value was adjusted to 3.8-4.5, the temperature was raised to 80-90℃, and the reaction was stirred for 2-3 hours. After cooling, filtration, washing, and drying, microcapsule powder was obtained.

[0037] Tests showed that the microcapsules had an average particle size of approximately 3 μm, a thermal decomposition temperature (TGA method) of 228℃, and a storage modulus of 1.5 GPa (DMA method) at 25℃. The color change temperature range was 15℃ to 28℃ (with the critical temperature for color fading upon heating being approximately 28℃, and the critical temperature for color development upon cooling being approximately 15 to 20℃ due to the hysteresis effect of the thermochromic system).

[0038] (2) Preparation of thermochromic ink The raw materials for thermochromic ink include 15wt% thermochromic microcapsules, 38.5% chlorinated polypropylene binder, 1wt% dispersant (BYK-110), 0.5wt% leveling agent (BYK-333), and 45wt% organic solvent (toluene / methyl ethyl ketone, mass ratio 1:1).

[0039] The binder, dispersant, leveling agent and organic solvent are mixed and stirred until the resin is completely dissolved; thermochromic microcapsules are added, and the mixture is stirred and ground at high speed to prepare a screen printing ink with a solid content of about 55%.

[0040] According to the contact angle meter test, the contact angle between the microcapsule wall material and the chlorinated polypropylene binder is 105°.

[0041] (3) Preparation of IMR functional thin films a. Use PET film with a thickness of 50μm, and the printed surface is corona treated with a surface tension ≥42dyn / cm.

[0042] b. Coat the corona-exposed surface of the PET film with an organosilicon release agent, with a wet film thickness of 10 μm, and then heat-cur it to form a release layer.

[0043] c. UV-curable acrylic resin is coated onto the release layer using a microgravure coating process, and then UV-cured to form a hardened layer with a dry film thickness of approximately 5 μm and a pencil hardness of 3H. DMA testing shows that the storage modulus of this hardened layer at 25°C is 1.2 GPa.

[0044] d. Apply the prepared thermochromic ink to the hardened layer, and form the set grid pattern by screen printing with a 400-mesh screen. Dry at 70°C for 3 minutes to form a thermochromic printing layer with a dry film thickness of about 12μm.

[0045] e. Apply a polyolefin adhesive to the thermochromic printing layer and dry at 80°C for 3 minutes to form an adhesive layer with a dry film thickness of approximately 20 μm.

[0046] (4) IMR injection molding: After cutting the aforementioned IMR film, it was placed in the injection mold of the air conditioner vent trim strip. ABS resin at 240±5℃ was injected, with an injection pressure of 80MPa, a holding pressure of 60MPa, and a holding time of 3 seconds. After cooling, the mold was opened, and the PET base film was peeled off from the edge of the product. The hardened layer, thermochromic printing layer, and adhesive layer were completely transferred to the surface of the plastic part.

[0047] The decorative strip prepared in this embodiment appears light blue in the grid pattern area at room temperature (25°C) (because the color concentration of the microcapsules is low, it appears visually close to gray after being superimposed on the gray ABS base color). When the air conditioner blows cold air (about 15°C) directly, the blue color deepens significantly. When the air conditioner blows warm air (about 40°C) or when touched by a finger, the blue color completely fades and returns to gray. The response is sensitive and the effect is completely reversible.

[0048] Example 2 This embodiment provides an IMR film for home appliance control panels that has an overall white background and changes color with temperature.

[0049] The difference between this embodiment and Embodiment 1 is that: The ink formulation consists of 8 wt% thermochromic microcapsules (same as in Example 1), 25 wt% titanium dioxide, 1 wt% dispersant (BYK-110), 22 wt% vinyl chloride-vinyl acetate copolymer binder, 0.5 wt% leveling agent (BYK-333), and 43.5 wt% organic solvent (butanone / cyclohexanone, mass ratio 1:1), resulting in a white thermochromic gravure printing ink with a solid content of approximately 55%. Testing showed that the contact angle between the microcapsule wall material and the binder was 98°.

[0050] The base film is a 38μm thick PET film; the thermochromic printing layer is printed on the entire surface using a 200 lines / inch gravure printing plate and dried at 80℃, with a dry film thickness of about 8μm. This printing layer has the dual functions of a white decorative base color and overall thermochromic effect; PC / ABS alloy is injected at 260℃ during injection molding.

[0051] The control panel obtained in this embodiment is pure white at room temperature. When the ambient temperature is below 20°C, the entire panel gradually turns light blue. When the temperature rises to above 40°C, it returns to pure white.

[0052] Example 3 This embodiment provides a dual-response thermochromic IMR film for automotive logo trim, which integrates fluorescent anti-counterfeiting function based on Embodiment 1.

[0053] The difference between this embodiment and Example 1 is that the microcapsule core material is composed of crystal violet lactone, bisphenol A, tetradecyl alcohol, and rhodamine-based red fluorescent probe in a molar ratio of 0.8:1.0:3.0:0.05, wherein the rhodamine-based red fluorescent probe is pre-coated with siloxane. Microcapsules with an average particle size of approximately 3 μm and a thermal decomposition temperature of 226°C were obtained. The storage modulus G' of the wall material at 25°C was also measured. 壁 The value is 1.4 GPa.

[0054] The ink formulation consists of: 12 wt% thermochromic microcapsules, 39.5 wt% transparent polyurethane binder, 2 wt% blue dye, 1 wt% dispersant (BYK-110), 0.5 wt% leveling agent (BYK-333), and 45 wt% organic solvent (butanone / cyclohexanone, mass ratio 1:1). The ink is deep blue at low temperatures and becomes transparent at high temperatures. The contact angle between the microcapsule wall material and the binder is 95°.

[0055] In the preparation method, a metallic silver base color is first printed on the hardened layer using ordinary ink gravure printing, and then the above-mentioned dark blue thermochromic ink is precisely overprinted on the brand logo area using a 300-mesh nickel foil electroforming screen to form a patterned thermochromic printing layer; during IMR injection molding, polypropylene PP material at 220℃ is injected.

[0056] The finished cap obtained in this embodiment is metallic silver. At room temperature (25℃), the logo area is a deep blue, covering the silver background underneath. When heated or touched, the logo becomes transparent, revealing the silver background. Under 365nm ultraviolet light, regardless of whether the logo is in a colored or colorless state, the red fluorescence signal of the Rhodamine fluorescent probe can be observed, realizing the integration of thermochromic and fluorescent anti-counterfeiting functions.

[0057] Example 4 The difference between this embodiment and Embodiment 1 is that: the wall material is urea-formaldehyde resin, the thermal decomposition temperature (TGA method) of the microcapsules is 218℃, and the storage modulus G' of the wall material at 25℃ is... 壁 The hardness is 1.1 GPa. The hardened layer is made of thermosetting acrylic resin with a pencil hardness of approximately 2H and a storage modulus G' of 0.9 GPa at 25°C.

[0058] The thermochromic microcapsules obtained in Example 1 were directly added to the wood grain gravure printing ink at a ratio of 5 wt%, and stirred evenly to prepare a thermochromic wood grain ink. This ink was then used on the hardened layer to form a wood grain pattern with thermochromic function in a single gravure printing process, omitting the steps of separately preparing the thermochromic ink and printing the color-changing layer as in Example 1. The printing layer thickness and drying conditions were the same as in Example 1. ABS resin was injected at 240±5℃.

[0059] The IMR film prepared in this embodiment has a clear wood grain texture and thermochromic properties, and can withstand injection molding temperatures above 220°C.

[0060] Comparative Example 1 The difference between this comparative example and Example 1 is that the microcapsules were prepared using a complex coagulation method with a gelatin-gum arabic composite as the wall material, resulting in microcapsules with a thermal decomposition temperature of only 180°C. All other preparation processes and application conditions are identical to those in Example 1.

[0061] The product prepared in this comparative example completely lost its color-changing function after injection molding at 240℃. Microscopic observation showed that a large number of microcapsules ruptured, indicating that if the heat resistance of the microcapsule wall material does not meet the requirement of temperatures above 220℃, even if the film structure is the same, it cannot withstand the high temperature of IMR injection molding.

[0062] Comparative Example 2 The difference between this comparative example and Example 1 is that polyvinyl butyral (PVB) was used as a binder when preparing the thermochromic ink. The contact angle between this binder and the microcapsule wall material was approximately 65° (<90°). The remaining preparation process and application conditions were exactly the same as in Example 1.

[0063] The inks in this comparative example showed significant sedimentation and aggregation after standing for 15 minutes; screen printing resulted in screen clogging and blurred pattern edges; and after injection molding transfer, the color-changing effect was uneven, with some areas showing no color change response. Microscopic observation revealed a large aggregation of microcapsules with significant differences in distribution density. This indicates that when the contact angle is <90°, the microcapsules and the binder exhibit excessively good interfacial compatibility, leading to wetting aggregation and preventing the achievement of a stable dispersion and uniform color-changing effect.

[0064] Comparative Example 3 The difference between this comparative example and Example 1 is that the hardening layer uses a high-rigidity UV-curable resin, whose storage modulus G' after curing at 25°C is... 硬 The energy storage modulus G' of the microcapsule wall material is 2.5 GPa. 壁 It is 1.5 GPa, or G' 硬 >G' 壁 The remaining preparation process and application conditions are exactly the same as in Example 1.

[0065] In this comparative example, although the color-changing function was partially preserved after injection molding, the color contrast decreased significantly and the response speed slowed down. Microscopic observation revealed that a large number of microcapsules were crushed or cracked.

[0066] Comparative Example 4 The difference between this comparative example and Example 3 is that the fluorescent dye (rhodamine-based red fluorescent probe) was not pretreated with siloxane coating and was directly added to the core material for microencapsulation. All other preparation processes and application conditions were exactly the same as in Example 3.

[0067] The products obtained in Examples 1-4 and Comparative Examples 1-4 were tested, and the results are shown in the table below: IMR process feasibility: Place the IMR film in the injection mold and injection mold it at the set temperature (ABS 240℃, PC / ABS 260℃, PP 220℃). Observe whether the pattern is completely transferred and whether the base film can be easily peeled off after demolding.

[0068] Color change performance after injection molding: Visual inspection and colorimeter measurement to observe the appearance and fading of blue color. Surface hardness: According to ASTM D3363 standard, a pencil hardness tester was used with a load of 750g and a 45° angle scratch test. The highest hardness without scratching the coating was taken as the result.

[0069] Abrasion resistance: In accordance with ASTM F2357 standard, the RCA paper tape abrasion tester was used with a load of 175g to record the number of cycles when the coating was worn through.

[0070] Alcohol resistance test: Use a cotton ball soaked in anhydrous alcohol to wipe the coating surface 50 times with a load of 500g, and observe the changes in appearance.

[0071] Cycle life: The sample was placed between 15℃ (30 min) and 35℃ (30 min) for temperature change and the number of cycles was recorded when ΔE*ab dropped to below 70% of the initial value.

[0072] Fluorescence function: Excitation with a 365nm UV lamp was used, and the fluorescence intensity retention rate of the sample after injection molding was calculated based on the fluorescence intensity of the microcapsule powder before injection molding (100%).

[0073]

[0074] The results show that all embodiments can successfully complete the IMR process and obtain complete transfer prints. Examples 1-3 all exhibited sensitive and reversible color-changing response, surface hardness above 3H, excellent abrasion and chemical resistance, and a cycle life exceeding 5000 cycles under different application scenarios. Example 3, in particular, achieved fluorescent function integration (fluorescence intensity retention >90%) while maintaining the above performance. Example 4, while reducing costs, still achieved good color-changing effects and a cycle life exceeding 3000 cycles, meeting the needs of cost-sensitive applications.

[0075] In Comparative Example 1, the microcapsule wall material had insufficient heat resistance, resulting in a complete loss of color-changing function after injection molding. In Comparative Example 2, the contact angle was less than 90°, and microcapsule aggregation led to severely uneven color-changing effects. In Comparative Example 3, the modulus was mismatched, and the curing shrinkage stress of the hardened layer and the thermal shock of injection molding could not be effectively buffered by the microcapsules, resulting in a large number of microcapsules rupturing and a significant decrease in color-changing performance and cycle life. In Comparative Example 4, the fluorescent dye was not protected against heat, and its fluorescent function was basically lost after injection molding.

[0076] In summary, this invention solves the problems of high-temperature thermal decomposition, dispersion instability, and mechanical failure of thermochromic materials in the IMR process through the synergistic effect of heat-resistant microcapsule wall material design, contact angle control, and modulus matching. This allows the IMR film to maintain excellent process compatibility and long-term durability while also possessing high-precision patterning capabilities. Furthermore, the high-heat-resistant microcapsule platform of this invention provides unified synergistic protection for different functional materials, eliminating the need for separate protection schemes for each functional material. This allows all functional units to withstand the high temperatures of IMR injection molding, providing a feasible technical path for multi-response function integration.

[0077] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A thermochromic in-mold transfer film, characterized in that, It includes a base film, a release layer, a hardening layer, a thermochromic printing layer, and an adhesive layer that are stacked in sequence; The thermochromic printing layer comprises a pattern formed by printing ink, wherein the printing ink comprises thermochromic microcapsules uniformly dispersed therein; The wall material of the thermochromic microcapsule is melamine-formaldehyde resin or urea-formaldehyde resin, and the thermal decomposition temperature of the thermochromic microcapsule is higher than 220°C.

2. The thermochromic in-mold transfer film according to claim 1, characterized in that, The printing ink also includes a binder resin, and the contact angle between the wall material of the thermochromic microcapsule and the binder resin is ≥90°.

3. The thermochromic in-mold transfer film according to claim 2, characterized in that, The energy storage modulus of the hardened layer at 25°C is G'. 硬 The storage modulus of the wall material of the thermochromic microcapsule at 25°C is G'. 壁 , and G' 壁 ≥G' 硬 .

4. The thermochromic in-mold transfer film according to any one of claims 1-3, characterized in that, The core material of the thermochromic microcapsule contains a leuco dye, a color developer, and a solvent, wherein the leuco dye is crystal violet lactone. The average particle size of the thermochromic microcapsules is 1~10μm.

5. The thermochromic in-mold transfer film according to claim 4, characterized in that, The core material of the thermochromic microcapsule also contains a fluorescent dye; when the thermochromic microcapsule is in a decolorized state, the fluorescent dye emits fluorescence under ultraviolet light excitation; when it is in a color-developing state, the fluorescence of the fluorescent dye is masked by the color of the leuco dye. The fluorescent dye is pre-coated with siloxane to prevent thermal degradation at injection molding temperatures above 220°C.

6. The thermochromic in-mold transfer film according to claim 1, characterized in that, The solid content of the printing ink is 40-60%, and the thermochromic microcapsules account for 5-20% of the total mass of the printing ink.

7. The thermochromic in-mold transfer film according to claim 1, characterized in that, The hardened layer is made of UV-curable resin or thermosetting resin, with a pencil hardness of not less than 2H; and / or, The base film is a polyethylene terephthalate film or a polycarbonate film with a thickness of 20~200μm; and / or, The printing ink also contains at least one of colored pigments, metallic powders, or pearlescent powders.

8. A method for preparing a thermochromic in-mold transfer film according to any one of claims 1-7, characterized in that, Includes the following steps: S1: Sequentially coat the release layer and the hardening layer onto the base film; S2: The thermochromic printing layer is formed on the hardened layer by printing ink containing thermochromic microcapsules through a printing process; S3: Apply an adhesive layer to the thermochromic printing layer.

9. The method for preparing the thermochromic in-mold transfer film according to claim 8, characterized in that, In step S2, the curing conditions for the thermochromic printing layer are: temperature 60~90℃, time 1~5 minutes.

10. The application of the thermochromic in-mold transfer film according to any one of claims 1-7 in an in-mold decoration process, characterized in that, The in-mold decoration process includes at least one of in-mold transfer printing, in-mold labeling, and insert injection molding. In application, the thermochromic in-mold transfer film is placed in the mold, and after injection molding, the thermochromic printing layer is bonded to the surface of the plastic part; the injection molding temperature is 200~300℃, and the holding pressure is 50~150MPa.

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