A kind of Co-Ni-MOF / aramid composite system-based thermochromic paper and its preparation method

Abstract

The application discloses a preparation method of a thermochromic paper based on a Co-Ni-MOF / aramid composite system, which realizes the structural and functional integration of a high-performance functional composite material through an integrated process route of "in-situ functionalization synthesis-multifield synergistic dispersion-radiation coupling curing". The method comprises three core steps: a microwave-assisted solvothermal method is used to synthesize Co-Ni-MOF nanocrystals with amino or epoxy groups grafted on the surface; through a direct current field pretreatment, a high-energy ultrasonic crushing and a microfluidic high-pressure shear synergistic dispersion process, the nanocrystals and aramid solution are uniformly compounded; after the composite slurry is coated into a film, a multi-stage programmed heat treatment is carried out to realize solvent removal, aramid curing and chemical bonding of the MOF-aramid interface. The obtained thermochromic paper has high mechanical strength, fast color change response and excellent cycle stability. The method is controllable, and the product has application prospects in the fields of intelligent packaging, flexible temperature sensing and anti-fake identification.

Description

Technical Field

[0001] This invention belongs to the field of special composite material technology, specifically relating to a thermochromic paper based on a Co-Ni-MOF / aramid composite system and its preparation method. Background Technology

[0002] With the rapid development of the Internet of Things, artificial intelligence and wearable electronics, thermochromic materials, which can respond quickly and intuitively to changes in ambient temperature, have shown important application value in fields such as intelligent temperature indication in cold chain logistics, overheat warning of electronic devices, building energy conservation, military camouflage and passive temperature monitoring in aerospace.

[0003] Metal-organic frameworks (MOFs), as novel thermochromic material systems, possess advantages such as strong structural designability, large specific surface area, and abundant porous environment. By introducing thermosensitive metal centers (such as Co(II) and Ni(II)) or utilizing host-guest charge transfer, sensitive and reversible color changes can be achieved. However, existing MOF-based thermochromic materials are mostly micron / nano-scale crystalline powders, which are brittle, lack self-supporting capabilities, and are difficult to directly process into flexible macroscopic devices. They need to be combined with polymer matrices before use.

[0004] Traditional composite methods face several technical bottlenecks: First, MOF particles have high surface energy, resulting in poor interfacial compatibility with general polymer matrices, leading to easy agglomeration and reduced color uniformity and mechanical properties. Second, MOFs and matrices are bonded solely by van der Waals forces, resulting in weak interfacial bonding and easy debonding under mechanical / thermal stress, leading to functional loss. Third, general polymer matrices (such as PVB and PMMA) have low mechanical strength and thermal stability, failing to meet the requirements of harsh environments.

[0005] Aramid fibers (meta-aramid / para-aramid) possess ultra-high tensile strength, excellent thermal stability (long-term operating temperature exceeding 200℃), chemical stability, and flame retardancy, making them an ideal matrix for high-performance paper-based composite materials. However, combining MOFs with aramid fibers still faces key challenges: the aramid solution has extremely high viscosity, which restricts the Brownian motion of MOF particles, making it difficult to overcome agglomeration using traditional dispersion methods; the aramid molecular chains are chemically inert, making it difficult to form effective chemical bonds with the unmodified MOF surface, resulting in prominent interface problems.

[0006] In the prior art, such as the nickel organic complex / polyurethane electrospun nanofiber membrane disclosed in patent CN118147821A, although it achieves flexibility, it has a porous and low-density structure with low mechanical strength and low process efficiency, which cannot meet the needs of large-scale preparation of dense and high-strength paper-based materials.

[0007] Therefore, it is urgent to develop a novel preparation strategy to solve the interfacial compatibility and bonding force problems between MOF and aramid matrix, achieve uniform and stable dispersion of MOF, and prepare thermochromic composite paper with integrated structure and function, excellent mechanical properties and sensitive color-changing function. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a thermochromic paper based on a Co-Ni-MOF / aramid composite system and its preparation method, thereby solving the aforementioned technical problems in the prior art.

[0009] The objective of this invention can be achieved through the following technical solutions: A method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system includes the following steps: S1. Preparation of surface-functionalized Co-Ni-MOF nanocrystals: Using pyromellitic acid as an organic ligand and cobalt and nickel salts as metal sources, Co-Ni-MOF nanocrystals with amino or epoxy groups grafted on their surface are synthesized by a solvothermal method in the presence of a silane coupling agent; wherein the molar ratio of cobalt salt to nickel salt is 1:10 to 10:1. S2. Preparation of high-uniformity composite slurry: The surface-functionalized Co-Ni-MOF nanocrystals obtained in S1 are dispersed in an organic solvent and subjected to DC electric field pretreatment, high-energy probe ultrasonic crushing, and microfluidic high-pressure shear mixing in sequence. The treated Co-Ni-MOF nano suspension is mixed with aramid solution to obtain Co-Ni-MOF / aramid composite slurry. S3. Paper Forming and Thermal Curing: The composite slurry is formed into a wet film, subjected to multi-stage heat treatment, and finally cooled and peeled off to obtain thermochromic paper.

[0010] Furthermore, in S1, the solvothermal method is a microwave-assisted solvothermal method, with a reaction temperature of 120-180℃ and a reaction time of 30-90 min; The cobalt salt is one or more of cobalt nitrate, cobalt acetate, or cobalt chloride; The nickel salt is one or more of nickel nitrate, nickel acetate, or nickel chloride.

[0011] Furthermore, in S1, the silane coupling agent has a bifunctional structure, with one end being a group that can react with the surface of the MOF crystal and the other end being an active functional group that can chemically react with the aramid matrix.

[0012] Furthermore, in S2, the electric field strength for the DC electric field pretreatment is 100-300V / cm, and the treatment time is 10-30min.

[0013] Furthermore, in S2, the preprocessing also includes ultrasonic processing of the probe after DC electric field processing, with an ultrasonic power of 300-500W and a processing time of 30-60min; The ultrasonic processing of the probe is performed in an ice-water bath, and the ultrasonic working mode is 2 seconds of ultrasound followed by 1 second of intermittent ultrasound.

[0014] Further, in S2, the mixing process is carried out in a microfluidic mixer, where the dispersion of the surface-functionalized Co-Ni-MOF nanocrystals and the aramid solution are mixed at an inlet pressure of 5-15 MPa and a shear rate of 10. 4 -10 5 s -1 Dynamic shear mixing is performed within a microfluidic mixer; during mixing, a Y-shaped or cross-shaped channel is followed by a serpentine mixing unit.

[0015] Further, the organic solvent is N-methylpyrrolidone or N,N-dimethylacetamide; the mass fraction of the aramid solution is 8-10%.

[0016] Furthermore, in S3, the multi-stage heat treatment process specifically includes: S301, First stage: Heating at 60-90℃ for 0.5-2 hours to remove most of the solvent; S302, Second stage: Heating at 100-150℃ for 0.5-1.5h to further remove residual solvent and promote the pre-arrangement of aramid molecular chains; S303, Third Stage: Simultaneously subject to ultraviolet irradiation or plasma treatment at 150-300℃ for 0.1-0.5h to achieve aramid curing and chemical bonding at the MOF-aramid interface; the ultraviolet light wavelength is 365nm and the intensity is 50-100mW / cm². 2 Furthermore, the thermochromic paper contains 5-20% Co-Ni-MOF by mass and has a thickness of 20-200 μm; the paper exhibits reversible thermochromic properties within a temperature range of 40-160℃, with a color change response time ≤5s and a tensile strength ≥50MPa.

[0017] The thermochromic paper is used in the fields of smart packaging, flexible temperature sensing, or anti-counterfeiting labels.

[0018] The beneficial effects of this invention are: 1. This invention, through molecular-level design, seamlessly integrates the inorganic functional unit MOF with the high-performance organic matrix aramid, resulting in a novel material that combines structure and function, rather than a simple mixture. It possesses both the intelligent color-changing function of MOF and the ultra-high mechanical properties, thermal stability, and chemical stability of the aramid matrix.

[0019] 2. This invention allows for convenient adjustment of the thermochromic response temperature of the material (e.g., continuously adjustable within the range of 40-160℃) by precisely controlling the molar ratio of the Co / Ni bimetallic compounds, to meet the needs of different application scenarios (such as body temperature monitoring, overheat warning for electronic devices, and temperature measurement of industrial pipelines). By adjusting the loading of MOF in the composite paper (e.g., 5-20wt%), the depth and contrast of the color can be effectively controlled.

[0020] 3. The aramid matrix used in this invention possesses excellent hydrophobicity and extremely high thermal stability, providing a robust "protective shell" for the internal MOF nanocrystals. This protective shell effectively shields against fluctuations in ambient humidity, making the material's color-changing behavior more stable and reliable, and greatly expanding its application range in harsh environments such as humidity and high temperature.

[0021] 4. The core interfacial chemical bonding strategy of this invention enables a strong chemical bond between the MOF and the aramid fiber. This strong interaction ensures that the MOF functional units will not detach from the matrix even under repeated bending, friction, or thermal cycling, thus giving the composite paper an ultra-long service life and excellent mechanical durability.

[0022] 5. The microwave solvothermal synthesis method used in this invention has a short time and uniform grain size; the casting to paper + programmed curing process is mature and suitable for the mass production of dense, high-strength thermochromic paper, which can be used in smart packaging, flexible temperature measurement, anti-counterfeiting labels and other scenarios. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0024] Figure 1 This is a schematic diagram of the overall process flow of the present invention; Figure 2 This is a SEM image of microwave-assisted in-situ surface functionalized Co-Ni-MOF nanocrystal material according to an embodiment of the present invention; Figure 3 This is the XRD pattern of the Co-Ni-MOF nanocrystal material of Example 1 of the present invention; Figure 4 These are the XRD patterns and physical images (embedded images) of the thermochromic paper of the Co-Ni-MOF / aramid composite system in Embodiment 1 of the present invention. Figure 5 These are DSC images of the Co-Ni-MOF nanocrystal materials from Examples 1 and 2 of this invention.

[0025] Figure 6This is a graph showing the transmittance variation at different temperatures in Example 1 of the present invention at wavelengths of 400-1000 nm. Figure 7 Optical photographs of the color of the thermochromic paper of the Co-Ni-MOF / aramid composite system in Example 1 of this invention at different temperatures. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0027] like Figure 1 As shown, the present invention employs a method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system, comprising the following three core steps: S1. Microwave-assisted in-situ surface functionalization of Co-Ni-MOF nanocrystals: The key innovation of this step lies in introducing "interface engineering" design at the initial stage of MOF crystal growth, pre-laying "chemical anchor points" for subsequent chemical bonding with the aramid matrix. The specific operation is as follows: Using pyromellitic acid or its derivatives as organic ligands, and a mixture of soluble cobalt salts (such as cobalt nitrate, cobalt acetate, and cobalt chloride) and nickel salts (such as nickel nitrate, nickel acetate, and nickel chloride) as metal sources, one or more silane coupling agents are simultaneously introduced into a solvent (such as DMF, ethanol, or a mixture thereof). The silane coupling agent has a bifunctional structure, with one end being a group that can react with the surface of MOF crystals (such as triethoxysilyl groups), and the other end being an active functional group that can chemically react with the aramid matrix (such as amino or epoxy groups). The silane coupling agent is γ-aminopropyltriethoxysilane (KH-550) or γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560).

[0028] The above-described mixed reaction system was placed in a microwave reactor for microwave-assisted solvothermal synthesis. Compared to traditional oil bath heating, microwave heating is faster, more uniform, and more efficient, significantly shortening the reaction time and producing MOF nanocrystals with smaller size and narrower distribution. Under the action of a microwave field, the reaction system can reach the specified temperature (e.g., 120-180℃) within tens of minutes, completing crystal nucleation and growth. During this process, a silane coupling agent is grafted in situ onto the surface of the newly formed MOF crystals, forming an organically functionalized shell. This shell not only improves the surface affinity of the MOF through its organic segments, initially suppressing its tendency to aggregate during subsequent dispersion, but more importantly, its exposed active functional groups (-NH2 or epoxy groups) provide the possibility for subsequent formation of strong covalent bonds with the aramid matrix.

[0029] By precisely controlling the molar ratio of cobalt salt to nickel salt in the reaction system (ranging from 1:10 to 10:1), the electronic structure and coordination environment of the obtained Co-Ni-MOF solid solution crystal can be systematically adjusted, thereby achieving "programmable" precise control of its thermochromic transition temperature (Tc) in a wide temperature range of 40-160℃.

[0030] S2. Preparation of composite slurry by multi-field synergistic dispersion of electric field-ultrasound-microfluidic: This step is crucial for achieving unprecedented uniform dispersion of MOF nanocrystals in an extremely viscous aramid solution, and it employs an innovative multi-physics synergistic strategy: DC electric field pretreatment: The surface-functionalized MOF nanocrystals prepared in S1 are initially dispersed in an organic solvent (such as N-methylpyrrolidone, NMP). Subsequently, a moderate-intensity DC electric field (e.g., 100-300 V / cm) is applied to the suspension. Due to the surface charge on the MOF nanocrystals caused by functionalization, electrophoretic movement occurs under the applied electric field. This directional migration effectively suppresses random collisions and aggregation between particles, forming an ordered, weakly interacting pre-dispersion state, laying the foundation for subsequent higher-energy dispersion processes. The experimental setup can be designed as a container containing parallel plate electrodes, with the electric field strength and treatment time precisely controlled by an adjustable DC power supply.

[0031] High-energy probe ultrasonic fragmentation: Next, the suspension, which has been pretreated with an electric field, undergoes high-energy probe ultrasonic treatment (power 300-500W). The local high temperature, high pressure and strong shear force generated by ultrasonic cavitation effectively break down the remaining soft aggregates, allowing the MOF nanocrystals to be stably dispersed in a state close to primary particles, forming a uniform nano-suspension.

[0032] Microfluidic high-pressure shear mixing: Finally, the above MOF nanosuspension and the pre-prepared aramid solution are pumped into a specially designed microfluidic mixing chip at a precise flow rate ratio using two independent precision pumps. Driven by high inlet pressure (5-15 MPa), the two fluids are forced to undergo intense laminar folding, stretching, and diffusion within micrometer-scale channels (e.g., Y-type or cross-type channels followed by serpentine mixing units), generating extremely high shear rates (up to 10). 4 -10 5 s -1 This extreme shear environment can instantly overcome the high viscosity barrier of aramid solution, forcibly and uniformly "kneading" MOF nanocrystals into the aramid molecular chain network, achieving molecular-level mixing.

[0033] S3. Multi-stage heat treatment and radiation-assisted interface curing: This step aims to transform a homogeneous composite pulp into a solid composite paper with excellent overall performance, and in the process, complete the crucial interfacial chemical bonding.

[0034] The composite slurry obtained from S2 is uniformly deposited on the substrate using a doctor blade coating method or a casting method. Subsequently, the wet film undergoes a carefully designed multi-stage heat treatment process: S301, First stage (low temperature solvent removal): Treat at a relatively low temperature (e.g., 60-90℃) for 0.5-2 hours. The purpose is to gently remove most of the high-boiling-point solvent (e.g., NMP) to avoid defects such as skin formation or pinholes on the membrane surface due to excessive solvent evaporation.

[0035] S302, Second Stage (Medium-Temperature Densification): Heat to 100-150℃ and hold for 0.5-1.5 hours. This stage aims to further remove residual solvent, while utilizing molecular thermal motion to promote the stretching and pre-alignment of aramid molecular chains, preparing for the formation of a dense network structure.

[0036] S303, Third Stage (High-Temperature Radiation-Coupled Curing): The temperature is further increased to 150-300℃, supplemented by ultraviolet (UV) irradiation (e.g., 365nm wavelength) or plasma treatment. This is another key innovation of the present invention. In this stage, the high temperature not only completely removes the binding solvent and initiates the final curing of the aramid matrix, but more importantly, the high-energy particles in the high-energy UV photons or plasma can effectively activate the functional groups on the MOF surface (e.g., opening the rings of epoxy groups) and certain sites on the aramid molecular chains, promoting chemical reactions between them to form stable covalent bonds (e.g., amide bonds, ether bonds, etc.). This "radiation-assisted curing" process firmly "welds" the originally physically dispersed MOF nanocrystals into the aramid matrix network, constructing a super-strong interfacial bonding layer.

[0037] After processing, the film is naturally cooled to room temperature and peeled off from the substrate to obtain the target product—thermochromic paper based on the Co-Ni-MOF / aramid composite system.

[0038] Example 1: The method for preparing thermochromic paper with a Co / Ni molar ratio of 1:1 and a MOF loading of 10wt% includes the following steps: S1, Synthesis of Surface-Aminated Co-Ni-MOF Nanocrystals Weigh 1.455 g (5 mmol) of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 1.454 g (5 mmol) of nickel nitrate hexahydrate (Ni(NO3)2·6H2O), and dissolve them in 150 mL of N,N-dimethylformamide (DMF), denoted as solution A. Weigh 2.542 g (10 mmol) of pyromellitic acid (H4btec) and 1.2 mL of the silane coupling agent γ-aminopropyltriethoxysilane (KH-550), and dissolve them in 150 mL of anhydrous ethanol, denoted as solution B. Under vigorous stirring, slowly add solution B dropwise to solution A, and continue stirring for 30 minutes to form a homogeneous precursor solution. After mixing A and B, transfer the mixture to a microwave reactor and react at 160 °C for 60 min. After the reaction, allow it to cool naturally to room temperature, and wash the resulting pink precipitate three times each with DMF and anhydrous ethanol by centrifugation (8000 rpm, 10 min). The washed product was placed in a vacuum drying oven and dried at 80°C for 12 hours to obtain surface amino-functionalized Co-Ni-MOF nanocrystal powder.

[0039] from Figure 3 As can be seen, the diffraction peaks are located at 8.3° and 10.8°, which correspond to the (011) and (002) lattice planes of Co-Ni-MOF, respectively.

[0040] S2. Preparation of composite slurry by synergistic dispersion of electric field-ultrasound-microfluidics 1.0 g of the dried MOF powder was dispersed in 100 mL of N-methylpyrrolidone (NMP) containing 0.1 wt% polyethylene glycol octylphenyl ether (Triton X-100). First, the suspension was placed between parallel plate electrodes (1 cm apart) and treated with a DC electric field of 200 V / cm for 20 minutes. Then, the electric field-treated suspension was placed in an ice-water bath and subjected to high-energy ultrasonic treatment using a titanium alloy probe (6 mm in diameter). The ultrasonic power was set to 400 W, the operating mode was 2 seconds of ultrasonic treatment followed by a 1-second interval, and the total treatment time was 40 minutes, resulting in a stable MOF nano-suspension.

[0041] Separately, 10g of meta-aramid (PMIA) short-cut fibers (6mm in length) were dissolved in 100g of NMP and mechanically stirred in an oil bath at 60℃ until completely dissolved, to obtain a transparent, viscous aramid spinning solution with a mass fraction of approximately 9%.

[0042] The MOF suspension and aramid solution were pumped into a Y-shaped microfluidic mixing chip (channel dimensions: 1 mm wide × 0.5 mm deep) at a volume ratio of 1:5 using two independent precision injection pumps, with the inlet pressure controlled at 10 MPa. Efficient mixing was achieved in the vortex mixing zone of the chip, and a uniform Co1Ni1-MOF / PMIA composite slurry was collected at the outlet.

[0043] S3, Multi-stage heat treatment and UV-assisted curing The composite slurry was poured into a four-sided adjustable doctor blade applicator on the glass plate. The doctor blade gap was adjusted to 400 μm, and the slurry was applied at a constant speed of approximately 5 cm / s to form a wet film of uniform thickness. The glass plate with the wet film was then placed in a programmable temperature-controlled oven and processed according to the following stages: S301, First stage: Set the temperature to 80℃ and process for 2 hours.

[0044] S302, Second stage: Raise the temperature to 130℃, turn on the hot air circulation, and process for 1 hour.

[0045] S303, Third Stage: Transfer the sample to an area equipped with a UV lamp (main wavelength 365nm, intensity 80mW / cm²). 2 In a high-temperature oven, it is simultaneously subjected to ultraviolet irradiation and heat treatment at 200℃ for 0.1 hours.

[0046] After the procedure is completed, turn off the heat source and UV lamp, and allow the sample to cool naturally to room temperature (about 25°C) in the oven. Peel the cured composite film off the glass substrate to obtain a self-supporting thermochromic paper with a thickness of about 100±5μm.

[0047] from Figure 4 As shown in the upper right corner of the embedded image, the Co-Ni-MOF / aramid composite color-changing paper has a pink film-like structure at room temperature. This indicates that the Co-Ni-MOF and aramid have been successfully composited, as can be seen from the XRD pattern analysis.

[0048] Example 2: A method for preparing thermochromic paper using a Co / Ni molar ratio of 3:1 and a MOF loading of 8wt% (this embodiment aims to demonstrate the control of the color-changing temperature by adjusting the Co / Ni ratio) specifically includes the following steps: S1. Synthesis of surface-aminated Co-Ni-MOF nanocrystals: The amounts of the metal salt raw materials were adjusted as follows: Co(NO3)2·6H2O 2.183 g (7.5 mmol), Ni(NO3)2·6H2O 0.727 g (2.5 mmol), dissolved in 150 mL LMF. Pyromellitic acid 2.542 g (10 mmol), KH2O... 5501.2 mL was added to 150 mL of anhydrous ethanol. The subsequent mixing, microwave reaction (160 °C, 60 min), centrifugation washing, and vacuum drying processes were exactly the same as in Example 1, finally yielding surface amino-functionalized Co3Ni-MOF nanocrystals.

[0049] S2. Preparation of composite slurry by synergistic dispersion of electric field-ultrasound-microfluidics: To achieve an 8 wt% MOF loading, the raw material amounts were adjusted: 0.89 g of the dried Co3Ni-MOF powder was weighed, 10 g of aramid was used, and NMP solvent and other additives were adjusted proportionally. The DC electric field pretreatment, high-energy ice-water bath ultrasonication, and microfluidic high-pressure mixing process parameters were completely consistent with those in Example 1, resulting in a uniform composite slurry.

[0050] S3, Multi-stage heat treatment and UV-assisted curing The coating process, including the three-stage programmed temperature heat treatment and UV-coupled curing, was exactly the same as in Example 1. The final product was Co3Ni with uniform thickness and a lower thermochromic temperature. MOF / aramid composite paper.

[0051] Specific reference Figure 2 As shown, Figure 2 (a) and (b) SEM images of Co-Ni-MOF nanocrystal materials. As can be seen from the images, the microstructure of the prepared Co-Ni-MOF is nanorods. The nanorods are constructed from multiple layers of nanosheets with a diameter of approximately 10-50 nm and a length of 200 nm, reaching 3 micrometers.

[0052] from Figure 5 The DSC plots show that both curves exhibit an upward trend within the temperature range of 40℃ to 160℃, indicating that the sample continuously absorbs heat during the heating process. A key comparative result is that, throughout the entire temperature range, the heat flux of the sample with a Co / Ni molar ratio of 1:1 (black curve) is consistently higher than that of the sample with a molar ratio of 3:1 (red curve). This indicates a significant difference in the thermal behavior of the two material ratios, with the 1:1 ratio sample exhibiting a stronger endothermic effect at the same temperature.

[0053] from Figure 6 As can be seen from the data, the transmittance of the Co-Ni-MOF / aramid composite thermochromic paper at 700 nm visible light wavelength is 50.71% at 25℃ and 22.64% at 160℃, yielding ΔT.可见光 =28.07%, the transmittance at a near-light wavelength of 900nm at 25℃ is 50.15%, and at 160℃ it is 25.87%, thus ΔT is obtained. 近红外光 =24.28%, demonstrating wide-band optical modulation capability.

[0054] from Figure 7 As can be seen, the thermochromic paper of the Co-Ni-MOF / aramid composite system can achieve reversible color change from pink to purple by heating and cooling at 25℃ and 160℃.

[0055] Comparative Example 1 (to demonstrate the key role of surface functionalization): Composite paper prepared from unfunctionalized MOFs includes the following steps: S1. During the synthesis process, no silane coupling agent KH-550 was added, and all other raw materials and process parameters were the same as in Example 1, resulting in unfunctionalized Co-Ni-MOF.

[0056] S2 and S3 follow the method of Example 1 exactly, using this unmodified MOF for composite pulp preparation and papermaking processes.

[0057] Results analysis: During the S2 dispersion process, even after multiple synergistic treatments, the resulting pulp still exhibited significant stratification and sedimentation after standing for 1 hour. Upon microscopic observation after final paper formation, numerous micron-sized MOF aggregates were visible, with extremely uneven color distribution.

[0058] Comparative Example 2 (to demonstrate the superiority of the multi-field collaborative dispersion strategy): Composite paper prepared by traditional mechanical stirring and dispersion process S1. The same raw materials as in Example 1 are used, namely surface-functionalized Co-Ni-MOF and aramid.

[0059] S2. The electric field pretreatment, high-energy ultrasound, and microfluidic mixing steps are omitted. Instead, MOF powder is directly added to the aramid solution and mixed using a high-speed mechanical stirrer (1000 rpm) at room temperature for 12 hours.

[0060] S3. The papermaking process is the same as in Example 1.

[0061] Results Analysis: The paper surface was rough and contained agglomerates. Performance: Poor color uniformity; tensile strength was 35 MPa, showing significant decrease after 500 cycles. Table 1 compares the main performance parameters of the above examples and comparative examples.

[0062] Table 1

[0063] As shown in Table 1, the composite paper prepared in Example 1 is far superior in all performance aspects. Its tensile strength of up to 62.5 MPa is close to that of pure aramid paper, proving that the strong interfacial bonding effectively transfers stress. Its rapid and uniform color change and excellent cycle stability and durability are directly attributed to the uniform dispersion of MOF and its strong chemical bonding with the matrix.

[0064] In contrast, Comparative Example 1, lacking interfacial chemical anchors, exhibited an extremely thin interface between the MOF and aramid, resulting in poor mechanical properties and easy detachment of functional units. Although Comparative Example 2 utilized a functionalized MOF, traditional dispersion methods failed to overcome the high viscosity of the aramid system, leading to agglomeration and similarly poor performance. This fully demonstrates the necessity and superiority of the two core technologies of this invention: "surface functionalization" and "multi-field synergistic dispersion."

[0065] Comparing Example 1 and Example 2, it can be seen that by adjusting the Co / Ni molar ratio from 1:1 to 3:1, the color change temperature is significantly reduced from 75℃ to 55℃, proving that the method of the present invention can conveniently achieve "programmable" control of the thermal response behavior of materials to adapt to different application needs.

[0066] The thermochromic paper prepared by this invention has reversible thermochromic properties in the temperature range of 40–160℃, with a color change response time ≤5s, tensile strength ≥50MPa, no significant performance decay after 2000 thermal cycles, and excellent bending stability. It can be used in fields such as smart packaging, flexible temperature sensing, and anti-counterfeiting labels.

[0067] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system, characterized in that, Includes the following steps: S1. Preparation of surface-functionalized Co-Ni-MOF nanocrystals: Using pyromellitic acid as an organic ligand and cobalt and nickel salts as metal sources, Co-Ni-MOF nanocrystals with amino or epoxy groups grafted on their surface are synthesized by a solvothermal method in the presence of a silane coupling agent; wherein the molar ratio of cobalt salt to nickel salt is 1:10 to 10:

1. S2. Preparation of high-uniformity composite slurry: The surface-functionalized Co-Ni-MOF nanocrystals obtained in S1 are dispersed in an organic solvent and subjected to DC electric field pretreatment, high-energy probe ultrasonic crushing, and microfluidic high-pressure shear mixing in sequence. The treated Co-Ni-MOF nano suspension is mixed with aramid solution to obtain Co-Ni-MOF / aramid composite slurry. S3. Paper Forming and Thermal Curing: The composite slurry is formed into a wet film, subjected to multi-stage heat treatment, and finally cooled and peeled off to obtain thermochromic paper.

2. The method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system according to claim 1, characterized in that, In S1, the solvothermal method is a microwave-assisted solvothermal method, with a reaction temperature of 120-180℃ and a reaction time of 30-90 min; The cobalt salt is one or more of cobalt nitrate, cobalt acetate, or cobalt chloride; The nickel salt is one or more of nickel nitrate, nickel acetate, or nickel chloride.

3. The method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system according to claim 1, characterized in that, In S1, the silane coupling agent has a bifunctional structure, with one end being a group that can react with the surface of the MOF crystal and the other end being an active functional group that can chemically react with the aramid matrix.

4. The method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system according to claim 1, characterized in that, In S2, the electric field strength for the DC electric field pretreatment is 100-300V / cm, and the treatment time is 10-30min.

5. The method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system according to claim 1, characterized in that, In S2, the preprocessing also includes ultrasonic processing of the probe after DC electric field processing, with an ultrasonic power of 300-500W and a processing time of 30-60min. The ultrasonic processing of the probe is performed in an ice-water bath, and the ultrasonic working mode is 2 seconds of ultrasound followed by 1 second of intermittent ultrasound.

6. The method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system according to claim 1, characterized in that, In S2, the mixing process is carried out in a microfluidic mixer, where the dispersion of the surface-functionalized Co-Ni-MOF nanocrystals and the aramid solution are mixed at an inlet pressure of 5-15 MPa and a shear rate of 10. 4 -10 5 s -1 Dynamic shear mixing is performed within a microfluidic mixer; during mixing, a Y-shaped or cross-shaped channel is followed by a serpentine mixing unit.

7. The method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system according to claim 1, characterized in that, The organic solvent is N-methylpyrrolidone or N,N-dimethylacetamide; the mass fraction of the aramid solution is 8-10%.

8. The method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system according to claim 1, characterized in that, In S3, the multi-stage heat treatment process specifically includes: S301, First stage: Heating at 60-90℃ for 0.5-2 hours to remove most of the solvent; S302, Second stage: Heating at 100-150℃ for 0.5-1.5h to further remove residual solvent and promote the pre-arrangement of aramid molecular chains; S303, Third Stage: Simultaneously subject to ultraviolet irradiation or plasma treatment at 150-300℃ for 0.1-0.5h to achieve aramid curing and chemical bonding at the MOF-aramid interface; the ultraviolet light wavelength is 365nm and the intensity is 50-100mW / cm². 2 .

9. The method for preparing thermochromic paper based on a Co-Ni-MOF / aramid composite system according to claim 8, characterized in that, The thermochromic paper contains 5-20% Co-Ni-MOF by mass and has a thickness of 20-200 μm. The paper exhibits reversible thermochromic properties within a temperature range of 40-160℃, with a color change response time ≤5s and a tensile strength ≥50MPa.

10. The application of the thermochromic paper as described in claim 9 in the fields of smart packaging, flexible temperature sensing, or anti-counterfeiting labels.

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