Double-layer color-changing microcapsule as well as preparation method and application thereof

Abstract

The invention discloses a double-layer color-changing microcapsule as well as a preparation method and application thereof, and relates to the technical field of microcapsules. The preparation method comprises the following steps: dissolving allyl chitosan and sodium alginate in water, adding a fat-soluble component to be embedded, shearing, then adding a composite cross-linking system solution, carrying out constant-temperature stirring reaction, centrifuging to take precipitate, washing and drying to obtain a first microcapsule; dispersing the first microcapsule in absolute ethyl alcohol, adding a color-changing material and a first cross-linking agent for cross-linking reaction, centrifuging, taking precipitate, and drying to obtain the first microcapsule coated with the color-changing material; adding the first microcapsule coated with the color-changing material, a polymer monomer and a second cross-linking agent into water for emulsification, and adding an initiator for polymerization reaction to obtain the double-layer color-changing microcapsule. The particle size D50 of the double-layer color-changing microcapsule provided by the invention is about 200 nm; the structure is compact, high in strength and crack-free; the color change response is fast; a good slow-release effect is achieved; the method is suitable for different to-be-embedded components and can also be suitable for different temperature-sensitive or light-sensitive color-changing materials.

Description

Technical Field

[0001] This invention relates to the field of microcapsule technology, and in particular to a bilayer color-changing microcapsule, its preparation method, and its application. Background Technology

[0002] Microcapsules are a technology that encapsulates solid, liquid, or gaseous substances in tiny containers, typically ranging in diameter from micrometers to millimeters. The encapsulation principle involves forming an outer shell through physical or chemical methods (such as spray drying, condensation, and interfacial polymerization) to encapsulate the core substance. The outer shell can be designed as a semi-permeable membrane or a completely sealed structure, and release is triggered by external conditions such as temperature, pH, and pressure. The core substance of the microcapsule is isolated by a protective outer shell made of polymeric or other materials, achieving functions such as sustained release, protection, and controlled release. Microcapsules are widely used in pharmaceuticals, food, agriculture, and daily chemicals.

[0003] Color-changing microcapsules are functional micro-containers containing phase change materials. They achieve reversible color changes in response to external stimuli such as temperature or light. The core principle of color-changing microcapsules is that the internal materials (such as thermosensitive pigments and photochromic dyes) undergo molecular structure changes under external stimulation, resulting in a color transition. Therefore, based on their color-changing mechanisms, color-changing microcapsules are classified into two types: thermosensitive and photosensitive.

[0004] Color-changing microcapsules have broad application prospects. In the consumer goods sector, they can be used to manufacture products such as color-changing mugs (heat-sensitive), temperature-sensitive phone cases (touch-sensitive), and business cards (heat-sensitive color-changing); they can also be used to add "white-to-black" microparticles to microcapsules to achieve a color-changing effect when applied; and they can be used in fabric printing to achieve dynamic colors under temperature or light (such as temperature-sensitive T-shirts and light-sensitive color-changing clothing). In the industrial and technological fields, they can be used as color-changing paint for automobiles. They can also be embedded in spacesuits to regulate temperature by absorbing / releasing heat.

[0005] In existing technologies, microcapsule preparation mainly involves single-layer wall coating, which suffers from poor wall strength and mechanical properties, as well as poor weather resistance, limiting the widespread application of microcapsules. Furthermore, the preparation methods are cumbersome, requiring multiple reaction steps, and involve large amounts of organic solvents, resulting in poor environmental impact.

[0006] The structural design of microcapsules enables the controlled release of the encapsulated ingredients. Taking D-limonene, used for mosquito repellency, as an example, when textiles come into contact with the human body or are in different environmental conditions, the porous structure of the microcapsules is affected by external factors (such as temperature, humidity, and mechanical friction), causing changes in pore size and porosity, thereby regulating the release rate of the encapsulated ingredient. Under normal temperature and humidity conditions, the encapsulated D-limonene is released slowly, maintaining a continuous mosquito-repellent effect.

[0007] When color-changing materials are selected, the outer color-changing material will show different colors in different temperature ranges as the ambient temperature rises or the human body sweats. At the same time, the release rate of microcapsules will also increase accordingly to enhance the mosquito repellent effect.

[0008] However, integrating the three functions of D-limonene—slow release, biomonitoring, and thermal comfort management—into a single-layer microcapsule presents a serious functional conflict. The release of D-limonene requires a certain porosity, which weakens temperature conduction; the introduction of color-changing materials can block the release channels of D-limonene, affecting its mosquito-repellent effect. Summary of the Invention

[0009] To address the aforementioned problems, this invention provides a bilayer color-changing microcapsule, its preparation method, and its applications. The bilayer color-changing microcapsule provided by this invention has a dense, crack-free structure, exhibits rapid color-changing response, and demonstrates excellent sustained-release effects; it is applicable not only to different encapsulated components but also to different temperature-sensitive or light-sensitive color-changing materials. Specifically, this is achieved through the following techniques.

[0010] A method for preparing bilayer color-changing microcapsules, characterized by comprising the following steps:

[0011] Allyl chitosan and sodium alginate were dissolved in water, and the fat-soluble component to be encapsulated was added. The mixture was then sheared to obtain an emulsion.

[0012] A composite cross-linking system solution was added to the emulsion, and the mixture was stirred at a constant temperature. The precipitate was collected by centrifugation, washed, and dried to obtain the first microcapsule.

[0013] The first microcapsule was dispersed in anhydrous ethanol, and a color-changing material and a first cross-linking agent were added to carry out a cross-linking reaction. The precipitate was collected by centrifugation and dried to obtain the first microcapsule coated with the color-changing material.

[0014] The first microcapsule coated with the color-changing material, the polymer monomer, and the second crosslinking agent were emulsified in water; an initiator was added and polymerization was carried out in a protective atmosphere to obtain the bilayer color-changing microcapsule.

[0015] The composite crosslinking system includes any one or more of 2,4-hexadienal, benzoyl formaldehyde, 4-hydroxyphenylglyoxal, and terephthalaldehyde, and also includes MgCl2 or CaCl2.

[0016] The first crosslinking agent is terephthalaldehyde or glutaraldehyde.

[0017] The polymer monomer is any one or more of propylene sulfonate, acrylamide lactone derivative, and styrene.

[0018] The second crosslinking agent is a propylene-based crosslinking agent.

[0019] Furthermore, the preparation method of the allyl chitosan is as follows: dissolve chitosan in acetic acid solution, add methanol and stir evenly, add benzaldehyde-methanol solution dropwise at 20-40℃, react for 8-12 h to obtain a system containing gel solid;

[0020] Add sodium hydroxide solution to the system containing the gel solid and stir; add anhydrous ethanol until all the colloid precipitates out, and collect the colloid;

[0021] The colloid was washed with anhydrous ethanol, dried, and ground to obtain the allyl chitosan.

[0022] Furthermore, the preparation method of the allyl chitosan is as follows:

[0023] Chitosan was dissolved in acetic acid solution, methanol was added and stirred until homogeneous, and benzaldehyde-methanol solution was added dropwise at 60°C. The reaction was carried out for 6 h to obtain a system containing gel solids.

[0024] Add sodium hydroxide solution to the system containing the gel solid and stir; add anhydrous ethanol until all the colloid precipitates out, and collect the colloid;

[0025] The colloid was washed with anhydrous ethanol, dried, and ground to obtain the allyl chitosan.

[0026] Furthermore, the mass ratio of allyl chitosan, sodium alginate, and the component to be encapsulated is (3-5.4):(1.8-2.8):1.

[0027] Furthermore, the mass ratio of the allyl chitosan, sodium alginate, and the component to be encapsulated is 3.3:1.7:1.

[0028] Furthermore, the mass of the effective solids in the composite cross-linking system solution is 0.5-2% of the total mass of allyl chitosan and sodium alginate.

[0029] Furthermore, the mass of the effective solids in the composite cross-linking system solution is 0.2-1% of the total mass of allyl chitosan and sodium alginate.

[0030] Furthermore, the concentration of the color-changing material in the system after adding the color-changing material is 0.2-1.8 g / L.

[0031] Furthermore, the concentration of the color-changing material in the system after its addition is 0.5 g / L.

[0032] Furthermore, in the crosslinking reaction system, the mass fraction of the first crosslinking agent is 0.6-1.8%.

[0033] Furthermore, in the system of the crosslinking reaction, the mass fraction of the crosslinking agent is 1%.

[0034] Furthermore, the mass ratio of the first microcapsule, polymer monomer, second crosslinking agent, and initiator encapsulated in the color-changing material is 1:(3.5-4.9):(0.35-0.42):(0.05-0.12).

[0035] Furthermore, the mass ratio of the first microcapsule, polymer monomer, second crosslinking agent, and initiator encapsulated in the color-changing material is 1:4:0.39:0.08.

[0036] Furthermore, the conditions for the isothermal stirring reaction are: under constant temperature conditions of 15-45℃, stirring at 500-1000 r / min for 30-90 min.

[0037] Furthermore, the conditions for the isothermal stirring reaction are: stirring at 800 r / min for 90 min under isothermal conditions of 16°C.

[0038] Furthermore, the cross-linking reaction conditions are 30-75℃ for 3-10 h.

[0039] Furthermore, the crosslinking reaction is carried out at 40°C for 4 hours.

[0040] Furthermore, the polymerization reaction is carried out at 40-60°C for 3-8 hours.

[0041] Furthermore, the polymerization reaction is carried out at 60°C for 4 hours.

[0042] This invention selects allyl chitosan (as the shell of the first microcapsule, which has the potential to copolymerize with polymer monomers by introducing double bonds) and sodium alginate as the inner wall material to embed the component to be embedded. Then, the color-changing material is adsorbed on the surface of the inner wall material. Finally, 2-acrylamide-2-methylpropanesulfonic acid, styrene and N-isopropylacrylamide are used to form the polymer outer wall material, and finally a double-layer color-changing microcapsule is prepared.

[0043] The method for preparing the above-mentioned bilayer color-changing microcapsules provided by this invention is not particularly limited in terms of the components to be encapsulated and the color-changing materials. That is, any component that needs to be encapsulated can be prepared into a corresponding microcapsule structure using the method of this invention.

[0044] Optionally, the color-changing material selected in this invention can be any one of commonly used color-changing materials such as crystal violet lactone (CVL), triphenylmethane blue, coumarin 6, and fluorescein isothiocyanate.

[0045] Optionally, the polymer monomers used in the polymerization reaction are divided into water-soluble monomers and fat-soluble monomers. The water-soluble monomers are any one or more of 2-acrylamido-2-methylpropanesulfonic acid, α-hydroxyethyl methacrylate, and dimethylaminopropylacrylamide. The fat-soluble monomers are styrene, methyl methacrylate, and butyl acrylate.

[0046] Optionally, the propylene-based crosslinking agent (second crosslinking agent) used in the polymerization reaction is any one of N-tert-butylacrylamide, N-octadecylacrylamide, N-(2-amino-2-oxoethyl)acrylamide, and N-[tris(hydroxymethyl)methyl]acrylamide.

[0047] Alternatively, the initiator used in the polymerization reaction can be any one of ammonium persulfate (APS), ascorbic acid-sodium sulfite, or potassium persulfate. These compounds are commonly used initiators for polymerization reactions.

[0048] The present invention also provides a bilayer color-changing microcapsule prepared by any of the above preparation methods.

[0049] The present invention also provides the application of the above-mentioned bilayer color-changing microcapsules as an embedding carrier for the component to be embedded.

[0050] Furthermore, the component to be encapsulated is D-limonene or a plant extract containing D-limonene.

[0051] Take, for example, the component to be encapsulated containing 50% D-limonene. D-limonene is a natural mosquito repellent ingredient. Its mosquito-repelling mechanism mainly involves interfering with the olfactory receptors of mosquitoes, preventing them from sensing the attractants such as carbon dioxide and lactic acid emitted by the human body, thereby achieving a mosquito-repelling effect. D-limonene has a good repellent effect on a variety of mosquitoes and is safe and harmless to humans, without causing skin irritation.

[0052] The bilayer color-changing microcapsule structure provided by this invention enables the controlled release of D-limonene. When textiles come into contact with the human body or are in different environmental conditions, the porous structure of the wall material in the inner layer of the microcapsule is affected by external factors (such as temperature, humidity, mechanical friction, etc.), causing changes in pore size and porosity, thereby regulating the release rate of D-limonene. Under normal temperature and humidity conditions, D-limonene is released slowly, maintaining a continuous mosquito-repellent effect. When the ambient temperature rises or the human body sweats, the color-changing material on the outer side changes color, and the release rate of D-limonene inside the microcapsule also increases accordingly to enhance the mosquito-repellent effect and meet the mosquito-repellent needs in different environments.

[0053] Taking crystal violet lactone, a color-changing material, as an example, when the ambient temperature is above 28°C and the humidity is above 60%, the color of crystal violet lactone changes from blue to light blue or light purple, at which point the probability of mosquito bites increases by 80%. Therefore, by observing the color change of the double-layered color-changing microcapsules provided by this invention, not only can the mosquito repellent needs of different environments be met, but the changes in human body temperature and the number of external mosquitoes can also be intuitively understood, realizing real-time monitoring of human body temperature and warning of disease vectors.

[0054] The bilayer color-changing microcapsule of this invention integrates three major functions: sustained release of the encapsulated component to exert its efficacy, biomonitoring, and thermal comfort management. The sustained release of the encapsulated component does not affect temperature transfer, and the introduction of the color-changing material does not block the sustained release channel, nor does it affect the efficacy of the encapsulated component.

[0055] Specifically, taking D-limonene as an example, in the bilayer color-changing microcapsules of this invention, the inner layer structure focuses on controlled release for mosquito repellency: the pore size of the porous structure is precisely controlled at 200-400 nm, ensuring that D-limonene is released at an ideal rate while preventing the outer color-changing material from seeping in and clogging the channels. The outer layer structure focuses on signal response: the color-changing material is distributed in the outer layer in the form of an interpenetrating network, forming color-developing units with a diameter of 200-500 nm. This structural design allows for rapid color change triggered by temperature changes ≥28℃, with a response time ≤10 seconds, while not affecting the release of D-limonene from the inner layer.

[0056] When the ambient temperature exceeds 28°C, the outer layer changes color. Simultaneously, the increased temperature intensifies the chain movement of the inner polymer material, increasing porosity by 30% and raising the D-limonene release rate to 1.8-2.5 μg / h. This linkage mechanism of the bilayer color-changing microcapsules of this invention extends the effective mosquito-repellent time of textiles at 35°C by 40% compared to room temperature. As sweat from the human body permeates into the inner layer of the microcapsules, the porous structure swells, further increasing porosity by 20% and raising the D-limonene release rate to 2.4-3 μg / h, thereby enhancing the mosquito-repellent effect in high-temperature environments where the human body sweats easily.

[0057] Compared with the prior art, the advantages of the present invention are: the bilayer color-changing microcapsules prepared by the present invention have a particle size D 50 Approximately 200nm; dense structure, high strength, and no cracks; fast color change response; good sustained-release effect; suitable not only for different encapsulated components, but also for different temperature-sensitive or light-sensitive color-changing materials.

[0058] When the ambient temperature exceeds 28℃, the outer heat-sensitive material changes color. Simultaneously, the increased temperature intensifies the chain segment movement of the inner polymer material, increasing porosity by 30%, which enhances the release rate of the encapsulated component. As sweat penetrates into the inner layer of the microcapsule, the inner layer porosity further increases, further enhancing the release rate of the encapsulated component. Attached Figure Description

[0059] Figure 1 The infrared analysis results are for the bilayer color-changing microcapsules prepared in Example 1.

[0060] Figure 2 The infrared analysis results are for the allyl chitosan prepared in Example 1.

[0061] Figure 3 The results of TG analysis are for the bilayer color-changing microcapsules prepared in Example 1.

[0062] Figure 4 The images shown are scanning electron microscope (SEM) images of the bilayer color-changing microcapsules prepared in Example 1. The left image is a 5000x magnification image, and the right image is an 8000x magnification image.

[0063] Figure 5 The results are TG and DTG (thermogravimetric analysis) of the first microcapsule in Example 1.

[0064] Figure 6 The particle size distribution of the bilayer color-changing microcapsules prepared in Example 1.

[0065] Figure 7 The change in DRS reflectance of the double-layer color-changing microcapsules before and after 30℃.

[0066] Figure 8 The color changes of the bilayer color-changing microcapsules are shown at ambient temperatures <30℃ and >30℃.

[0067] Figure 9 The 1-10 h mosquito repellency rate of double-layer color-changing microcapsules encapsulating citrus essential oil (50% D-limonene content).

[0068] Figure 10 The cumulative release rate of bilayer color-changing microcapsules containing citrus essential oil (50% D-limonene content) over 24 hours at different pH levels was determined.

[0069] Figure 11 The effects of different composite crosslinking systems on the encapsulation efficiency of bilayer color-changing microcapsules are shown. The composite crosslinking systems are: 2,4-hexadienal (EE) + magnesium chloride, benzoyl formaldehyde (PEN) + calcium chloride, 4-hydroxyphenylglyoxal (TTE) + calcium chloride, and terephthalaldehyde (TAG) + calcium chloride.

[0070] Figure 12 The effect of different initiators on polymerization time is shown. The initiators are ascorbic acid-sodium sulfite (AA-SHS), ammonium persulfate (APS), and potassium persulfate (KPS).

[0071] Figure 13 The results show the effect of different water-soluble monomers on the particle size of bilayer color-changing microcapsules in the polymer monomers. Among them, 2-acrylamide-2-methylpropanesulfonic acid is abbreviated as AMPS, α-hydroxyethyl methacrylate is abbreviated as MHEA, and dimethylaminopropylacrylamide is abbreviated as NNDA.

[0072] Figure 14 The results show the effect of different lipid-soluble monomers on the particle size of bilayer color-changing microcapsules in the polymer monomers. The lipid-soluble monomers are styrene (St), methyl methacrylate (MAA), and butyl acrylate (BA).

[0073] Figure 15 The effects of different allyl crosslinking agents (second crosslinking agents) on the particle size of bilayer color-changing microcapsules are shown. The allyl crosslinking agents are: N-isopropylacrylamide (NLA), N-tert-butylacrylamide (nTBA), N-octadecylacrylamide (NNOA), N-(2-amino-2-oxoethyl)acrylamide (NAGA), and N-[tris(hydroxymethyl)methyl]acrylamide (NAT). Detailed Implementation

[0074] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0075] In some embodiments of the present invention, a method for preparing a bilayer color-changing microcapsule includes the following steps:

[0076] (1) Dissolve allyl chitosan and sodium alginate in water, add the fat-soluble component to be encapsulated, and shear to obtain an emulsion;

[0077] (2) Add the composite cross-linking system solution to the emulsion, stir the reaction at a constant temperature, centrifuge to collect the precipitate, wash and dry it to obtain the first microcapsule;

[0078] (3) The first microcapsule is dispersed in anhydrous ethanol, the color-changing material is added and dispersed evenly, the first cross-linking agent is added to carry out the cross-linking reaction, the precipitate is centrifuged and dried to obtain the first microcapsule coated with the color-changing material;

[0079] (4) The first microcapsule coated with the color-changing material, the polymer monomer and the second crosslinking agent are added to water for emulsification; an initiator is added and polymerization reaction is carried out in a protective atmosphere to obtain the double-layer color-changing microcapsule.

[0080] The composite crosslinking system includes any one or more of 2,4-hexadienal, benzoyl formaldehyde, 4-hydroxyphenylglyoxal, and terephthalaldehyde, and also includes MgCl2 or CaCl2.

[0081] In the above preparation method, the first crosslinking agent is terephthalaldehyde or glutaraldehyde;

[0082] In the above preparation method, the polymer monomer is any one or more of propylene sulfonate, acrylamide lactone derivative, and styrene.

[0083] Polymer monomers are classified into water-soluble monomers and fat-soluble monomers. Water-soluble monomers include any one or more of 2-acrylamido-2-methylpropanesulfonic acid, α-hydroxyethyl methacrylate, and dimethylaminopropylacrylamide. Fat-soluble monomers include styrene, methyl methacrylate, and butyl acrylate.

[0084] In the above preparation method, the second crosslinking agent is an propylene-based crosslinking agent; for example, any one or more of N-tert-butylacrylamide, N-octadecylacrylamide, N-(2-amino-2-oxoethyl)acrylamide, and N-[tris(hydroxymethyl)methyl]acrylamide can be selected.

[0085] In the above preparation method, the initiator used can be any one of ammonium persulfate (APS), ascorbic acid-sodium sulfite, or potassium persulfate. These compounds are commonly used initiators for polymerization reactions.

[0086] Furthermore, the preparation method of the allyl chitosan is as follows:

[0087] Chitosan was dissolved in acetic acid solution, methanol was added and stirred until homogeneous, and benzaldehyde-methanol solution was added dropwise at 20-40℃. The reaction was carried out for 8-12 h to obtain a system containing gel solids.

[0088] Add sodium hydroxide solution to the system containing the gel solid and stir; add anhydrous ethanol until all the colloid precipitates out, and collect the colloid;

[0089] The colloid was washed with anhydrous ethanol, dried, and ground to obtain the allyl chitosan.

[0090] Furthermore, the preparation method of the allyl chitosan is as follows:

[0091] Chitosan was dissolved in acetic acid solution, methanol was added and stirred until homogeneous, and benzaldehyde-methanol solution was added dropwise at 60°C. The reaction was carried out for 6 h to obtain a system containing gel solids.

[0092] Add sodium hydroxide solution to the system containing the gel solid and stir; add anhydrous ethanol until all the colloid precipitates out, and collect the colloid;

[0093] The colloid was washed with anhydrous ethanol, dried, and ground to obtain the allyl chitosan.

[0094] Optionally, in the above preparation method, the mass ratio of allyl chitosan, sodium alginate and the component to be encapsulated is (3-5.4):(1.8-2.8):1.

[0095] Specifically, the mass ratio of allyl chitosan, sodium alginate, and the component to be encapsulated is 3.3:1.7:1.

[0096] Optionally, the mass of the effective solids in the composite cross-linking system solution is 0.5-2% of the total mass of allyl chitosan and sodium alginate.

[0097] Specifically, the mass of the effective solids in the composite cross-linking system solution is 0.2-1% of the total mass of allyl chitosan and sodium alginate.

[0098] Optionally, the concentration of the color-changing material in the system after adding the color-changing material is 0.2-1.8 g / L.

[0099] Specifically, the concentration of the color-changing material in the system after its addition is 0.5 g / L.

[0100] Optionally, in the crosslinking reaction system, the mass fraction of the first crosslinking agent is 0.6-1.8%.

[0101] Specifically, the crosslinking agent has a mass fraction of 1%.

[0102] Optionally, the mass ratio of the first microcapsule, polymer monomer, second crosslinking agent and initiator encapsulated in the color-changing material is 1:(3.5-4.9):(0.35-0.42):(0.05-0.12).

[0103] Specifically, the mass ratio of the first microcapsule, polymer monomer, second crosslinking agent, and initiator encapsulated in the color-changing material is 1:4:0.39:0.08.

[0104] Optionally, in the above preparation method, the conditions for the isothermal stirring reaction are: under constant temperature of 15-45℃, the reaction is stirred at 500-1000 r / min for 30-90 min.

[0105] Specifically, the conditions for the isothermal stirring reaction are: stirring at 800 r / min for 90 min at a constant temperature of 16℃.

[0106] Optionally, in the above preparation method, the crosslinking reaction is carried out at 30-75℃ for 3-10 h.

[0107] Specifically, the cross-linking reaction was carried out at 40°C for 4 hours.

[0108] Optionally, in the above preparation method, the polymerization reaction conditions are 40-60℃ for 3-8 h.

[0109] Specifically, the polymerization reaction was carried out at 60°C for 4 hours.

[0110] This invention selects allyl chitosan and sodium alginate as the inner wall material to embed the components to be embedded, and then adsorbs the color-changing material on the surface of the inner wall material; finally, 2-acrylamide-2-methylpropanesulfonic acid, styrene and N-isopropylacrylamide are used to form a polymer outer wall material, and finally a double-layer color-changing microcapsule is prepared.

[0111] The method for preparing the above-mentioned bilayer color-changing microcapsules provided by this invention is not particularly limited in terms of the components to be encapsulated and the color-changing materials. That is, any component that needs to be encapsulated can be prepared into a corresponding microcapsule structure using the method of this invention.

[0112] Optionally, the color-changing material selected in this invention can be any one of commonly used color-changing materials such as crystal violet lactone (CVL), triphenylmethane blue, coumarin 6, and fluorescein isothiocyanate.

[0113] Optionally, the initiator used in the polymerization reaction can be any one of ammonium persulfate (APS), ascorbic acid-sodium sulfite, or potassium persulfate. These compounds are commonly used initiators for polymerization reactions.

[0114] The functions of 2-acrylamide-2-methylpropanesulfonic acid, styrene, and N-isopropylacrylamide are:

[0115] In the following specific implementation examples, citrus essential oil (containing 50% D-limonene) was selected as the encapsulation component for the experiment. D-limonene can be obtained by self-isolation and purification from citrus peel, or it can be directly purchased as a commercial product. Commonly used methods in the field can be used for the self-isolation and purification of D-limonene, such as enzymatic hydrolysis or subcritical fluid extraction. The choice of self-isolation and purification method does not affect the relevant performance of the bilayer color-changing microcapsules of this application.

[0116] Citrus essential oil, as a natural food byproduct, contains a variety of bioactive substances and possesses multiple physiological functions such as antioxidation, blood sugar reduction, and blood lipid reduction. D-limonene in citrus essential oil exhibits excellent free radical scavenging properties and good insect and mosquito repellent effects. Therefore, the following specific implementation case examines the mosquito repellent performance of a double-layered color-changing capsule encapsulated with citrus essential oil to test its sustained-release effect on the citrus essential oil (the encapsulation material).

[0117] In the following specific implementation examples, crystal violet lactone (CVL) was selected as the color-changing material. Crystal violet lactone (CVL) is a functional leuco dye with temperature- and pressure-sensitive color-changing properties. As the temperature increases, crystal violet lactone gradually changes from blue to bluish-violet.

[0118] Example 1

[0119] The preparation method of allyl chitosan used in this embodiment is as follows:

[0120] (1) Take 5.00 g of chitosan and add it to a 1 L three-necked flask. Add 350 mL of 1% acetic acid solution and stir mechanically (e.g., for 3 h) until the chitosan is completely dissolved and becomes clear and transparent.

[0121] (2) Add 350 mL of methanol slowly in batches and stir until a homogeneous solution is formed.

[0122] After heating to 60℃, 100 mL of 20% benzaldehyde-methanol solution was added dropwise using a constant pressure dropping funnel. After the addition was complete, the reaction was allowed to proceed for 6 h, during which a large amount of gel solid was formed.

[0123] (3) After the reaction is complete, add 100 mL of 1% sodium hydroxide solution to the system and stir thoroughly for 20 min; pour into 1 L of anhydrous ethanol, a large amount of colloid will precipitate, and filter to collect the colloid.

[0124] (4) The colloid was repeatedly soaked in anhydrous ethanol to completely remove benzaldehyde. The colloid was dried and ground to obtain allyl chitosan.

[0125] The double-layer color-changing capsule provided in this embodiment is prepared by the following method:

[0126] (1) Dissolve 0.9 g allyl chitosan (ACS, degree of deacetylation ≥90%) and 0.6 g sodium alginate (SA) in 100 mL of deionized water, and adjust the pH of the solution to 4.0 to obtain the solution.

[0127] Add 0.2 g of citrus essential oil to the above solution (i.e., the mass ratio of allyl chitosan, sodium alginate and the component to be encapsulated is 4.5:2:1), mix well, and emulsify at 8000 rpm for 5 min to form a homogeneous and stable emulsion.

[0128] (2) Add 20 mL of CaCl2 solution (1% by mass, pH=6.0) and 0.5 mL of terephthalaldehyde-ethanol solution (20% by mass) to the emulsion, and let it stand at 4℃ for 2 h to allow sodium alginate to crosslink and solidify.

[0129] Centrifuge at 5000 rpm for 10 min, collect the precipitate (first microcapsule), wash three times with deionized water and dry (vacuum drying at 40℃) to obtain the first microcapsule solid powder.

[0130] (3) Disperse all the prepared first microcapsule solid powders in 100 mL of anhydrous ethanol, add 0.05 g crystal violet lactone (CVL, i.e., concentration of 0.5 g / L), ultrasonically disperse (300 W, 5 min) until uniform, and homogenize at 20000 r / min for 10 min.

[0131] 10 mL of the first crosslinking agent, terephthalaldehyde (concentration of 1% in the crosslinking reaction system), was slowly added under stirring at 500 rpm. The crosslinking reaction was carried out at 40℃ for 2 h, allowing CVL to be adsorbed onto the surface of the microcapsules. The precipitate was collected by centrifugation (5000 rpm, 10 min) and dried (vacuum drying at 40℃) to obtain the first microcapsules coated with the color-changing material.

[0132] (4) Add 0.8 g of the first microcapsule coated with the color-changing material, 0.7 g of 2-acrylamide-2-methylpropanesulfonic acid (AMPS), 2.7 g of styrene (St) and 0.32 g of N-isopropylacrylamide (NIPAM) to 100 mL of deionized water and emulsify by ultrasonication (600 W, 10 min).

[0133] 0.1 g of ammonium persulfate (APS) initiator was added, and polymerization was carried out at 60 °C for 4 h under a nitrogen atmosphere to obtain a reaction solution containing the aforementioned bilayer color-changing microcapsules. Specifically, the mass ratio of the first microcapsule coated with the color-changing material, 2-acrylamide-2-methylpropanesulfonic acid, styrene, N-isopropylacrylamide, and initiator was 1:0.875:3.375:0.4:0.125.

[0134] (5) The bilayer color-changing microcapsules were separated from the reaction solution by centrifugation (5000 rpm, 10 min) and vacuum filtration. They were washed three times with anhydrous ethanol to remove residual monomers and other impurities, and then dried under vacuum at 60°C to obtain the bilayer color-changing microcapsules.

[0135] Infrared analysis of the bilayer color-changing microcapsules yielded the following results: Figure 1 As shown. It can be seen that 1653 cm -1 The stretching vibration peak corresponding to the carbonyl group (-C=O) of the residual amide bond in the second shell molecule indicates the presence of 2-acrylamido-2-methylpropanesulfonic acid in the polymer monomer; 1595 cm⁻¹ -1 The peak at that position is the stretching vibration peak of the amino group (-NH2), indicating the presence of the -NH2 group of the monomer N-isopropylacrylamide. Compared to Figure 2 The FTIR spectrum of unmodified chitosan, with a diameter of 1618 cm⁻¹. -1 The peak intensity at 1526 cm⁻¹ showed a significant increase, which is because the carbonyl peak at that location was superimposed with the double bond peak from the allyl group. -1 The peak at that point did not change, indicating the presence of amino groups in the modified allyl chitosan.

[0136] The particle size of the prepared bilayer color-changing microcapsules was observed, and the results are as follows: Figure 6 As shown. The average particle size D of the microcapsules was measured by dynamic light scattering method. 50 ≈200nm; the microcapsules exhibit a sharp single-peak particle size distribution, with a statistically significant PDI < 0.1, indicating a monodisperse state. The calculated particle size is D. 90 =227nm. The microstructure of the bilayer color-changing microcapsules is as follows: Figure 4 As shown, scanning electron microscopy results reveal that the bilayer structure of the bilayer color-changing microcapsules is dense and crack-free.

[0137] In the above-described method for preparing bilayer color-changing microcapsules in this embodiment, under a nitrogen atmosphere, the bilayer color-changing microcapsules undergo two stages of degradation compared to allyl chitosan: the first stage occurs around 100°C, mainly involving the removal of moisture; the second stage occurs between 200-400°C, resulting from the decomposition, oxidation, and combustion of the second layer of microcapsule wall material, which can be attributed to the dehydration of the comonomer and the depolymerization of the polymer unit.

[0138] Thermogravimetry (TG) analysis was performed on the bilayer color-changing microcapsules. The results are as follows: Figure 3 , 5As shown in Table 1 below, compared with allyl chitosan, the second layer of microcapsule wall material significantly improved thermal stability due to the introduction of 2-acrylamido-2-methylpropanesulfonic acid, styrene and N-isopropylacrylamide, with the residual char content increasing from 33.1% to 37.1%.

[0139] Table 1

[0140] like Figure 3 and 5 As shown, the quality decline caused by the leakage of citrus essential oil occurs at around 200℃. This temperature range coincides with the boiling point of D-limonene (176℃), which demonstrates that the prepared bilayer color-changing microcapsules have good slow volatilization properties.

[0141] like Figure 7 , 8 As shown, the bilayer microcapsule powder exhibits a color change at 30℃. Specifically, when the ambient temperature T < 30℃, the bilayer microcapsule is a blue powder with a maximum absorption wavelength of λmax = 490 nm; when the ambient temperature T > 30℃, the bilayer microcapsule is a light blue powder with a maximum absorption wavelength of λmax = 505 nm.

[0142] like Figure 9 As shown, the double-layer color-changing microcapsules achieved a mosquito repellency rate of 96.1% after 2 hours and 93.6% after 5 hours using a Y-type olfactory meter.

[0143] like Figure 10 As shown, the cumulative release rate of the prepared bilayer color-changing microcapsules in buffer solution (pH=7.4, 0.2 mol / L disodium hydrogen phosphate, 0.1 mol / L citric acid) reached 72.7% after 24 hours, which meets the requirements for sustained release.

[0144] Example 2: Effect of different composite crosslinking systems on the encapsulation efficiency of bilayer color-changing microcapsules

[0145] This embodiment studies the effect of different composite crosslinking systems on the encapsulation efficiency of bilayer color-changing microcapsules. The preparation method of the bilayer color-changing capsules is basically the same as in Example 1, with the difference being:

[0146] (1) The mass ratio of allyl chitosan, sodium alginate and the component to be encapsulated is 4.5:2:1. The effective content in the composite crosslinking system solution is 1% of the total mass of allyl chitosan and sodium alginate.

[0147] (2) The composite crosslinking systems are: 2,4-hexadienal (abbreviated as EE) + magnesium chloride, benzoyl formaldehyde (abbreviated as PEN) + calcium chloride, 4-hydroxyphenylglyoxal (abbreviated as TTE) + calcium chloride, and terephthalaldehyde (abbreviated as TAG) + calcium chloride.

[0148] The results are as follows Figure 11 As shown, the encapsulation effect is best when the crosslinking agent is terephthalaldehyde (TAG) + (calcium chloride).

[0149] Example 3: Effect of different initiators on polymerization reaction time

[0150] This embodiment is used to study the effect of different initiators on polymerization reaction time. The preparation method of the bilayer color-changing capsule is basically the same as in Example 1, with the difference being:

[0151] (1) The mass ratio of allyl chitosan, sodium alginate and the component to be encapsulated (citrus essential oil, or lemongrass essential oil, wormwood oil or pepper oil) is 4.5:2:1. The effective content in the composite cross-linking system solution is 1% of the total mass of allyl chitosan and sodium alginate.

[0152] (2) The mass ratio of the first microcapsule (0.8 g) coated with the color-changing material, 2-acrylamide-2-methylpropanesulfonic acid, styrene, N-isopropylacrylamide, and the initiation system is 1:1.4:3.2:0.39:0.054.

[0153] (3) The initiators are ascorbic acid-sodium sulfite (AA-SHS), ammonium persulfate (APS), and potassium persulfate (KPS).

[0154] The results are as follows Figure 12 As shown, in the polymerization reactions involving the three initiators, the effective reaction time was longest for ascorbic acid-sodium sulfite and shortest for potassium persulfate.

[0155] Example 4: Effect of different water-soluble monomers in the polymer monomer on the particle size of bilayer color-changing microcapsules

[0156] This embodiment investigates the effect of different water-soluble monomers in the polymer monomer on the particle size of bilayer color-changing microcapsules. The preparation method of the bilayer color-changing capsules is basically the same as in Example 1, with the difference being:

[0157] The mass ratio of the first microcapsule (0.8 g) coated with the color-changing material, water-soluble monomer, styrene, N-isopropylacrylamide, and potassium persulfate is 1:1.2:3.5:0.38:0.08. The water-soluble monomers selected are 2-acrylamido-2-methylpropanesulfonic acid (AMPS), α-hydroxyethyl methacrylate (MHEA), and dimethylaminopropylacrylamide (NNDA).

[0158] The results are as follows Figure 13As shown, the particle sizes of the bilayer color-changing microcapsules prepared using the three water-soluble monomers all meet the requirements. Among them, the particle size of the bilayer color-changing microcapsules prepared using 2-acrylamide-2-methylpropanesulfonic acid (AMPS) is 227 nm.

[0159] Example 5: Effect of different oil-soluble monomers on the particle size of bilayer color-changing microcapsules

[0160] This embodiment investigates the effect of different oil-soluble monomers in the polymer monomers on the particle size of bilayer color-changing microcapsules. The preparation method of the bilayer color-changing capsules is basically the same as in Example 1, with the difference being:

[0161] The mass ratio of the first microcapsule (0.8 g) coated with the color-changing material, 2-acrylamide-2-methylpropanesulfonic acid, oil-soluble monomer, N-isopropylacrylamide, and potassium persulfate is 1:1.8:2.6:0.36:0.075. The oil-soluble monomers selected are styrene (abbreviated as St), methyl methacrylate (abbreviated as MAA), and butyl acrylate (abbreviated as BA).

[0162] The results are as follows Figure 14 As shown, the particle sizes of the bilayer color-changing microcapsules prepared using the three oil-soluble monomers all meet the requirements. Among them, the particle size of the bilayer color-changing microcapsules prepared with styrene (abbreviated as St) is 206 nm.

[0163] Example 6: Effect of different second crosslinking agents (propylene-based crosslinking agents) on the particle size of bilayer color-changing microcapsules

[0164] This embodiment investigates the effect of different second crosslinking agents (propylene-based crosslinking agents) on the particle size of bilayer color-changing microcapsules. The preparation method of the bilayer color-changing capsules is basically the same as in Example 1, with the following differences:

[0165] The mass ratio of the first microcapsule (0.8 g) coated with the color-changing material, 2-acrylamido-2-methylpropanesulfonic acid, styrene, allyl crosslinking agent, and potassium persulfate is 1:1.8:2.6:0.36:0.075. The allyl crosslinking agents selected are: N-isopropylacrylamide (NLA), N-tert-butylacrylamide (nTBA), N-octadecylacrylamide (NNOA), N-(2-amino-2-oxoethyl)acrylamide (NAGA), and N-[tris(hydroxymethyl)methyl]acrylamide (NAT).

[0166] The results are as follows Figure 15 As shown, the particle sizes of the bilayer color-changing microcapsules prepared using five allyl crosslinking agents all met the requirements. Among them, the particle size of the bilayer color-changing microcapsules prepared with N-isopropylacrylamide (NLA) was 264 nm.

[0167] The above detailed embodiments describe the implementation of the present invention; however, the present invention is not limited to the specific details described in the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

Claims

1. A method for preparing a double-layer color-changing microcapsule, characterized in that, Includes the following steps: Allyl chitosan and sodium alginate were dissolved in water, and the fat-soluble component to be encapsulated was added. The mixture was then sheared to obtain an emulsion. A composite cross-linking system solution was added to the emulsion, and the mixture was stirred at a constant temperature. The precipitate was collected by centrifugation, washed, and dried to obtain the first microcapsule. The first microcapsule was dispersed in anhydrous ethanol, and a color-changing material and a first cross-linking agent were added to carry out a cross-linking reaction. The precipitate was collected by centrifugation and dried to obtain the first microcapsule coated with the color-changing material. The first microcapsule coated with the color-changing material, the polymer monomer, and the second crosslinking agent were emulsified in water; an initiator was added and polymerization was carried out in a protective atmosphere to obtain the bilayer color-changing microcapsule. The composite crosslinking system includes any one or more of 2,4-hexadienal, benzoyl formaldehyde, 4-hydroxyphenylglyoxal, and terephthalaldehyde, and also includes MgCl2 or CaCl2. The first crosslinking agent is terephthalaldehyde or glutaraldehyde; The polymer monomers are propylene sulfonate, acrylamide lactone derivatives, and styrene-propylene. The second crosslinking agent is a propylene-based crosslinking agent.

2. The method for preparing bilayer color-changing microcapsules according to claim 1, characterized in that, The preparation method of the allyl chitosan is as follows: Chitosan was dissolved in acetic acid solution, methanol was added and stirred until homogeneous, and benzaldehyde-methanol solution was added dropwise at 20-40℃. The reaction was carried out for 8-12 h to obtain a system containing gel solids. Add sodium hydroxide solution to the system containing the gel solid and stir; add anhydrous ethanol until all the colloid precipitates out, and collect the colloid; The colloid was washed with anhydrous ethanol, dried, and ground to obtain the allyl chitosan.

3. The method for preparing bilayer color-changing microcapsules according to claim 1, characterized in that, The mass ratio of allyl chitosan, sodium alginate, and the component to be encapsulated is (3-5.4):(1.8-2.8):1; Furthermore, the mass ratio of the allyl chitosan, sodium alginate, and the component to be encapsulated is 3.3:1.7:

1.

4. The method for preparing bilayer color-changing microcapsules according to claim 1, characterized in that, The effective solids in the composite cross-linking system solution are 0.5-2% of the total mass of allyl chitosan and sodium alginate. Furthermore, the mass of the effective solids in the composite cross-linking system solution is 0.2-1% of the total mass of allyl chitosan and sodium alginate.

5. The method for preparing bilayer color-changing microcapsules according to claim 1, characterized in that, The concentration of the color-changing material in the system after its addition is 0.2-1.8 g / L; Furthermore, the concentration of the color-changing material in the system after adding the color-changing material is 0.5 g / L.

6. The method for preparing bilayer color-changing microcapsules according to claim 1, characterized in that, In the crosslinking reaction system, the mass fraction of the first crosslinking agent is 0.6-1.8%; Furthermore, the mass fraction of the first crosslinking agent is 1%.

7. The method for preparing bilayer color-changing microcapsules according to claim 1, characterized in that, The mass ratio of the first microcapsule, polymer monomer, second crosslinking agent, and initiator encapsulated in the color-changing material is 1:(3.5-4.9):(0.35-0.42):(0.05-0.12); Furthermore, the mass ratio of the first microcapsule, polymer monomer, second crosslinking agent, and initiator encapsulated in the color-changing material is 1:4:0.39:0.

08.

8. The method for preparing bilayer color-changing microcapsules according to claim 1, characterized in that, The conditions for the isothermal stirring reaction are: under constant temperature of 15-45℃, stirring at 500-1000 r / min for 30-90 min. Furthermore, the conditions for the isothermal stirring reaction are: stirring at 800 r / min for 90 min under isothermal conditions of 16℃. The crosslinking reaction is carried out at 30-75°C for 3-10 hours; further, the crosslinking reaction is carried out at 40°C for 4 hours. The polymerization reaction is carried out at 40-60℃ for 3-8 hours; further, the polymerization reaction is carried out at 60℃ for 4 hours.

9. A double-layered color-changing microcapsule, characterized in that, It is prepared by the preparation method described in any one of claims 1-8.

10. The use of a bilayer color-changing microcapsule prepared by the preparation method according to any one of claims 1-8, or the bilayer color-changing microcapsule of claim 9, as an embedding carrier for the component to be embedded; Furthermore, the component to be encapsulated is D-limonene or a plant extract containing D-limonene.

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