A multi-color responsive ink based on temperature-sensitive color-changing microspheres and a printing application method thereof

Abstract

The application discloses a kind of multi-color response ink based on temperature-sensitive color-changing microspheres and printing application method thereof, belong to the field of intelligent anti-fake functional materials.The ink includes temperature-sensitive color-changing microspheres 20-30 parts, connecting material resin 40-50 parts, solvent 20-30 parts, auxiliary agent 2-5 parts by weight;The temperature-sensitive color-changing microspheres are gradient density core-shell structure, adjacent shell layer is covalently linked by grafting double bond copolymerization of silane coupling agent, and the outer shell layer is fixed with the up-conversion nanoparticle-spiropyran composite functional unit connected by amide bond through free radical copolymerization.The ink of the application has body temperature sensitivity, ultraviolet light, near-infrared light three independent multi-color response, and the light transmittance at 610 nm wavelength is more than 87%.The light response performance retention rate is more than 90% after 1000 times rubbing, solves the defects that the existing anti-fake ink structure stability is poor, chromogenicity and mechanical strength are not compatible, adapts mainstream printing process such as silk screen, intaglio, digital inkjet, and can be widely used in high-end anti-fake, intelligent packaging field.

Description

Technical Field

[0001] This invention belongs to the field of intelligent anti-counterfeiting functional materials and printing technology, specifically relating to a multicolor responsive ink based on thermochromic microspheres and its printing application method. Background Technology

[0002] With the continuous upgrading of anti-counterfeiting technology, intelligent responsive anti-counterfeiting inks, due to their dynamic color-changing and strong verifiability, have become one of the core materials for anti-counterfeiting in high-end packaging, documents, and luxury goods. Among them, thermochromic and photochromic inks are the most widely used categories due to their fast response speed and high recognizability. Integrating multiple triggering modes and multi-color rendering effects into a single ink system enhances the anti-counterfeiting level and raises the barrier to counterfeiting, which is currently the core research and development direction of the industry.

[0003] Existing multi-color responsive anti-counterfeiting inks are mostly prepared by simple physical blending of thermochromic and photochromic materials, or by using composite microcapsules with a "core-shell-satellite" structure as functional fillers. The closest existing technology is Chinese invention patent CN121319909A, published on April 17, 2020, entitled "A Composite Color-Changing Agent with a Core-Shell-Satellite Structure and Its Preparation Method," which discloses a composite color-changing agent with a thermochromic material as the core, a transparent polymer as the shell, and a photochromic material as a satellite embedded on the shell surface. This achieves dual response to temperature and ultraviolet light, but still has inherent defects that cannot be avoided: First, poor structural stability. The photochromic material adheres to the surface of the microcapsules through physical adsorption and is easily detached during ink grinding, printing shearing, and daily friction, leading to rapid decay of the color-changing response function and a significant reduction in ink lifespan. Second, there is an irreconcilable contradiction between color development performance and mechanical strength. To protect the thermochromic core material, a method is used... The dense polymer shell severely blocks the transmission of the core layer's color-changing signal, resulting in small color difference and low color saturation. Thinning the shell to improve light transmittance reduces the microcapsule's shear resistance, making it prone to breakage and failure during printing. Third, the triggering mode is limited; most existing products can only achieve dual responses to temperature and ultraviolet light. Ultraviolet light penetration is extremely poor, making it impossible to achieve concealed anti-counterfeiting verification through the packaging film. Furthermore, it poses a certain degree of biological damage, resulting in low anti-counterfeiting levels and easy counterfeiting. Fourth, the multi-color effect is limited; it cannot achieve independent multi-level color development from multiple trigger sources, resulting in insufficient anti-counterfeiting dimensions. Fifth, printing adaptability is poor; existing functional microspheres have poor dispersion in ink systems, easily agglomerating and clogging the screen, making them unsuitable for various mainstream printing processes such as screen printing, gravure printing, and digital inkjet printing, severely limiting large-scale application.

[0004] Furthermore, Chinese patent CN113842375A, published on December 27, 2022, entitled "A Gradient Capsule Wall Microcapsule and Its Preparation Method and Application", discloses a sustained-release microcapsule with a gradient cross-linked capsule wall. Its core purpose is to regulate the drug release rate, which is completely unrelated to the technical needs of solving the contradiction between strength and light transmittance in the field of anti-counterfeiting. Chinese patent CN104610963B, published on August 17, 2016, entitled "A Near-Infrared Light-Controlled Upconversion Fluorescent Switch Material and Its Preparation Method", discloses an upconversion-spiropyran material covalently linked by amide bonds. It is only used for fluorescent switch anti-counterfeiting. Its core is to utilize the fluorescence emission of the upconversion particles themselves. It has never disclosed the use of its ultraviolet emission to drive photochromism, nor has it combined with thermosensitive color-changing microspheres to form a gradient core-shell structure to achieve multicolor response.

[0005] The aforementioned technological biases are commonly reflected in classic textbooks in this field, such as *Microcapsule Technology and Applications* (Chemical Industry Press, 2019) and industry reviews, such as "Research Progress of Intelligent Response Anti-counterfeiting Microcapsules" (*Fine Chemicals*, Vol. 39, No. 5, 2022). Existing technologies generally consider a dense shell to be the only way to balance the mechanical strength and encapsulation stability of microcapsules, while porous structures lead to a decrease in shell mechanical properties and an increase in light scattering loss. Therefore, optimization focuses on modifying the material of the dense shell, without ever proposing to simultaneously address the contradiction between strength and transmittance through gradient pore structures, let alone achieving independent multi-color anti-counterfeiting responses from multiple trigger sources. Overall, existing technologies lack the technological inspiration to combine the above-mentioned technologies to solve the inherent defects of existing anti-counterfeiting inks, and the long-standing technological biases in this field remain unresolved. Summary of the Invention

[0006] In view of this, this invention addresses the inherent defects of existing multicolor anti-counterfeiting inks, such as poor structural stability, incompatibility between color development performance and mechanical strength, single triggering mode, low anti-counterfeiting level, insufficient multicolor dimensions, and poor printability. It proposes a multicolor responsive ink based on thermochromic microspheres and its printing application method. Through the core design of thermochromic microspheres with a gradient density core-shell structure, the invention simultaneously solves the above-mentioned technical pain points and achieves a unified approach of multi-trigger multicolor response, high stability, high color development, and compatibility with all printing processes.

[0007] The multi-color response described in this invention refers to the ink achieving differentiated color development effects through three independent triggering methods: body temperature-sensitive, ultraviolet light, and near-infrared light. These effects include reversible temperature-sensitive color change from deep blue to colorless, ultraviolet photochromic color change from colorless to blue, and cascaded color development from colorless to blue. The three response modes can be triggered independently or combined to present a multi-dimensional, multi-color anti-counterfeiting effect, significantly improving the counterfeiting threshold and anti-counterfeiting level.

[0008] The technical solution of this invention is implemented as follows: This invention provides a multicolor responsive ink based on thermochromic microspheres, which, by weight, consists of the following components: 20-30 parts thermochromic microspheres, 40-50 parts binder resin, 20-30 parts solvent, and 2-5 parts additives. The thermochromic microspheres have a core-shell structure, including a thermochromic core layer and a gradient density composite shell layer covering the thermochromic core layer. The thermosensitive color-changing core layer is composed of fatty alcohol phase change materials with carbon chain lengths of C14-C18, lactone-based leuco dyes, and phenolic color developers. The gradient density composite shell consists of a dense inner shell, a gradient porosity intermediate shell, and a porous functional outer shell, from the inside out. Adjacent shells are connected by covalent bonding. The dense inner shell is a dense, non-porous layer composed of polyurethane and nano-silica, with a thickness of 50-100 nm, a porosity of 0-5%, and a Young's modulus of 1-2 GPa. Its surface is grafted with methacryloyloxypropyltrimethoxysilane to form methacryloyloxy double bond active sites. The gradient porosity intermediate shell is a composite layer of methyl methacrylate-hydroxyethyl methacrylate copolymer and nano-silica, with a thickness of 100-200 nm. Its porosity increases continuously radially outward from 5% at the junction with the dense inner shell to 25-35% at the junction with the porous functional outer shell. The porous functional outer shell is an interconnected open porous structure of polymethyl methacrylate-nano silica composite matrix, with a thickness of 100-150 nm, a porosity of 40-50%, and a pore size of 100-200 nm. The introduction of nano silica into the outer shell can further enhance the mechanical strength of the porous structure, prevent the collapse of the pores during printing and cutting, and optimize the refractive index matching effect of the shell, thereby further improving the light transmittance and color saturation. The porous functional outer shell layer has polymerizable composite functional units fixed on its pore inner walls and polymer framework via free radical copolymerization; the polymerizable composite functional units are surface-carboxylated NaYF4:Yb. 3+ Er 3+ The product is obtained by grafting a methacryloyloxy double bond onto the end of the @NaYF4 core-shell upconversion nanoparticles and the spiropyran derivative SP-3 modified with an amino group via an amide bond; the intermolecular distance between the upconversion nanoparticles and the spiropyran derivative SP-3 is 2-5 nm.

[0009] Furthermore, the phase change material is one or a mixture of two or more of tetradecyl alcohol, hexadecyl alcohol, and octadecyl alcohol, and the color-changing temperature of the thermosensitive color-changing microspheres is 25-60℃; the lactone-type leuco dye is at least one of crystal violet lactone and malachite green lactone, and the phenolic color developer is at least one of bisphenol A and p-phenylphenol. Here, the melting point of the phase change material directly determines the triggering temperature of the thermosensitive color change. By adjusting the carbon chain length and mixing ratio of the fatty alcohol, the triggering requirements of different application scenarios can be precisely matched, such as the 38-42℃ range triggered by human body temperature, which can achieve universal anti-counterfeiting verification without additional tools; its core principle is: when the ambient temperature is lower than the melting point of the phase change material, the color developer and the leuco dye molecules are in close contact and electron transfer occurs, and the leuco dye opens its ring to form a conjugated color-changing system; when the temperature rises above the melting point, the phase change material melts and destroys the contact environment between the two, the electron transfer is interrupted, the dye closes its ring and decolorizes, realizing a reversible thermosensitive color-changing response, constituting a first-dimensional multicolor color-changing channel.

[0010] Furthermore, the thermochromic core layer, by weight, comprises 80-95 parts of fatty alcohol phase change material, 1-5 parts of lactone-based leuco dye, and 3-15 parts of phenolic color developer. This formulation ensures the color saturation, response speed, and cycle stability of the thermochromic effect, avoids incomplete decolorization due to excessive color developer, and avoids background color interference due to excessive leuco dye, thus ensuring the stability of the color development effect of the thermosensitive channel.

[0011] Furthermore, the binder resin is one or a mixture of two or more of epoxy acrylate, vinyl chloride resin, and waterborne polyurethane; the solvent is at least one of deionized water, dipropylene glycol methyl ether acetate, ethanol, and ethyl acetate. The binder resin can be selected according to the printing method. For example, epoxy acrylate is used for screen printing to ensure adhesion and abrasion resistance after curing; vinyl chloride resin is used for gravure printing to meet the fast-drying requirements; and waterborne polyurethane is used for digital inkjet printing to ensure smooth inkjet printing, achieving full compatibility of the same ink system with mainstream printing processes. The selection of the solvent takes into account both environmental friendliness and compatibility with the resin and microspheres, avoiding solvent corrosion and damage to the microsphere shell and ensuring ink storage stability.

[0012] Furthermore, the additives consist of at least one of dispersant, leveling agent, defoamer, and ultraviolet absorber; the average particle size of the thermosensitive color-changing microspheres is 1-5 μm; the fineness of the inks used for screen printing and gravure printing is ≤15 μm, and the secondary dispersion particle size of the microspheres in the inks used for digital inkjet printing is ≤1 μm. The selection and dosage of these additives can be adjusted according to the printing process and application scenario. Dispersants can improve the uniformity of dispersion of the thermosensitive color-changing microspheres in the ink system, avoiding printing clogging and uneven color development caused by agglomeration; leveling agents and defoamers can ensure the smoothness of the printed film, avoiding pinholes and craters; ultraviolet absorbers can block ultraviolet light in the environment under non-verification conditions, preventing accidental triggering of spiropyran materials and improving the storage and use stability of the ink. The key to limiting the microsphere particle size and ink specifications is to adapt to the precision requirements of mainstream printing processes. A microsphere particle size of 1-5μm can ensure that it is not easily broken during ink grinding, and at the same time avoid problems such as screen clogging and line breakage during printing. The specification limit corresponding to the printing process can ensure the fineness of the printed pattern, realize the printing and forming of high-precision anti-counterfeiting patterns, and at the same time ensure uniform and consistent color development.

[0013] Furthermore, the polymerizable composite functional unit also incorporates an energy transfer bridge, which is one of ethylenediamine, propylenediamine, butylenediamine, or hexamethylenediamine, or CdSe / ZnS core-shell quantum dots with surface dicarboxyl groups. This energy transfer bridge design allows for more precise control of the distance between the upconversion nanoparticles and spiropyran molecules, while optimizing the matching degree between the donor emission spectrum and the acceptor absorption spectrum. Through a fluorescence resonance energy transfer mechanism, the excitation energy generated by upconversion is directly transferred to the spiropyran molecules without being emitted into free space as photons. This significantly improves the response efficiency and colorimetric sensitivity of near-infrared light excitation, further reducing the difficulty of replication and forming a second and third-dimensional multicolor colorimetric channel.

[0014] Furthermore, the nano-silica particles in the porous functional outer shell layer have a particle size of 10-30 nm and are added at a rate of 1%-5% of the mass of polymethyl methacrylate. This particle size and addition amount can maximize the mechanical strength and optical transmittance of the outer shell layer without affecting the pore structure.

[0015] Furthermore, the multi-color responsive ink possesses three independent color development response modes: a deep blue triggered by body temperature (37-42℃). The three response modes—colorless reversible thermosensitive color-changing response, colorless → blue photochromic response triggered by 365nm ultraviolet light, and colorless → blue cascaded photochromic response triggered by 980nm near-infrared light—do not interfere with each other and can independently or in combination present a multi-color anti-counterfeiting effect.

[0016] Furthermore, the cascaded photochromic response triggered by the 980nm near-infrared light can penetrate a 1mm thick PET film or a 2mm thick anti-counterfeiting paper to complete the color development, and is not affected by the obstruction of the surface non-transparent medium, thus realizing concealed anti-counterfeiting verification.

[0017] This invention also provides a printing application method for the above-mentioned multicolor responsive ink based on thermochromic microspheres, comprising the following steps: Step 1: Ink Preparation: Mix the binder resin, solvent, and additives, and stir at 800 rpm and 25°C for 30 minutes to disperse evenly. Then add thermosensitive color-changing microspheres and disperse at 2000 rpm for 30 minutes. For inks used in screen printing and gravure printing, grind them to a fineness of ≤15μm using a three-roll mill. For inks used in digital inkjet printing, disperse them using high-speed shearing until the secondary dispersion particle size of the microspheres is ≤1μm to obtain multi-color responsive inks. Step 2 Printing and forming: Using screen printing, gravure printing or digital inkjet printing, the multi-color responsive ink is printed on the surface of the substrate. After curing by the corresponding curing process, a printed product with anti-counterfeiting mark is obtained. Step 3: Multi-level and multi-color anti-counterfeiting verification: The printed materials are sequentially subjected to 37-42℃ body temperature touch temperature-sensitive color change verification, 365nm ultraviolet light irradiation color development verification, and 50mW 980nm near-infrared light penetration color development verification. Multi-dimensional anti-counterfeiting identification is achieved through different triggering methods.

[0018] This invention has the following core innovations and beneficial effects compared to the prior art: 1. This invention, through a gradient density core-shell structure design with continuously increasing radial porosity, breaks the long-standing technical prejudice in the field that "only a dense shell can balance the mechanical strength and encapsulation stability of microspheres." In the three covalently connected shell structure, the dense inner shell provides core mechanical support, while the gradient middle shell achieves a continuous and smooth transition of refractive index, eliminating Fresnel reflection loss at the interface between the dense layer and the porous layer. While maintaining a high Young's modulus of 1-2 GPa, the shell transmittance is increased from 58% in traditional dense shells to 87%, fundamentally solving the inherent contradiction between mechanical strength and color rendering transmittance in existing technologies, and providing an optical basis for multicolor rendering effects.

[0019] 2. This invention constructs a dual-stable structure through interlayer covalent bonding design and functional unit copolymerization fixation design. On the one hand, by introducing copolymerizable double bonds between the shell layers through a silane coupling agent, molecular-level covalent bonding between the polyurethane inner shell layer and the polyacrylate intermediate shell layer is achieved, completely avoiding interlayer delamination. On the other hand, through modification of polymerizable double bonds, the composite functional units are covalently grafted onto the polymer backbone during the polymerization of the outer shell layer, rather than being physically adsorbed or embedded. This completely solves the pain points of easy detachment and functional decay of photochromic materials in the prior art. After 1000 rubs, the photoresponse performance retention rate reaches more than 92%, ensuring the long-term stability of the multicolor response effect.

[0020] 3. This invention constructs a three-level independent multicolor response system, with the temperature-sensitive channel achieving deep blue. The colorless and reversible color change is achieved by using the ultraviolet channel to achieve colorless to blue color development and the near-infrared channel to achieve colorless to blue penetrating color development. The three response modes do not interfere with each other and can be triggered independently or combined to present a multi-color anti-counterfeiting effect. This breaks through the limitation of traditional anti-counterfeiting inks that can only achieve two-color changes, and upgrades the anti-counterfeiting level from Level 2 verification to Level 3 graded multi-color verification, which greatly increases the threshold for counterfeiting.

[0021] 4. The gradient shell structure and covalently bonded composite functional unit of the present invention form a significant synergistic effect: the high transmittance of the gradient shell provides an efficient transmission channel for near-infrared excitation light and multicolor color development signals, and the molecular-level spacing control of covalent bonding realizes efficient fluorescence resonance energy transfer. The combination of the two achieves for the first time 980nm near-infrared light driven microcapsule photochromism, which can penetrate 2mm paper and 1mm plastic film to complete concealed multicolor anti-counterfeiting verification, breaking through the scene limitations of traditional ultraviolet light excitation.

[0022] 5. The ink formula of this invention is compatible with a variety of mainstream printing processes such as screen printing, gravure printing, and digital inkjet printing. It can be applied on a large scale without modifying existing production lines and has strong industrial adaptability in fields such as high-end anti-counterfeiting and smart packaging. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] I. General Testing Methods 1. Microsphere size distribution: The particle size distribution was measured using a Malvern Zetasizer NanoZS90 laser particle size analyzer at a test temperature of 25℃. The dispersion medium was deionized water. The average value was taken from three parallel tests. 2. Shell morphology and thickness: The shell morphology and thickness were measured using a FEITecnai G2F20 transmission electron microscope (TEM). The microspheres were embedded in epoxy resin and prepared by ultrathin sectioning. Twenty intact microspheres were randomly selected to calculate the average thickness of each shell. 3. Porosity and porosity gradient: Focused ion beam (FIB) slicing combined with scanning transmission electron microscopy (STEM) electron energy loss spectroscopy (EELS) analysis was used. A test point was taken every 50 nm along the radial direction of the shell to statistically analyze the porosity change trend. The average value was taken for two parallel tests. 4. Young's modulus: Tested using an Agilent G200 nanoindenter, with a maximum load of 1 mN, a loading / unloading rate of 0.05 mN / s, and a holding time of 10 s. Ten microspheres were randomly selected for testing, and the average value was taken. 5. Transmittance: The transmittance of the microsphere / resin composite film at a wavelength of 610 nm (the characteristic peak of the temperature-sensitive core layer) was tested using a Shimadzu UV-3600 UV-Vis spectrophotometer. The film thickness was uniformly 50 μm. 6. Intermolecular distance measurement: The average distance between the upconversion nanoparticles and spiropyran molecules was calculated using Förster resonance energy transfer spectroscopy combined with dynamic light scattering. 7. Color difference value ΔE: The color difference was measured using a Minolta CR-10Plus colorimeter. The initial state of the sample was used as a reference. The Lab value after the response was measured, and the color difference value ΔE was calculated. Five parallel tests were conducted and the average value was taken. The test environment was 25℃, 50% relative humidity, and a dark environment. 8. Friction resistance: Tested according to GB / T7706-2008 "Relief Printing" standard, with a friction load of 2N and a friction speed of 60 times / min, the retention rate of the light response performance of the sample after 1000 friction cycles was tested. 9. Cyclic stability: 3000 temperature cycles (20℃) were completed using a high and low temperature alternating chamber. At 50℃, with a single cycle of 30s, 5000 light-switching cycles were completed using a UV lamp (30s irradiation / 30s darkness), and the retention rate of the color change performance of the sample after the cycles was tested. 10. Response performance testing: Thermosensitive response was triggered using a 38℃ constant temperature water bath; UV response was tested at 365nm and 10mW / cm². 2 UV lamp irradiation, near-infrared response using 980nm, 50mW / cm 2 Continuous laser irradiation was performed, and the response completion time and color difference value were recorded. 11. Storage stability: The ink was sealed and stored in a light-proof environment at 25°C for 6 months. The fineness / particle size, response color difference value, and performance retention rate of the ink were tested before and after storage. 12. Printing adhesion: Tested according to GB / T9286-1998 "Cross-cut test for paint and varnish film", using a 1mm grid, with grade 0 being the best.

[0025] II. Implementation Examples and Performance Verification Results Example 1: Preparation of Basic Thermosensitive Color-Changing Microspheres and Ink (corresponding to all technical features of claim 1) Step 1: Preparation of initial microspheres for thermosensitive color-changing core layer 90g of cetyl alcohol, 2g of crystal violet lactone, and 8g of bisphenol A were added to a sealed reactor and heated to 80℃±2℃ for constant temperature melting. The mixture was stirred at 300rpm for 30min to obtain a homogeneous oil phase. 1g of sodium dodecyl sulfate was added to 100g of deionized water and stirred at 25℃ to obtain an aqueous phase. The oil phase was poured into the aqueous phase at a uniform rate and emulsified at high speed of 10000rpm for 5min to obtain a homogeneous oil-in-water emulsion. 2g of isophorone diisocyanate was added to the emulsion, and high-purity nitrogen was introduced for protection. The mixture was stirred at 60℃±1℃ for 30min. Then, 10g of an aqueous solution containing 0.5g of ethylenediamine was slowly added dropwise at a rate of 0.5mL / min. After the addition was completed, the reaction was continued at this temperature for 2h. After the reaction was completed, the mixture was naturally cooled to 25℃, centrifuged at 3000rpm, and washed three times with deionized water to obtain a white slurry A of initial microspheres with a solid content of 20%.

[0026] Step 2: Fabrication of polymerizable composite functional units a. Take 50 mg of oleylamine-modified NaYF4:Yb 3+ Er 3+ @NaYF4 nanoparticles (particle size 25±5nm) were dispersed in 50mL of chloroform, and polyacrylic acid (Mw=1800, the density of carboxyl sites on the nanoparticle surface was determined to be 0.8mmol / g by acid-base titration) was added. The mixture was stirred vigorously at 25℃ and 500rpm for 24h to complete ligand exchange. The product was collected by centrifugation at 8000rpm and redispersed in deionized water to obtain a hydrosol of upconversion nanoparticles with carboxylated surface. b. Add 20 mg of amino-terminated spiropyran derivative SP-3, 30 mg of EDC, and 15 mg of NHS to the above hydrosol, wherein the molar ratio of EDC to carboxyl groups on the surface of upconversion nanoparticles is 1.2:1, and the molar ratio of NHS to carboxyl groups is 0.6:1. Adjust the pH of the system to 6.0 ± 0.2 with 0.1 mol / L dilute hydrochloric acid, and react at 25°C in the dark with stirring at 300 rpm for 12 h. c. Add 5 mg of glycidyl methacrylate to the reaction system, stir at 300 rpm for 6 h in the dark at 25 °C to introduce a methacryloyloxy double bond at the end of the product; after the reaction is completed, dialyze the product using a dialysis bag with a molecular weight cutoff of 3500 for 24 h to obtain an aqueous solution of polymerizable composite functional unit.

[0027] Step 3: Construction of Gradient Density Composite Shell a. Inner shell reinforcement and active site introduction: Take all of slurry A, dilute with deionized water to a solid content of 5%, add 0.5g γ-methacryloyloxypropyltrimethoxysilane (KH-570) and 2g tetraethyl orthosilicate, adjust the pH of the system to 9.5±0.2 with ammonia, stir at 25℃ and 300rpm for 12h to deposit a dense polyurethane-nano silica composite inner shell on the surface of the initial microspheres, and introduce methacryloyloxy double bond active sites on the shell surface at the same time; after the reaction is completed, centrifuge at 3000rpm and wash twice with deionized water to obtain slurry B; b. Construction of the middle and outer shell layers by continuous gradient polymerization: Two sets of pre-emulsions were prepared. Solution A was a low-porogen pre-emulsion: 10g methyl methacrylate, 1g hydroxyethyl methacrylate, 0.2g nano silica (particle size 20nm), 1g cross-linked PMMA porogen microspheres (particle size 50nm, cross-linking degree 8%), 0.1g AIBN initiator, 0.25g sodium dodecyl sulfate, and 25g deionized water. The mixture was stirred at 500rpm for 30min until homogeneous. Solution B was a high-porogen pre-emulsion: 10g methyl methacrylate, 1g hydroxyethyl methacrylate, 0.2g nano silica (particle size 20nm, added at 2% of the mass of methyl methacrylate), 5g cross-linked PMMA porogen microspheres (particle size 50nm, cross-linking degree 8%), 0.1g AIBN initiator, 0.25g sodium dodecyl sulfate, and 2g deionized water. 5g of deionized water was emulsified by stirring at 500rpm for 30min. 1.5g of the polymerizable composite functional unit aqueous solution prepared in step 2 was added evenly to solution A and solution B and stirred until homogeneous. Slurry B was added to a four-necked flask, and the temperature was raised to 65℃±1℃ in a water bath under high-purity nitrogen protection. Solution A and solution B were respectively placed into two synchronously operating constant-pressure dropping funnels, and continuous gradient dropping was completed within 4h: only solution A was added at the beginning of the dropping phase, and then the dropping acceleration rate of solution A was linearly decreased and the dropping acceleration rate of solution B was linearly increased within the next 4h, with the total dropping acceleration rate linearly increasing from 0.5mL / min to 2.5mL / min. Only solution B was added at the end of the dropping phase, and the stirring speed and temperature were maintained at 200rpm throughout the process. After the dropping was completed, the reaction was kept at this temperature for 1h, and then naturally cooled to 25℃. The pore-forming agent was completely removed by washing three times with tetrahydrofuran to obtain crude thermosensitive color-changing microspheres.

[0028] Step 4: Post-processing and ink preparation The crude microspheres were centrifuged and washed three times each with deionized water and anhydrous ethanol, and then spray-dried at an inlet temperature of 120℃ and an outlet temperature of 60℃ to obtain basic thermochromic microsphere powder. By weight, 25 parts of thermochromic microspheres, 45 parts of epoxy acrylate resin, 25 parts of dipropylene glycol methyl ether acetate, 31 parts of dispersant BYK-16, 0.3 parts of leveling agent BYK-333, 0.2 parts of defoamer BYK-022, and 3.5 parts of ultraviolet absorber UV-9 were used to prepare a screen printing type multicolor responsive ink.

[0029] Performance test results for this embodiment: The microspheres have an average particle size of 2.5 μm; a dense inner shell with a thickness of 75 nm, a porosity of 3%, and a Young's modulus of 1.6 GPa; a gradient-pore intermediate shell with a thickness of 150 nm and a porosity that continuously increases from 5% to 30%; a porous functional outer shell with a thickness of 120 nm, a porosity of 45%, and an average pore size of 150 nm; an average intermolecular distance of 3.2 nm between the upconversion nanoparticles and the spiropyran derivative; a transmittance of 87% at 610 nm wavelength; a thermosensitive response ΔE = 22 (38℃, 3s), an ultraviolet response ΔE = 20 (365 nm, 3s), and a near-infrared response ΔE = 18 (980 nm, 5s); a light response retention rate of 92% after 1000 rubs; a performance retention rate of 94% after 3000 temperature cycles; a near-infrared response ΔE = 15 after 2 mm of paper penetration; a performance retention rate of 91% after 6 months of storage; and a printing adhesion rating of 0.

[0030] Example 2: Preparation of thermochromic microspheres with different color-changing temperatures Only the phase change material in step 1 was changed; all other preparation processes and raw material ratios remained completely consistent with Example 1. Three types of microspheres were prepared, and the performance test results are as follows:

[0031] Example 3: Preparation of microspheres with different ratios of temperature-sensitive core layer Only the raw material ratio of the temperature-sensitive core layer in step 1 was changed; all other preparation processes and raw material ratios remained completely consistent with Example 1. Three types of microspheres were prepared, and the performance test results are as follows:

[0032] Example 4: Preparation of Thermosensitive Color-Changing Microspheres with Different Shell Parameters Only the process parameters in step 3 were adjusted; the remaining preparation processes and raw material ratios were completely consistent with those in Example 1. Four types of microspheres were prepared, and the performance test results are as follows:

[0033] Example 5: Preparation of composite functional unit microspheres with different intermolecular spacings Only the molar ratio of ethylenediamine in step 2 was adjusted to regulate the intermolecular distance between the upconversion nanoparticles and spiropyran. The rest of the preparation process and raw material ratios were completely consistent with Example 1. Three types of microspheres were prepared, and the performance test results are as follows:

[0034] Example 6: Preparation of Thermosensitive Color-Changing Microspheres Containing Energy Transfer Bridges Only the preparation process of the composite functional unit in step 2 was changed, introducing short-chain diamines and quantum dot energy transfer bridges respectively. The remaining steps were completely consistent with Example 1. The performance test results are as follows:

[0035] Example 7: Preparation of outer shell microspheres with different nano-silica parameters Only the particle size and amount of nano-silica in step 3 were adjusted; the rest of the preparation process and raw material ratios were completely consistent with Example 1. Three types of microspheres were prepared, and the performance test results are as follows:

[0036] Example 8: Preparation of Multicolor Responsive Inks for Different Printing Types Only the ink formulation and dispersion / grinding process were adjusted; all other raw materials used were the thermosensitive color-changing microspheres prepared in Example 1. Three types of inks were prepared, and the performance test results are as follows:

[0037] Example 9: Verification of Near-Infrared Transmission Colorimetric Performance The ink prepared in Example 1 was printed into 20 μm thick samples. After being covered with media of different thicknesses, the samples were irradiated with a 50 mW 980 nm laser for 10 seconds to test the color development performance. The results are as follows:

[0038] Example 10: Printing Application Method of Multicolor Responsive Ink Step 1: Printing and Molding 1. Screen printing: Using a 300-mesh polyester screen, the ink prepared in Example 8-1 was printed onto the surface of coated paper and PET film. The squeegee pressure was 0.3 MPa. After printing, it was cured by a UV curing lamp with a curing energy of 800 mJ / cm². 2 This yields printed samples; 2. Gravure printing: Using a 175-line gravure printing roller, the ink prepared in Example 8-2 was printed on the surface of the cigarette pack aluminum foil paper. The printing speed was 120m / min and the drying temperature was 80℃ to obtain the printed sample. 3. Digital inkjet printing: Using a piezoelectric inkjet printhead, the ink prepared in Examples 8-3 is printed on the surface of the document anti-counterfeiting paper. The printing resolution is 1200×1200dpi, and the paper is dried with hot air at 60℃ to obtain a printed sample.

[0039] Step 2: Multi-level, multi-color anti-counterfeiting verification 1. Level 1 Temperature Sensitive Verification: Touch the anti-counterfeiting pattern area of ​​the printed sample with your finger for 3 seconds and observe the color change from dark blue to colorless. The pattern should return to its original color within 10 seconds after you remove your finger to complete the initial universality verification. 2. Secondary UV verification: Irradiate the anti-counterfeiting pattern area with a 365nm UV banknote detector lamp for 3 seconds, observe the color development effect of the pattern from colorless to blue, and after being placed in the dark for 30 seconds, it will return to its original color, thus completing the routine anti-counterfeiting verification; 3. Level 3 Near Infrared Verification: Illuminate the anti-counterfeiting pattern area with a 50mW 980nm laser pointer. It can directly penetrate PET packaging film and 2mm thick paper. Illuminate for 5 seconds and observe the color development effect of the pattern changing from colorless to blue. After being placed in the dark for 60 seconds, it returns to its original color, completing the advanced anti-counterfeiting verification.

[0040] Performance test results of this embodiment: The sample patterns of the three printing methods are complete, without broken lines or plate blockage, with uniform color development, normal three-level response, and no pattern peeling or significant attenuation of response performance after 1000 rubs.

[0041] III. Comparative Content (Strictly Adhering to the Single Variable Principle) Comparative Example 1 is the closest to the existing technology reference sample (core-shell-satellite structure, CN121319909A). Step 1: Preparation of thermosensitive color-changing core layer microspheres: The process is completely consistent with Step 1 of Example 1; Step 2 Satellite structure loading: Disperse the spiropyran photochromic material in ethanol, add the above microsphere slurry, stir at room temperature for 4 hours, so that the photochromic material is embedded on the shell surface through physical adsorption, and centrifuge and wash to obtain the product; Step 3 Ink Preparation: Using the same formulation and process as in Example 1, a control ink was obtained.

[0042] Comparative Example 2: Uniform and dense shell control sample (no gradient structure) The only difference is that the gradient density composite shell is replaced with a single uniform dense shell, while the other raw materials, proportions, and processes are completely the same as in Example 1: the continuous gradient drop-addition process is cancelled, and the pre-emulsion is added to the reaction system at a uniform rate within 1 hour to polymerize and form a uniform dense shell, thus obtaining control microspheres and ink.

[0043] Comparative Example 3: Physically hybrid functional unit control sample (without amide bond covalent linkage) The only difference was that the covalently bonded polymerizable composite functional units were replaced with an equal mass of physically mixed carboxylated upconversion nanoparticles and spiropyran derivatives. All other raw materials, ratios, and processes were completely consistent with those in Example 1, resulting in control microspheres and ink.

[0044] Comparative Example 4: Control sample without upconversion nanoparticles (no near-infrared response) Only the upconversion nanoparticles in the composite functional unit were removed, and only an equal mass of spiropyran derivative was loaded. All other raw materials, ratios, and processes were completely consistent with those in Example 1, resulting in control microspheres and ink.

[0045] Comparative Example 5: Control sample without a gradient intermediate shell (no refractive index matching layer) Only the intermediate shell layer with gradient pores was removed, and a porous functional outer shell layer was directly coated on the outside of the dense inner shell layer. All other raw materials, proportions, and processes were completely consistent with those in Example 1, resulting in control microspheres and ink.

[0046] Comparative Example 6: Control sample without interlayer covalent bonding (only double bond active sites removed). In the inner shell layer reinforcement step, KH-570 of the same amount as in Example 1 was added and reacted for 12 hours. Then, hydroxyethyl methacrylate was added to seal the methacryloyloxy double bonds on the surface. All other raw materials, proportions, and process steps were completely consistent with Example 1. The only variable was whether there were copolymerizable double bond active sites between the layers, resulting in control microspheres and ink.

[0047] Comparative Example 7: Physical embedding control of functional units (without polymerizable double bond modification) Only the polymerizable double bond modification step of the composite functional unit was omitted, while the other raw materials, proportions, and processes were completely consistent with those in Example 1. The composite functional unit was only physically embedded and fixed in the outer shell layer to obtain control microspheres and ink.

[0048] IV. Core Performance Comparison and Creative Demonstration Table 1. Comparison of core performance between Example 1 and Comparative Examples 1-7

[0049] Creative reasoning conclusion: 1. The comparison between Comparative Example 2 and Example 1 demonstrates that the gradient density shell design, while maintaining a Young's modulus comparable to that of the dense shell, increases the transmittance from 58% to 87%, breaking the technical bias in this field; the transmittance of the sample without a gradient intermediate shell in Comparative Example 5 is only 65%, proving that continuous matching of the refractive index of the gradient intermediate shell is the core to achieving high transmittance, solving the inherent contradiction of the inability to simultaneously achieve strength and transmittance in existing technologies, and providing core optical support for multicolor color rendering effects.

[0050] 2. Comparison of Comparative Example 6 and Example 1 demonstrates that the interlayer covalent bonding design increases the Young's modulus of the microspheres from 0.9 GPa to 1.6 GPa, and the performance retention rate after 1000 rubs increases from 63% to 92%, fundamentally solving the problem of interlayer delamination, ensuring the structural stability of the microspheres during the printing process, and ensuring long-term stability of the multi-color response effect.

[0051] 3. Comparison of Comparative Examples 7 and 3 with Example 1 demonstrates that the copolymerization and fixation design of the composite functional unit increases the performance retention rate after friction from 71% to 92%, and the control of the molecular spacing of covalent bonding increases the near-infrared response ΔE from 8 to 18, achieving an efficiency improvement that is unpredictable to those skilled in the art. This completely solves the problem of easy detachment of photochromic materials and ensures the multicolor rendering effect of the ultraviolet and near-infrared channels.

[0052] 4. The gradient shell structure and covalently bonded composite functional unit of the present invention form a significant synergistic effect: the high transmittance of the gradient shell provides a transmission channel for near-infrared light, and the molecular-level spacing control of covalent bonding realizes efficient energy transfer. The combination of the two realizes near-infrared multicolor anti-counterfeiting verification that penetrates 2mm paper, while the existing technology comparison samples do not have this capability, which greatly improves the anti-counterfeiting level and multicolor display dimension.

[0053] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A multicolor responsive ink based on thermochromic microspheres, characterized in that, By weight, it consists of the following components: 20-30 parts of thermosensitive color-changing microspheres, 40-50 parts of binder resin, 20-30 parts of solvent, and 2-5 parts of additives. The thermochromic microspheres have a core-shell structure, including a thermochromic core layer and a gradient density composite shell layer covering the thermochromic core layer. The thermosensitive color-changing core layer is composed of fatty alcohol phase change materials with carbon chain lengths of C14-C18, lactone-based leuco dyes, and phenolic color developers. The density composite shell consists of a dense inner shell, a gradient porosity intermediate shell, and a porous functional outer shell, from the inside out. Adjacent shells are connected by covalent bonding. The dense inner shell is a dense, non-porous layer composed of polyurethane and nano-silica, with a thickness of 50-100 nm, a porosity of 0-5%, and a Young's modulus of 1-2 GPa. Its surface is grafted with methacryloyloxypropyltrimethoxysilane to form methacryloyloxy double bond active sites. The gradient porosity intermediate shell is a composite layer of methyl methacrylate-hydroxyethyl methacrylate copolymer and nano-silica, with a thickness of 100-200 nm. Its porosity increases continuously radially outward from 5% at the junction with the dense inner shell to 25-35% at the junction with the porous functional outer shell. The porous functional outer shell layer is an interconnected open porous structure of polymethyl methacrylate-nano silica composite matrix, with a thickness of 100-150 nm, a porosity of 40-50%, and a pore size of 100-200 nm. The porous functional outer shell layer has polymerizable composite functional units fixed on its pore inner walls and polymer framework via free radical copolymerization; the polymerizable composite functional units are surface-carboxylated NaYF4:Yb. 3+ Er 3+ The product is obtained by grafting a methacryloyloxy double bond onto the end of the @NaYF4 core-shell upconversion nanoparticles and the spiropyran derivative SP-3 modified with an amino group via an amide bond; the intermolecular distance between the upconversion nanoparticles and the spiropyran derivative SP-3 is 2-5 nm.

2. The multicolor responsive ink based on thermochromic microspheres according to claim 1, characterized in that, The phase change material is one or a mixture of two or more of tetradecyl alcohol, hexadecyl alcohol, and octadecyl alcohol; the color change temperature of the thermosensitive color-changing microspheres is 25-60℃; the lactone leuco dye is at least one of crystal violet lactone and malachite green lactone; and the phenolic color developer is at least one of bisphenol A and p-phenylphenol.

3. The multicolor responsive ink based on thermochromic microspheres according to claim 2, characterized in that, The thermosensitive color-changing core layer, by weight, consists of 80-95 parts of fatty alcohol phase change material, 1-5 parts of lactone leuco dye, and 3-15 parts of phenolic color developer.

4. The multicolor responsive ink based on thermochromic microspheres according to claim 1, characterized in that, The binder resin is one or a mixture of two or more of epoxy acrylate, chloroacetic acid resin, and waterborne polyurethane; the solvent is at least one of deionized water, dipropylene glycol methyl ether acetate, ethanol, and ethyl acetate.

5. The multicolor responsive ink based on thermochromic microspheres according to claim 1, characterized in that, The additives are composed of at least one of dispersant, leveling agent, defoamer, and ultraviolet absorber; the average particle size of the thermosensitive color-changing microspheres is 1-5 μm; the fineness of the inks for screen printing and gravure printing is ≤15 μm, and the secondary dispersion particle size of the microspheres in the inks for digital inkjet printing is ≤1 μm.

6. The multicolor responsive ink based on thermochromic microspheres according to claim 1, characterized in that, The polymerizable composite functional unit is further provided with an energy transfer bridge, which is one of ethylenediamine, propylenediamine, butanediamine, hexamethylenediamine, or CdSe / ZnS core-shell quantum dots with surface dicarboxyl groups modified.

7. The multicolor responsive ink based on thermochromic microspheres according to claim 1, characterized in that, The nano-silica particles in the porous functional outer shell layer have a diameter of 10-30 nm and are added at a rate of 1%-5% of the mass of polymethyl methacrylate.

8. The multicolor responsive ink based on thermochromic microspheres according to claim 1, characterized in that, The multi-color responsive ink has three independent color rendering response modes: a deep blue triggered by body temperature (37-42℃). The three response modes—colorless reversible thermosensitive color-changing response, colorless → blue photochromic response triggered by 365nm ultraviolet light, and colorless → blue cascaded photochromic response triggered by 980nm near-infrared light—do not interfere with each other and can independently or in combination present a multi-color anti-counterfeiting effect.

9. The multicolor responsive ink based on thermochromic microspheres according to claim 8, characterized in that, The cascaded photochromic response triggered by the 980nm near-infrared light can penetrate a 1mm thick PET film or a 2mm thick anti-counterfeiting paper to complete the color development, and is not affected by the obstruction of the surface non-transparent medium.

10. A printing application method for a multicolor responsive ink based on thermochromic microspheres according to any one of claims 1 to 9, characterized in that, Includes the following steps: Step 1: Ink Preparation: Mix the binder resin, solvent, and additives, and stir at 800 rpm and 25°C for 30 minutes to disperse evenly. Then add thermosensitive color-changing microspheres and disperse at 2000 rpm for 30 minutes. For inks used in screen printing and gravure printing, grind them to a fineness of ≤15μm using a three-roll mill. For inks used in digital inkjet printing, disperse them using high-speed shearing until the secondary dispersion particle size of the microspheres is ≤1μm to obtain multi-color responsive inks. Step 2 Printing and forming: Using screen printing, gravure printing or digital inkjet printing, the multi-color responsive ink is printed on the surface of the substrate. After curing by the corresponding curing process, a printed product with anti-counterfeiting mark is obtained. Step 3: Multi-level and multi-color anti-counterfeiting verification: The printed materials are sequentially subjected to 37-42℃ body temperature touch temperature-sensitive color change verification, 365nm ultraviolet light irradiation color development verification, and 50mW 980nm near-infrared light penetration color development verification. Multi-dimensional anti-counterfeiting identification is achieved through different triggering methods.

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