Temperature-sensitive liquid crystal microcapsule based on microfluidic technology, preparation method of temperature-sensitive liquid crystal microcapsule and temperature-sensitive liquid crystal film

Abstract

The invention relates to the technical field of temperature-sensitive liquid crystal membranes, and discloses a temperature-sensitive liquid crystal microcapsule based on a microfluidic technology, a preparation method of the temperature-sensitive liquid crystal microcapsule and a temperature-sensitive liquid crystal membrane.The preparation method of the temperature-sensitive liquid crystal microcapsule comprises the steps that a continuous phase, an intermediate phase and a dispersed phase are prepared, and the flow velocity of three-phase fluid is controlled through microfluidics, therefore, the flow of each phase of liquid in the glass chip is controlled, the purpose of controlling the particle size of the temperature-sensitive liquid crystal microcapsule is achieved, meanwhile, the temperature-sensitive liquid crystal microcapsule prepared by adopting a micro-fluidic technology is high in encapsulation efficiency, small in loss, short in reaction time and good in batch stability, and a temperature-sensitive liquid crystal film prepared by utilizing the temperature-sensitive liquid crystal microcapsule is free of granular sensation, uniform in color development and good in stability. The color developing effect is good.

Description

Technical Field

[0001] The present invention relates to the technical field of temperature-sensitive liquid crystal films, and in particular to a temperature-sensitive liquid crystal microcapsule based on microfluidics technology, a preparation method thereof, and a temperature-sensitive liquid crystal film. Background Art

[0002] The use of thermochromic liquid crystal (TLC) products in research and testing is growing. The main application areas are flow visualization and heat transfer research. These materials can be used for almost any temperature field indication and thermal mapping work. For example, cholesterol derivatives or cholesteric liquid crystals mixed with nematic phases and chiral agents can display the three primary colors of red, green and blue as the temperature changes within the operating temperature range. The fundamental reason for the color is the change in the pitch of the cholesterol liquid crystal mixture. This pitch will change with different external temperature, electric field and other conditions. At the appropriate temperature, its pitch will be close to a certain spectral wavelength, which will cause Bragg scattered light and present a certain color.

[0003] Temperature measurement can be performed based on the temperature-sensitive properties of cholesterol-based liquid crystals. Usually, a liquid crystal mixture with a specific temperature range can be obtained by mixing multiple cholesterol liquid crystals or mixing nematic liquid crystals with chiral agents. It can be used to display different temperatures by coating it on a specific substrate. However, this type of liquid crystal is sensitive to external factors (heat, oxygen, ultraviolet rays) and is not easy to preserve. Traditional emulsification techniques such as rapid stirring, homogenization, ultrasound and membrane emulsification have the advantages of simple operation and low cost, and can achieve higher yields; but their disadvantages are also obvious: the production particle size distribution is wide (CV<30%), the core material encapsulation rate is low, the loss is large, the reaction time for single batch preparation is long, and the batch stability is difficult to control.

[0004] The liquid crystal structure encapsulated in microcapsules can effectively solve the above problems. The main functions of liquid crystal microencapsulation are: (1) reduce the contact between liquid crystal and the external environment, thereby preventing deterioration and loss. (2) can ensure that the performance of temperature-sensitive liquid crystal products does not change during the service life. (3) turn liquid liquid crystal into solid powder, which is easy to process, transport and store. (4) eliminate the influence of some post-processing processes on liquid crystal, such as mechanical shearing during coating or mixing.

[0005] However, traditional methods for preparing liquid crystal microcapsules include spray drying, complex coacervation, interfacial polymerization, etc. These methods mainly control the process of droplet formation through mechanical stirring. The size and internal structure of the resulting microcapsules are difficult to accurately control, which affects the performance and application of the microcapsules.

[0006] It can be seen that the existing technology still needs to be improved and enhanced. Summary of the invention

[0007] In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a temperature-sensitive liquid crystal microcapsule based on microfluidic control technology, a preparation method thereof, and a temperature-sensitive liquid crystal film, aiming to prepare a temperature-sensitive liquid crystal film product with high sensitivity and good color rendering effect, ensure that the particle size of the thermochromic microcapsule meets the requirements of high monodispersity, and use microfluidic technology to achieve the purpose of controlling the particle size of the temperature-sensitive liquid crystal microcapsule.

[0008] In order to achieve the above object, the present invention adopts the following technical solutions:

[0009] A method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic control technology comprises the following steps:

[0010] S1. Preparation of the continuous phase: mixing polyvinyl alcohol and deionized water to obtain a polyvinyl alcohol aqueous solution;

[0011] S2. Preparation of the mesophase: A mixture containing an acrylate monomer, 2-hydroxy-2-methyl-1-phenyl-1-propanone and Span 80 were stirred to obtain an mesophase;

[0012] S3. Preparation of dispersed phase: cholesteryl oleyl carbonate, cholesteryl oleate, cholesteryl nonanoate, cholesteryl p-butyl benzoate and cholesteryl chloride were mixed and heated to obtain a liquid crystal mixture;

[0013] S4. Inject the continuous phase, intermediate phase and dispersed phase into the glass chip respectively, so that the dispersed phase fluid is injected into the capillary and then flows out, and is sheared and wrapped by the intermediate phase, and the formed droplets are collected in a container containing the continuous phase; irradiate the container with ultraviolet light to obtain a shell layer; obtain the temperature-sensitive liquid crystal microcapsules through centrifugation and drying.

[0014] In the method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic control technology, the mass fraction of the polyvinyl alcohol aqueous solution in step S1 is 1.5% to 2%; heating is used to dissolve the polyvinyl alcohol in water, and the heating temperature is 70 to 75°C.

[0015] The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic control technology, wherein the mixture containing acrylate monomers includes one or more of 1,6-hexanediol diacrylate, propane trimethanol triacrylate, and ethoxylated trimethylolpropane triacrylate.

[0016] The preparation method of the temperature-sensitive liquid crystal microcapsule based on the microfluidic control technology, wherein the mass fraction of 2-hydroxy-2-methyl-1-phenyl-1-propanone in the step S2 is 1.0% to 1.5%, and the mass fraction of Span 80 is 1.0% to 1.5%; the weight of the mixture containing the acrylate monomer is 10 to 13 g, the volume of the 2-hydroxy-2-methyl-1-phenyl-1-propanone is 0.09 to 0.18 mL, and the volume of the Span 80 is 0.10 to 0.20 mL 。

[0017] The preparation method of the temperature-sensitive liquid crystal microcapsules based on microfluidic control technology, wherein the addition ratio of the cholesteryl oleyl carbonate, the cholesteryl oleate, the cholesteryl nonanoate, the cholesteryl p-butyl benzoate and the cholesteryl chloride is (14-15): (9-10.2): (55-60): (9.4-10.5): (5-10.6).

[0018] The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic control technology, wherein the heating treatment in step S3 is performed by secondary heating; wherein the temperature of the first heating is 35°C to 80°C, and the heating time is 30min to 3h; and the temperature of the second heating is 60°C to 85°C.

[0019] The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic control technology, wherein the volume flow rate ratio of the dispersed phase, the continuous phase and the intermediate phase in step S4 is (1-10): (5-25): (200-1200).

[0020] A temperature-sensitive liquid crystal microcapsule based on microfluidic control technology is prepared by the preparation method of the temperature-sensitive liquid crystal microcapsule based on microfluidic control technology. The temperature-sensitive liquid crystal microcapsule based on microfluidic control technology has an average particle size of 17 to 41 μm, a particle size distribution of 1.5 to 3.2%, and an encapsulation rate greater than or equal to 96.5%.

[0021] A temperature-sensitive liquid crystal film, comprising a temperature-sensitive liquid crystal coating and an ink layer arranged in sequence from bottom to top; the temperature-sensitive liquid crystal coating is formed by coating a temperature-sensitive liquid crystal coating on a base film and then drying it; the ink layer is formed by drying an aqueous black ink; wherein, calculated by weight, the temperature-sensitive liquid crystal coating comprises the following components: 10 to 30 parts of the temperature-sensitive liquid crystal microcapsules based on microfluidic technology, 30 to 80 parts of an aqueous polyurethane resin, 2 to 6 parts of an aqueous crosslinking agent, 0.1 to 0.5 parts of a thixotropic agent, and 1 to 5 parts of a black color paste.

[0022] The temperature-sensitive liquid crystal film, wherein the thickness of the temperature-sensitive liquid crystal coating is 10-30 μm; the thickness of the ink layer is 5-25 μm; the temperature-sensitive liquid crystal coating and the water-based black ink are coated by micro-concave coating.

[0023] Beneficial effects:

[0024] The present invention provides a temperature-sensitive liquid crystal microcapsule based on microfluidic control technology, a preparation method thereof and a temperature-sensitive liquid crystal film. The temperature-sensitive liquid crystal microcapsule prepared by microfluidic technology can control the flow rate of the fluid through an injection pump, thereby controlling the flow rate of each phase of liquid in the glass chip, and achieving the purpose of controlling the particle size of the temperature-sensitive liquid crystal microcapsule. At the same time, the temperature-sensitive liquid crystal microcapsule prepared by microfluidic technology has high encapsulation rate, small loss, short reaction time and good batch stability. In contrast, while ensuring high monodispersity of microdroplets (CV < 5%), integrating thousands or even tens of thousands of microfluidic droplet generation units to run simultaneously can better meet the needs of industrial applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Optical microscope of the microcapsules prepared in Comparative Example 2-1 Figure 1 .

[0026] Figure 2 Optical microscope of the microcapsules prepared in Comparative Example 2-1 Figure 2 .

[0027] Figure 3 This is an optical microscope image of the microcapsules prepared in Comparative Example 3-1 after film formation.

[0028] Figure 4 This is an optical microscope image of the microcapsules prepared by the microfluidic method in Example 2-1.

[0029] Figure 5 This is an optical microscope image of the microcapsules prepared by the microfluidic method in Example 3-1 after film formation.

[0030] Figure 6 This is a schematic diagram of the microcapsules prepared in Comparative Example 3-1 after film formation.

[0031] Figure 7 This is a schematic diagram of the microcapsules prepared by the microfluidic method in Example 3-1 after film formation. DETAILED DESCRIPTION

[0032] The present invention provides a temperature-sensitive liquid crystal microcapsule based on microfluidic control technology, a preparation method thereof, and a temperature-sensitive liquid crystal film. In order to make the purpose, technical solution, and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0033] See also Figure 1 The present invention provides a method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic control technology, comprising the following steps:

[0034] (1) Preparation of the continuous phase: polyvinyl alcohol and deionized water are placed in a beaker and mixed, and the beaker containing the above solution is placed on a magnetic stirrer. Since polyvinyl alcohol needs 65°C to 75°C to completely dissolve in water, the heating temperature is set to 70°C to 75°C, and the stirring is continued for 30 minutes in the above temperature range, and the stirring speed is 1200r / min to dissolve the polyvinyl alcohol to obtain a polyvinyl alcohol aqueous solution. The above operation allows polyvinyl alcohol to be fully dissolved in water to form a certain concentration of PVA aqueous solution. Specifically, the mass fraction of the polyvinyl alcohol aqueous solution is 1.5% to 2%. Among them, the main function of polyvinyl alcohol is that it can use its own properties to adsorb on the outer surface of the generated droplets, that is, to form a protective film around the generated droplets, reduce the surface free energy, and prevent the continuous phase from aggregating or merging due to collisions between droplets before the mixture of the intermediate phase is completely solidified, so that the dispersed phase droplets can be evenly and stably dispersed in the system, which is conducive to the formation of microcapsules with uniform size and good morphology.

[0035] (2) Preparation of mesophase: 10-13 g of a mixture containing acrylate monomers, 0.09-0.18 mL of 2-hydroxy-2-methyl-1-phenyl-1-propanone (1.0%-1.5% by mass relative to the acrylate monomer) and 0.10-0.20 mL of Span 80 (1.0%-1.5% by mass relative to the acrylate monomer) are placed in a beaker, and the beaker is placed on a magnetic stirrer. Since the three solutes can be fused at room temperature, the mixture is uniformly stirred at a speed of 800 r / min for 10 minutes at a temperature of 25° C. to obtain an mesophase. Specifically, the mixture containing acrylate monomers includes one or more of 1,6-hexanediol diacrylate, propane trimethanol triacrylate, and ethoxylated trimethylolpropane triacrylate. Among them, the above-mentioned mixture containing acrylate monomers all have multiple polymerizable double bonds. Under the initiation of the photoinitiator, that is, under the irradiation of ultraviolet light, the photoinitiator will produce free radicals, which can quickly undergo photopolymerization reaction. The polymerization reaction makes the acrylate monomers interconnected and gradually form a high molecular polymer network, which constitutes the shell of the microcapsule. More specifically, the acrylate groups at both ends of the 1,6-diacrylate hexanediol ester molecule can participate in the polymerization, so that the molecular chains are interconnected; trimethoxy propane trimethylol triacrylate and ethoxylated trimethylol propane triacrylate can further enhance the degree of crosslinking due to their more acrylate functional groups, thereby constructing a strong and stable microcapsule shell, providing good mechanical properties for the microcapsule, and effectively protecting the internal liquid crystal mixture. In addition, different acrylate monomers contribute differently to the flexibility of the shell. The molecular structure of 1,6-hexanediol diacrylate is relatively regular, and the formed polymer chain segment has a certain rigidity, which helps to improve the strength of the microcapsule shell; while the ethoxylated trimethylolpropane triacrylate molecule contains ethoxy groups, which increases the flexibility of the molecular chain and can adjust the flexibility of the shell to a certain extent, making it have both sufficient strength and certain elasticity, and better adapt to different application environments and conditions. The above monomers can also improve the chemical resistance, thermal stability and other properties of the microcapsule.

[0036] Span 80, a non-ionic surfactant, has both hydrophilic hydroxyl groups and hydrophobic fatty acid ester groups in its molecular structure. In the mesophase, Span 80 can reduce the interfacial tension between the acrylate monomer mixture and the dispersed phase liquid crystal mixture, making it easier for the two to mix evenly and form a stable emulsion system. This helps to better disperse the liquid crystal mixture in the mesophase in the form of tiny droplets during the microfluidics process, laying the foundation for the subsequent formation of uniform microcapsules. At the same time, in the emulsion system, the presence of Span 80 can form a protective film on the surface of the liquid crystal droplets to prevent the droplets from agglomerating due to mutual collisions. Through the dual effects of this steric hindrance effect and reduced surface energy, the liquid crystal droplets can maintain a good dispersion state in the mesophase, thereby improving the preparation efficiency and quality of microcapsules and ensuring that the prepared microcapsules are uniform in size and stable in performance.

[0037] (3) Preparation of dispersed phase:

[0038] ① Mix cholesteryl oleyl carbonate, cholesteryl oleate, cholesteryl nonanoate, cholesteryl p-butyl benzoate and cholesteryl chloride in the following ratio (14-15): (9-10.2): (55-60): (9.4-10.5): (5-10.6).

[0039] Based on the color change principle, color change range and precision requirements of the temperature-sensitive liquid crystal coating, the ratio of the liquid crystal mixture is determined. Among them, the color change principle of the temperature-sensitive liquid crystal coating is as follows: liquid crystal material is a type of liquid that has the fluidity of liquid and retains the anisotropic and orderly arrangement of some crystalline material molecules, forming an intermediate state with some properties of both crystals and liquids, usually in the nematic phase, smectic phase and cholesteric phase. Among various types of liquid crystals, cholesteric liquid crystal CLC has become one of the most attractive liquid crystals due to its unique spiral structure and Bragg reflection. Since the spiral structure is formed by self-assembly of cholesteric liquid crystal CLC molecules through extremely weak interactions between molecules, cholesteric liquid crystal CLC molecules are easily reoriented under external stimuli, which causes a large change in its pitch, making it highly sensitive to external stimuli. The unique spiral structure of CLC, the external environment will affect the size of the pitch, such as temperature, electric field, magnetic field, pressure and chemical composition, which can easily cause the pitch P to change. Therefore, the pitch will change with the change of any of the above parameters, resulting in a corresponding change in the wavelength of the visible light that can be reflected. When the ambient temperature changes, the wavelength of the selected light scattering changes greatly. Generally, the pitch becomes smaller as the temperature increases, and the wavelength of the scattered light moves toward the short wave, and the color changes from red, orange, yellow, green to purple accordingly; when the temperature decreases, the color changes from purple to red.

[0040] Referring to Table 1, it can be seen that different proportions of raw materials of different cholesteric liquid crystal compounds can correspond to different color change temperature ranges and effectively adjust the temperature sensing range of microcapsules. In addition, different cholesteric liquid crystal compounds have different molecular structures and chemical properties. After mixing in this ratio, they can form a rich and diverse liquid crystal phase within a certain temperature range. For example, cholesteryl oleyl carbonate and cholesteryl oleate molecules contain unsaturated double bonds, which makes them different from cholesteryl nonanoate, cholesteryl parabutyl benzoate and cholesteryl chloride in terms of intermolecular interactions and arrangement. Through reasonable matching ratios, the characteristics of various liquid crystal compounds can be combined to broaden the existence range and stability of liquid crystal phases, thereby meeting the needs of different application scenarios for liquid crystal phases. Furthermore, the response characteristics of each cholesteric liquid crystal compound to external stimuli such as temperature, electric field, and magnetic field are different. Cholesteryl nonanoate has good thermal stability and moderate response speed, and accounts for a large proportion in the mixed system. It can be used as the main functional component to provide basic temperature sensing performance; the addition of cholesteryl oleyl carbonate and cholesteryl oleate may enhance the sensitivity of the liquid crystal system to temperature changes, causing liquid crystal phase transitions in a narrower temperature range and improving temperature sensing accuracy; cholesteryl p-butyl benzoate and cholesteryl chloride may play a role in adjusting the optical properties and viscosity of liquid crystals, thereby improving the overall display effect or the clarity of the temperature-sensing display.

[0041] ② The mixture is first heated at 35℃~80℃ for 30min~3h until the liquid crystal presents a uniform liquid state. It should be noted that the heating time should not be too short or too long (if the heating time is too short, it is easy to cause part of the liquid crystal material to not dissolve or mix, and if it is heated for a long time, it is easy to cause the liquid crystal material to deteriorate). At the same time, confirm its display temperature range, and confirm the color change at a specific heating and cooling rate.

[0042] ③The solution is then heated for the second time. The purpose of this step is that the mixture prepared in step ② may be in a liquid crystal state or solid state at a lower temperature. The second heating makes the mixture a low-viscosity liquid state, wherein the heating temperature is 60°C to 85°C, and then a liquid crystal mixture is obtained.

[0043] (4) The generated liquid crystal mixture is used as the dispersed phase, the PVA aqueous solution is used as the continuous phase, and the mixture containing acrylic acid monomers is used as the intermediate phase. The volume flow rate ratio of the dispersed phase, the continuous phase and the intermediate phase is (1-10): (5-25): (200-1200). When the dispersed phase flow rate is slower than the continuous phase and the intermediate phase flow rates, the dispersed phase droplets have more time to be finely split and shaped under the shear force of the continuous phase and the intermediate phase, and it is easy to form smaller and uniformly sized droplets, thereby generating smaller microcapsules. If the dispersed phase flow rate is too fast, exceeding the shear and wrapping capacity of the continuous phase and the intermediate phase, the dispersed phase droplets will not have time to fully split and refine, and the formed droplets will be larger and unevenly distributed in size, which will eventually make the size of the microcapsules larger and the consistency worse. The appropriate three-phase flow rate ratio can make the continuous phase and the intermediate phase evenly distributed around the dispersed phase droplets, exert uniform pressure on the droplets, so that the generated microcapsules have a regular morphology and are close to spherical. If the flow rate ratio of the three phases is unbalanced, such as the flow rate of the middle phase is too fast or too slow, the droplets may be subjected to uneven force during the flow process, causing the microcapsules to have irregular morphologies such as deformation, depression or rough surface, affecting the appearance quality and performance of the microcapsules. In addition, by controlling the volume flow rate ratio of the three phases, the distribution uniformity of the dispersed phase in the microcapsule can be adjusted. A reasonable flow rate ratio can make the dispersed phase evenly dispersed in the microcapsule, ensuring that the liquid crystal molecules or other active substances inside the microcapsule are evenly distributed, so that the microcapsules show consistent performance in the application, such as uniform temperature sensing performance.

[0044] ① Use a 100mL syringe to inhale the polyvinyl alcohol aqueous solution (continuous phase), use a 50mL syringe to inhale the intermediate phase, use a 10mL syringe to inhale the cholesteric liquid crystal mixture (dispersed phase), and fix the three syringes with three syringe pumps respectively.

[0045] ② Place the prepared glass chip on the stage of the microscope and connect the catheter of the glass chip.

[0046] ③ Start the injection pump to inject the dispersed phase fluid into the capillary with a diameter of 0.1 mm and then flow out. The dispersed phase is sheared and wrapped by the intermediate phase, and the droplets formed are of uniform size. The smaller the capillary diameter, the greater the shear force on the dispersed phase fluid when it flows out of the capillary, and the easier it is to be sheared into smaller droplets, because the smaller capillary limits the flow space of the fluid, so that the fluid is squeezed and sheared by the intermediate phase when it flows out, thus forming droplets of smaller size. When the capillary diameter is small, under certain flow rate and shear conditions, the flow of the dispersed phase fluid is more stable and uniform, and the droplets formed are more uniform in size. In addition, a smaller capillary diameter helps to improve the encapsulation efficiency of the microcapsules. Due to the smaller size of the droplets formed, in the subsequent encapsulation and curing process, the continuous phase and the intermediate phase are more likely to completely encapsulate the dispersed phase, reducing the leakage of the dispersed phase material, thereby improving the encapsulation rate of the microcapsules.

[0047] ④ Collect the droplets into a container (collecting bottle) filled with a polyvinyl alcohol aqueous solution with a mass fraction of 1.5% to 2%. The purpose of this step is to keep the collected droplets stable.

[0048] ⑤ The response wavelength of the photoinitiator is 365nm. Therefore, the collected droplets are irradiated with ultraviolet light emitted by an ultraviolet lamp with a wavelength of 365nm. The intensity of ultraviolet radiation with a wavelength of 365nm is measured by an irradiation intensity meter and is approximately 60-120mW / cm 2 , cumulative irradiation energy 300~1500mJ / cm 2 The mixture containing the acrylic acid ester monomer is polymerized under the irradiation of ultraviolet light of the above wavelength to obtain a stable shell structure; then, the thermosensitive liquid crystal microcapsule is obtained through centrifugation and drying.

[0049] Specifically, the average particle size of the temperature-sensitive liquid crystal microcapsules prepared based on the microfluidic technology is 17-41 μm, the particle size distribution is 1.5-3.2%, and the encapsulation rate is greater than or equal to 96.5%.

[0050] The particle size and distribution of microcapsules have a significant effect on the color rendering properties of cholesteric liquid crystal microcapsules. Microcapsules with a particle size of 3 to 30 μm generally have good color rendering properties. The narrower the distribution range of the microcapsule particle size, the better the color rendering properties. In general, the uniformity of microcapsules will affect the encapsulation rate of liquid crystals, the thickness of microcapsules walls, etc., and will affect the uniformity of heat transfer. At the same time, the size of microcapsules is different, especially when large particles exist, which will make the coating surface uneven. Therefore, the uniformity of microcapsules is an important prerequisite for uniform coating.

[0051] Specifically, the average particle size of the temperature-sensitive liquid crystal microcapsules is between 17 and 41 μm, and this particle size range is the result of the combined effect of multiple factors, such as the volume flow rate ratio of the dispersed phase, the continuous phase and the intermediate phase, and the capillary diameter, etc. The appropriate particle size has an important influence on the application performance of the microcapsules. Smaller particle sizes are more conducive to rapid thermal response, because smaller particles have a relatively large specific surface area and can exchange heat with the external environment more quickly, thereby more sensitively sensing temperature changes, and are suitable for occasions with high requirements for temperature sensing speed.

[0052] The above particle size distribution range indicates that the particle size of the prepared microcapsules is highly uniform. The smaller particle size distribution means that the particle size of most microcapsules is very close to the average particle size, which is crucial for the performance stability of the microcapsules. The appropriate three-phase flow rate ratio can make the droplet formation process more stable, thereby obtaining microcapsules with uniform particle size; the consistency of the capillary diameter and the uniform shearing effect on the flow of the dispersed phase fluid also help to maintain a low particle size distribution. In addition, the stability and uniformity of the components of each phase, such as the uniform concentration of the polyvinyl alcohol aqueous solution of the continuous phase and the uniform mixing of the various components of the intermediate phase, also play an important role in reducing the particle size distribution.

[0053] The encapsulation rate is greater than or equal to 96.5%, which is a fairly high level of encapsulation rate. The high encapsulation rate means that during the preparation process, most of the dispersed phase is effectively encapsulated inside the microcapsule to form a complete microcapsule structure.

[0054] A temperature-sensitive liquid crystal film, the temperature-sensitive liquid crystal film comprising a temperature-sensitive liquid crystal coating and an ink layer arranged in sequence from bottom to top; the temperature-sensitive liquid crystal coating is formed by coating the temperature-sensitive liquid crystal coating on a base film and then drying; the ink layer is formed by drying an aqueous black ink; wherein, calculated by weight, the temperature-sensitive liquid crystal coating comprises the following components: the temperature-sensitive liquid crystal microcapsules based on microfluidics technology (as the core component of the temperature-sensitive liquid crystal coating, determining the temperature-sensitive color change range and sensitivity): 10 to 30 parts, an aqueous polyurethane resin (as an adhesive, so that a single temperature-sensitive liquid crystal microcapsule forms a continuous coating, such as Covestro Model HYR-2350 product): 30-80 parts, water-based crosslinking agent (the water-based crosslinking agent is a melamine resin or an aziridine crosslinking agent, which can be cross-linked with the water-based polyurethane resin adhesive by thermal curing reaction, thereby improving the water resistance of the coating): 2-6 parts, thixotropic agent (used to prevent the sedimentation of the temperature-sensitive liquid crystal microcapsule particles and improve the coating performance): 0.1-0.5 parts, and black color paste (such as Clariant Colanyl, water-based color paste black T-30 can be selected): 1-5 parts.

[0055] The temperature-sensitive liquid crystal coating is applied to a base film such as PET or PC by micro-concave coating, and the thickness of the temperature-sensitive liquid crystal coating is controlled to be 10 to 30 μm. The film material after the temperature-sensitive liquid crystal coating is applied is dried and rolled up. The coating thickness will change the surface texture. The thin coating is matte and slightly rough. This is mainly because when the coating is too thin, the microcapsules cannot be completely covered and exposed; thicker coatings are easier to flow, making the surface smoother and brighter. At the same time, the coating thickness and surface texture affect the brightness and hue of the color produced, and also affect the temperature at which each color appears. Generally speaking, the thicker the coating, the lower the color rendering and the lower the temperature sensitivity. In addition, too thick a coating will cause the usually bright colors to appear milky white, which is more obvious at the red end of the spectrum. Therefore, the optimal dry film thickness is about 10 to 30 μm. To this end, a total wet film thickness of about 30 to 90 μm needs to be applied. The best effect may be achieved by gradually increasing the coating thickness and keeping it dry between two applications.

[0056] The water-based black ink is applied on the upper surface of the temperature-sensitive liquid crystal coating by micro-concave coating, and the thickness of the ink layer is controlled to be 5 to 25 μm. The film material coated with the water-based black ink is dried and rolled up to obtain a temperature-sensitive liquid crystal plastic film whose color changes at 20°C to 40°C. The ink layer acts as a protective layer and a heat-conducting layer. When the thickness of the ink layer is too thin, it cannot effectively protect the temperature-sensitive liquid crystal coating. When the thickness is too thick, the color temperature is easily shifted or the color sensitivity is reduced due to the low heat conduction efficiency.

[0057] In order to further illustrate a temperature-sensitive liquid crystal microcapsule based on microfluidic technology and a preparation method thereof and a temperature-sensitive liquid crystal film provided by the present invention, the following examples are provided.

[0058] Example 1

[0059] The corresponding relationship between the type and proportion of liquid crystal materials and the color change temperature range is shown in Table 1 below:

[0060]

[0061] Example 2 and Comparative Example 2-1

[0062] Comparative Example 2-1 (Traditional method for preparing microcapsules)

[0063] 10g of cholesteric liquid crystal mixture was mixed with 4g of methyl methacrylate (MMA) and 4g of ethylene glycol dimethacrylate (EGDMA), and ultrasonicated at 45°C until transparent to obtain an oil phase; 50mL of HSMA (10 (wt)%) was added to 400mL of deionized water, and the pH was adjusted to 5 by stirring to obtain an aqueous phase; the aqueous phase and the oil phase were mixed, and the emulsion was obtained by shearing with a high-speed shear emulsifier for 5min at a shear rate of 7000r / min. The emulsion was placed in a three-necked flask, 13g of potassium persulfate and 1.3g of sodium nitrite were added, the three-necked flask was placed in an oil bath at a constant temperature of 70°C, the mechanical stirring rate was set to 3000r / min, nitrogen was introduced at the same time, and a condenser reflux device was connected. After 12h, the emulsion in the bottle was centrifuged and washed three times with distilled water and dried to obtain liquid crystal microcapsules.

[0064] Among them, the coating of comparative example 2-1 has the same composition as the coating of the present invention, and the coating method can be selected from micro-concave or conventional screen printing.

[0065] The preparation conditions of the temperature-sensitive liquid crystal microcapsules of Example 2 and Comparative Example 2-1 are shown in Table 2 below:

[0066]

[0067]

[0068] Wherein, polydispersity index PDI = standard deviation of microcapsule particle size δ / average particle size of microcapsules d; encapsulation efficiency = mass of liquid crystal in microcapsules m 2 / Actual liquid crystal feed mass mL.

[0069] Example 3

[0070] The ratio of the temperature-sensitive liquid crystal coating of Example 3 and Comparative Example 3-1 (using the liquid crystal microcapsules prepared in Comparative Example 2-1) is shown in Table 3 below:

[0071]

[0072] As can be seen from Table 2, the average particle size of the temperature-sensitive liquid crystal microcapsules prepared by the microfluidic method of the present invention is 17-41 μm, the particle size distribution is 1.5-3.2%, and the encapsulation rate is greater than or equal to 96.5%. It can be seen that the particle size distribution of the temperature-sensitive liquid crystal microcapsules is uniform, the size is uniform, and the encapsulation rate is very high. Figure 1 and Figure 2 It can be seen that the particle size of the microcapsules prepared by conventional mechanical stirring method is uneven. Figure 3 It can be seen that the surface roughness of the temperature-sensitive liquid crystal film prepared by the conventional mechanical stirring method is high and presents an uneven state. Figure 4 It can be seen that the particle size of the temperature-sensitive liquid crystal microcapsules prepared by the microfluidic method is uniform. Figure 5It can be seen that the surface flatness of the temperature-sensitive liquid crystal film is high. Figure 6 It can be seen that the particle size of the temperature-sensitive liquid crystal film prepared by the conventional mechanical stirring method is different, the color is uneven, the graininess is serious, and the effect is poor. Figure 7 It can be seen that the temperature-sensitive liquid crystal film prepared by the microfluidic method has no granularity, uniform color development and good effect.

[0073] In summary, in order to obtain a temperature-sensitive liquid crystal film product with high sensitivity and good color rendering effect, the present invention puts forward higher requirements on the uniformity of the particle size of the temperature-sensitive liquid crystal microcapsules and the uniformity of the coating. The most important thing is that the particle size of the temperature-sensitive liquid crystal microcapsules should meet the requirements of high monodispersity. The temperature-sensitive liquid crystal microcapsules prepared by the present invention using microfluidic technology can control the flow rate of the fluid through an injection pump, thereby controlling the flow rate of each phase of liquid in the glass chip, and achieving the purpose of controlling the particle size of the temperature-sensitive liquid crystal microcapsules. At the same time, the temperature-sensitive liquid crystal microcapsules prepared by microfluidic technology have high encapsulation rate, small loss, short reaction time, and good batch stability. In contrast, while ensuring high monodispersity of microdroplets (CV <5%), integrating thousands or even tens of thousands of microfluidic droplet generation units and running them simultaneously can better meet the needs of industrial applications.

[0074] It is understandable that those skilled in the art can make equivalent substitutions or changes based on the technical solution and inventive concept of the present invention, and all these changes or substitutions should fall within the protection scope of the present invention.

Claims

1. A method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic technology, characterized in that: The steps include: S1. Preparation of the continuous phase: mixing polyvinyl alcohol and deionized water to obtain a polyvinyl alcohol aqueous solution; S2. Preparation of the mesophase: A mixture containing an acrylate monomer, 2-hydroxy-2-methyl-1-phenyl-1-propanone and Span 80 were stirred to obtain an mesophase; S3. Preparation of dispersed phase: cholesteryl oleyl carbonate, cholesteryl oleate, cholesteryl nonanoate, cholesteryl p-butyl benzoate and cholesteryl chloride were mixed and heated to obtain a liquid crystal mixture; S4. Injecting the continuous phase, the intermediate phase and the dispersed phase into the glass chip respectively, so that the dispersed phase fluid is injected into the capillary and then flows out, and is sheared and wrapped by the intermediate phase, and the formed droplets are collected in a container containing the continuous phase; The container is irradiated with ultraviolet light to obtain a shell layer; and the temperature-sensitive liquid crystal microcapsules are obtained through centrifugation and drying.

2. The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic technology according to claim 1, characterized in that: The mass fraction of the polyvinyl alcohol aqueous solution in step S1 is 1.5% to 2%; The polyvinyl alcohol is dissolved in water by heating at a temperature of 70 to 75°C.

3. The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic technology according to claim 1, characterized in that: The mixture containing acrylic acid ester monomers includes one or more of 1,6-hexanediol diacrylate, propane trimethanol triacrylate, and ethoxylated trimethylolpropane triacrylate.

4. The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic technology according to claim 1, characterized in that: In the step S2, the mass fraction of 2-hydroxy-2-methyl-1-phenyl-1-propanone is 1.0% to 1.5%, and the mass fraction of Span 80 is 1.0% to 1.5%; the weight of the mixture containing the acrylate monomer is 10 to 13 g, the volume of the 2-hydroxy-2-methyl-1-phenyl-1-propanone is 0.09 to 0.18 mL, and the volume of Span 80 is 0.10 to 0.20 mL.

5. The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic technology according to claim 1, characterized in that: The addition ratio of the cholesteryl oleyl carbonate, the cholesteryl oleate, the cholesteryl nonanoate, the cholesteryl p-butyl benzoate and the cholesteryl chloride is (14-15):(9-10.2):(55-60):(9.4-10.5):(5-10.6).

6. The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic technology according to claim 1, characterized in that: The heating treatment in step S3 is performed by double heating; wherein the temperature of the first heating is 35°C to 80°C, and the heating time is 30min to 3h; and the temperature of the second heating is 60°C to 85°C.

7. The method for preparing temperature-sensitive liquid crystal microcapsules based on microfluidic technology according to claim 1, characterized in that: In the step S4, the volume flow rate ratio of the dispersed phase, the continuous phase and the intermediate phase is (1-10):(5-25):(200-1200).

8. A temperature-sensitive liquid crystal microcapsule based on microfluidic technology, characterized in that: The microfluidic-based temperature-sensitive liquid crystal microcapsules are prepared by the preparation method of the microfluidic-based temperature-sensitive liquid crystal microcapsules according to any one of claims 1 to 7, wherein the average particle size of the microfluidic-based temperature-sensitive liquid crystal microcapsules is 17 to 41 μm, the particle size distribution is 1.5 to 3.2%, and the encapsulation rate is greater than or equal to 96.5%.

9. A temperature-sensitive liquid crystal film, characterized in that: The temperature-sensitive liquid crystal film includes a temperature-sensitive liquid crystal coating and an ink layer arranged in sequence from bottom to top; the temperature-sensitive liquid crystal coating is formed by coating the temperature-sensitive liquid crystal coating on a base film and then drying it; the ink layer is formed by drying an aqueous black ink; wherein, calculated by weight, the temperature-sensitive liquid crystal coating includes the following components: 10 to 30 parts of the temperature-sensitive liquid crystal microcapsules based on microfluidic technology as described in claim 8, 30 to 80 parts of an aqueous polyurethane resin, 2 to 6 parts of an aqueous cross-linking agent, 0.1 to 0.5 parts of a thixotropic agent, and 1 to 5 parts of a black color paste.

10. The temperature-sensitive liquid crystal film according to claim 9, characterized in that: The thickness of the temperature-sensitive liquid crystal coating is 10 to 30 μm; the thickness of the ink layer is 5 to 25 μm; the temperature-sensitive liquid crystal coating and the water-based black ink are coated by micro-concave coating.

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