Core-shell structure modified double-layer hydrogel self-healing material and preparation method thereof

Abstract

This invention discloses a core-shell structure-modified bilayer hydrogel self-healing material and its preparation method. The material comprises a nano-microscale wollastonite-modified hydrogel core layer and a calcium alginate shell layer formed by slow-release in-situ crosslinking with calcium ions. The hydrogel core layer is prepared by a suspension method. The shell layer is formed by adsorbing calcium ions in a calcium chloride solution through the hydrogel core layer and then slowly releasing and crosslinking with a sodium alginate solution. The sodium alginate chains are linked with the calcium... 2+ It forms an "egg box" structure; through the synergistic effect of the core and shell structure, functional zoning is achieved. The nano- and micro-sized wollastonite significantly improves the mechanical stability of the material. The shell layer can not only shield the cement-based high alkalinity and high ionic strength environment to protect the activity of the core layer, but also respond and trigger when cracks are generated or water invades. Combined with the synergistic healing effect of hydrogel swelling and sealing and wollastonite hydrolysis and mineralization, it can solve the problems of poor mechanical properties, insufficient hydrolysis resistance, easy premature release of healing agent, and difficulty in balancing stability and response sensitivity of traditional superabsorbent resins.

Description

Technical Field

[0001] This invention belongs to the field of self-healing technology of cement-based materials, and more specifically, relates to a core-shell structure modified bilayer hydrogel self-healing material and its preparation method. Background Technology

[0002] Cement-based materials, with their excellent mechanical properties, low cost, and widespread availability of raw materials, have become the most widely used and consumed building structural materials globally, finding extensive applications in housing construction, bridges, roads, water conservancy projects, and many other fields. However, the inherent brittleness, relatively low tensile strength, and unavoidable volume deformation during hydration and hardening of cement-based materials make them highly susceptible to microcracks during service life due to the influence of loads, temperature changes, wet-dry cycles, and other internal and external factors. The presence of these microcracks not only severely weakens the mechanical properties and structural integrity of cement-based materials but also provides penetration channels for external moisture and harmful ions (such as chloride and sulfate ions), accelerating structural corrosion and aging, significantly reducing the durability of cement-based materials, and even threatening the long-term safe service of the structure, leading to substantial economic losses and safety hazards.

[0003] To address the cracking problem in cement-based materials, self-healing technology has become a research hotspot in this field in recent years. Among them, the technical route using superabsorbent polymers (SAPs) as internal curing agents and healing agent carriers has attracted much attention due to its potential application value. SAPs can repair cracks by swelling and filling them themselves or by releasing loaded healing agents (chemical healing agents, biological agents, etc.) when cracks occur inside cement-based materials.

[0004] However, existing SAPs still have many shortcomings that urgently need to be addressed in practical applications: Insufficient mechanical stability: During the concrete mixing process, SAPs are subjected to strong shear forces and frictional collisions with aggregates, which can easily cause structural damage, making it impossible for them to maintain their complete functional form within the cement-based material. Insufficient hydrolysis resistance: During service, cement-based materials face repeated wet and dry cycles and water environment erosion. SAPs molecular chains are prone to hydrolysis and breakage in water media, which leads to the decline of their water absorption and swelling properties, destruction of structural integrity, and loss of the core function of subsequent crack repair. Poor precision in release of healing agents: For SAPs loaded with specific chemical healing agents or biological agents, structural damage during the mixing process or direct exposure to the complex environment of cement-based materials can lead to premature and non-selective release of the healing agents. This means that when cracks actually occur and need repair, the SAPs have already "run out of ammunition" and cannot achieve precise release of the healing agents as needed. Poor environmental adaptability: SAPs are directly exposed to the highly alkaline and high ionic strength environment formed in the early stage of cement hydration, which seriously affects their chemical stability. Not only is their own structure easily hydrolyzed and destroyed, but it may also cause the loaded healing agent to become inactive and lose its healing function. Stability and responsiveness are difficult to balance: If modifications are made solely to improve the structural and chemical stability of SAPs, it often leads to a decrease in their responsiveness to environmental changes such as crack formation and moisture intrusion, making it impossible to trigger the swelling or healing agent release process in a timely manner, thus affecting the crack repair effect.

[0005] Therefore, developing a novel hydrogel self-healing material that combines good mechanical stability, hydrolysis resistance, chemical stability, and environmental responsiveness, and can achieve precise on-demand release of the healing agent, is of great significance for improving the self-healing ability and long-term durability of cracks in cement-based materials. It is also a technical problem that urgently needs to be solved in the field of self-healing technology of cement-based materials. Summary of the Invention

[0006] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a method for preparing a core-shell structured modified bilayer hydrogel self-healing material. Through a core-shell structure design of "nano-micron-sized wollastonite-modified hydrogel core layer + calcium ion slow-release in-situ crosslinked calcium alginate shell layer" and a self-driven in-situ crosslinking preparation process based on osmotic pressure difference, the method significantly enhances the mechanical strength of the hydrogel by utilizing the needle-like and fibrous crystal structure of nano-micron-sized wollastonite, enabling it to resist shear forces and aggregate friction during concrete mixing and maintain structural integrity. Furthermore, the shell layer provides a shielding protection in the highly alkaline and high-ionic-strength environment of cement-based materials, preventing the core layer and any potentially loaded healing agent from over-excessive heat. It exhibits early inactivation and possesses environmentally triggered response characteristics when cracks occur or moisture invades, enabling precise release of the healing agent on demand. Combined with the synergistic healing effect of the hydrogel core layer swelling and sealing and the hydrolysis of nano- and micro-sized wollastonite to generate silicate products, it effectively solves the technical pain points of traditional superabsorbent polymers (SAPs), such as poor mechanical stability, insufficient hydrolysis resistance, premature release of the healing agent, and difficulty in balancing chemical stability and environmental response sensitivity. It significantly improves the self-healing rate and long-term durability of cracks in cement-based materials. Moreover, the preparation process is simple and controllable, requires no complex equipment, and is easy to scale up for production and application, demonstrating significant technical advantages and engineering application value.

[0007] To achieve the above objectives, one aspect of the present invention provides a core-shell structure modified bilayer hydrogel self-healing material, comprising a hydrogel core layer and a shell hydrogel, wherein the hydrogel core layer is a nano-microscale wollastonite modified hydrogel, comprising 75-80 parts by weight of hydrogel matrix and 20-25 parts by weight of nano-microscale wollastonite, and is prepared by reverse suspension method. The shell hydrogel is formed by in-situ crosslinking with calcium ions through slow release and is coated on the surface of the hydrogel core layer.

[0008] Further, the hydrogel matrix comprises, by weight, 25-30 parts monomer, 0.03-0.05 parts crosslinking agent, 0.2-0.5 parts initiator, and the remainder is solvent; The monomer is a combination of at least two of acrylic acid, acrylamide and sodium alginate; The crosslinking agent is N,N'-methylenebisacrylamide; The initiator is ammonium persulfate or potassium persulfate; The solvent is deionized water.

[0009] Furthermore, when the monomer is a combination of acrylic acid and sodium alginate, the proportions of the two in the total weight of the monomer are 80-85 parts of acrylic acid and 15-20 parts of sodium alginate, respectively.

[0010] Furthermore, the nano-microscale wollastonite is prepared by ball milling, with the specific preparation parameters being: the mass ratio of natural wollastonite to the grinding media is 1:3.5, and the dry milling time is 1 hour; the gradation of the grinding media is 5mm:3mm:1mm = 2:4:4.

[0011] Furthermore, the monomer of the shell hydrogel is sodium alginate; the sodium alginate cross-links and polymerizes with calcium ions seeping out from the core layer of the hydrogel to form the shell hydrogel, and the carboxyl groups on the sodium alginate chain form an "egg box" structure with Ca2+. Furthermore, the shell hydrogel plays a shielding and protective role in the highly alkaline and high ionic strength environment of cement-based materials, preventing premature deactivation of the hydrogel core layer. It can also rupture or swell when cracks occur or external moisture invades, triggering the swelling, filling, and healing effects of the hydrogel core layer.

[0012] Furthermore, the nano-microscale wollastonite has a needle-like or fibrous crystal structure, which can form a synergistic enhancement effect with the hydrogel matrix to improve the mechanical strength of the hydrogel. It can also hydrolyze in the crack environment to generate silicate products, which form a synergistic healing effect with the swelling and filling effect of the hydrogel core layer, thereby improving the density of crack repair.

[0013] A second aspect of the present invention provides a method for preparing a core-shell structure-modified bilayer hydrogel self-healing material, comprising the following steps: S1: Preparation of nano- and micro-scale wollastonite-modified hydrogel core layers; S2: Place the hydrogel core layer prepared in step S1 into a 0.5wt% calcium chloride solution and soak for 0.5–1.5 h to allow the hydrogel core layer to adsorb sufficient calcium ions and reach an adsorption-release equilibrium. S3: The hydrogel core layer soaked in step S2 is placed in a 2wt% sodium alginate solution. Calcium ions are slowly exuded by osmotic pressure and crosslinked in situ with sodium alginate to form a shell hydrogel, ultimately obtaining a core-shell structure modified bilayer hydrogel self-healing material.

[0014] Further, the preparation of the nano-microscale wollastonite-modified hydrogel core layer in step S1 specifically includes: S11. Prepare nano-micro wollastonite: Mix 75-80 parts by weight of hydrogel matrix and 20-25 parts by weight of nano-micro wollastonite; add 25-30 parts by weight of monomer of hydrogel matrix, 0.03-0.05 parts by weight of crosslinking agent and 0.2-0.5 parts by weight of initiator to deionized water; then add 20-25 parts by weight of the pre-prepared nano-micro wollastonite powder, turn on the magnetic stirrer and stir continuously at a speed of 500 rpm until a homogeneous and stable mixed solution is formed, thus obtaining the polymerization precursor; S12. Slowly drip the polymerization precursor into the oil phase, while turning on the ultrasonic equipment and controlling the stirring rate to 800 rpm, so that the polymerization precursor is uniformly dispersed in the oil phase to form a stable suspension; place the reaction vessel in a constant temperature environment of 65℃ and keep it at a constant temperature until the polymerization is complete to form a reaction product containing wollastonite modified hydrogel particles. S13. After the polymerization reaction is completed, the solid product is separated by filtration to remove the oil phase and unreacted impurities. The solid product is repeatedly washed with an appropriate amount of solvent to remove residual impurities. Then, it is dried in an 80°C oven to constant weight to obtain a nano-microscale wollastonite-modified hydrogel core layer.

[0015] Further, the preparation of nano-microscale wollastonite in step S11 includes: selecting natural wollastonite as raw material and matching it with a specific grinding media; the grinding media has a mass ratio of 5mm:3mm:1mm = 2:4:4; controlling the mass ratio of natural wollastonite to grinding media to be 1:3.5; and performing ball milling by dry milling, setting the ball milling time to 1 hour. Through this ball milling process, the natural wollastonite particles are pulverized to the nano-microscale level, and finally nano-microscale wollastonite powder is obtained.

[0016] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects: (1) The core-shell structure modified double-layer hydrogel self-healing material and its preparation method of the present invention can significantly improve the mechanical stability of hydrogel and ensure structural integrity during service: Specifically, nano-microscale wollastonite is prepared by ball milling and introduced into the hydrogel core layer as a modifying component. The unique needle-like and fibrous crystal structure of nano-microscale wollastonite forms a synergistic reinforcement effect with the hydrogel matrix, which greatly improves the mechanical strength of the hydrogel core layer. The synergistic effect of the hydrogel core layer and shell layer enables the self-healing material to resist shear force and aggregate friction and collision during concrete mixing, significantly reducing the structural damage rate. This effectively solves the technical defect of traditional superabsorbent polymers (SAPs) being easily damaged during mixing, ensuring that it maintains its complete functional form inside the cement-based material.

[0017] (2) The core-shell structure modified bilayer hydrogel self-healing material and its preparation method of the present invention achieve functional synergy through the bilayer core-shell structure. The shell layer is a calcium alginate gel layer formed by slow-release in-situ cross-linking of calcium ions. It is highly compatible with the hydrogel core layer and has good compatibility. In the high alkalinity and high ionic strength environment of the cement-based material in the early stage of hydration, it can play an effective shielding and protection role, avoid premature deactivation of the hydrogel core layer and the healing agent that may be loaded, and significantly improve the chemical stability of the material. At the same time, the shell layer has environmental trigger response characteristics. When the cement-based material cracks or external moisture invades, the shell layer can break or swell in time, ensuring that the hydrogel core layer responds quickly to environmental changes and triggers the swelling filling or healing agent release process. It effectively solves the technical contradiction that the stability and response sensitivity of traditional modified SAPs are difficult to balance.

[0018] (3) The core-shell structure modified double-layer hydrogel self-healing material and its preparation method of the present invention can achieve efficient and dense repair of cracks and improve the durability of cement-based materials. When cracks occur in cement-based materials, the shell layer of the hydrogel self-healing material of the present invention cracks or swells in response to environmental changes, and the hydrogel core layer rapidly swells to seal the cracks, while releasing the internal nano-micro-sized wollastonite. The nano-micro-sized wollastonite rapidly hydrolyzes in the crack environment to generate silicate products, which form a synergistic healing effect with the swelling and filling effect of the hydrogel, realizing the dense filling and mineralization repair of cracks, and significantly improving the self-healing rate of cracks. According to the test, the self-healing rate of the material of the present invention for 0.2 mm wide cracks can reach more than 85% after being cured in NaOH solution in a simulated high alkaline environment of cement-based materials for 28 days. It effectively blocks the penetration channels of external moisture and harmful ions, significantly improves the long-term durability of cement-based materials, and extends their service life.

[0019] (4) The core-shell structure modified bilayer hydrogel self-healing material and its preparation method of the present invention adopt the osmotic pressure difference self-driven in-situ crosslinking technology to prepare the core-shell structure. No additional crosslinking agent or complex equipment is required. The uniform and dense shell structure can be formed by adsorbing calcium ions in calcium chloride solution and then slowly releasing crosslinking in sodium alginate solution. The entire preparation process has clear parameters, controllable operation, good repeatability, low requirements for production equipment, controllable production cost, and can meet the needs of industrial-scale production. It has broad engineering application prospects.

[0020] (5) The core-shell structure modified bilayer hydrogel self-healing material and its preparation method of the present invention avoid the problem of premature and non-selective release of healing agent in traditional loaded SAPs during the stirring process or the initial stage of hydration through the shielding and protection effect of the shell layer; the shell layer will only start to respond and allow the core layer to swell or release the healing agent under the specific triggering condition of crack generation, which can realize the precise release of healing agent on demand, ensure that sufficient healing material can be provided during the critical period of crack repair, greatly improve the healing efficiency and resource utilization, and solve the technical pain point of "ammunition depletion" of traditional healing agent carriers. Attached Figure Description

[0021] Figure 1 This is a three-dimensional structural schematic diagram of a core-shell structure modified bilayer hydrogel self-healing material according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the microstructure of a core-shell structure modified bilayer hydrogel self-healing material according to an embodiment of the present invention; Figure 3 This is a flowchart illustrating a method for preparing a core-shell structured modified bilayer hydrogel self-healing material according to an embodiment of the present invention. Figure 4 This is a flowchart illustrating the preparation method of the nano-microscale wollastonite-modified hydrogel core layer in a method for preparing a core-shell structure modified bilayer hydrogel self-healing material according to an embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0023] like Figure 1 and Figure 2As shown, one aspect of the present invention provides a core-shell structure modified bilayer hydrogel self-healing material, comprising a hydrogel core layer and a shell hydrogel, wherein the hydrogel core layer is a nano-microscale wollastonite-modified hydrogel, and the shell hydrogel is formed by calcium ion slow-release in-situ crosslinking and coated on the surface of the hydrogel core layer; it can achieve a three-in-one functional integration of "mechanical reinforcement - environmental shielding - on-demand response", breaking through the inherent contradiction between stability and responsiveness of traditional hydrogel self-healing materials. The hydrogel core layer, by weight, comprises 75-80 parts of hydrogel matrix and 20-25 parts of nano-micro wollastonite, prepared by reverse suspension polymerization, which allows for precise control of the core layer particle size and pore structure, improving interfacial compatibility with cement-based matrices. The hydrogel matrix, by weight, comprises 25-30 parts of monomer, 0.03-0.05 parts of crosslinking agent, 0.2-0.5 parts of initiator, with the remainder being solvent. The monomer is a combination of at least two of acrylic acid, acrylamide, and sodium alginate, forming an interpenetrating network structure to optimize swelling performance and mechanical toughness. The crosslinking agent is N,N'-methylenebisacrylamide (BIS); the initiator is ammonium persulfate (APS) or potassium persulfate (KPS), which can achieve efficient polymerization under mild conditions; the solvent is deionized water; when the monomer is a combination of acrylic acid and sodium alginate, the proportions of the two in the total weight of the monomers are 80-85 parts acrylic acid and 15-20 parts sodium alginate, respectively. By adjusting the ratio of the two monomers, the core layer can simultaneously possess excellent water absorption and swelling capacity and ion response activity, providing a structural basis for subsequent shell crosslinking and healing triggering.

[0024] Furthermore, the nano-microscale wollastonite is prepared by ball milling. By optimizing the grinding parameters to retain its natural needle-like crystal structure, the specific preparation parameters are as follows: the mass ratio of natural wollastonite to the grinding media is 1:3.5, and the gradation of the grinding media is 5mm:3mm:1mm = 2:4:4, that is, a multi-gradation grinding media with a mass ratio of 5mm:3mm:1mm = 2:4:4 is used. The dry grinding time is 1 hour, so that the wollastonite particles are accurately crushed to the nano-microscale and the aspect ratio of the needle-like structure is maintained at 10-15:1. Compared with the ordinary ball milling process, its synergistic enhancement effect with the hydrogel matrix is ​​improved by more than 30%, effectively solving the technical pain point of traditional hydrogels being easily damaged under the shear force of concrete mixing.

[0025] Furthermore, the shell hydrogel uses sodium alginate as a monomer and innovatively utilizes an osmotic pressure-driven slow-release in-situ cross-linking mechanism for calcium ions. This allows calcium ions adsorbed in the hydrogel core layer to slowly seep out and cross-link with sodium alginate to form a dense shell. The carboxyl groups on the sodium alginate chains also interact with the calcium ions in the core layer. 2+ A stable "egg box" structure is formed, and the shell thickness can be precisely controlled between 50-100μm by soaking time. Compared with traditional coating processes, the interfacial bonding strength between the shell and the core is increased by 40%, and there is no risk of delamination.

[0026] Furthermore, the shell hydrogel possesses intelligent "active shielding-passive response" properties, forming a dense protective barrier in the highly alkaline (pH≈13) and high ionic strength environment of cement-based materials, effectively blocking OH groups. - It also prevents harmful ion penetration, avoids degradation and functional deactivation of the hydrogel core layer structure, and solves the problem of easy aging and disintegration of traditional hydrogels in the high-alkali environment of cement. At the same time, the shell layer is highly sensitive to mechanical stress and water intrusion when cracks occur, and can respond quickly through two paths: rupture or swelling, accurately triggering the swelling and filling of the core layer and the healing reaction, so as to achieve dynamic synchronization of "crack appearance - instant response".

[0027] Furthermore, the nano-microscale wollastonite has a needle-like and fibrous crystal structure. Its needle-like structure can form a synergistic system similar to "fiber-reinforced composite materials" with the hydrogel matrix, significantly improving the tensile strength and shear resistance of the hydrogel, so that the structural damage rate of the core layer under simulated concrete mixing conditions is less than 10%. More importantly, it can slowly hydrolyze in the crack water environment to generate silicate active products, forming a dual synergistic healing effect of "physical sealing-chemical mineralization" with the swelling physical filling of the hydrogel core layer. Compared with single swelling filling, the crack repair density is improved by more than 50%, effectively blocking the penetration of water and harmful ions, and significantly extending the service life of cement-based structures.

[0028] like Figure 3 As shown, a second aspect of the present invention provides a method for preparing a core-shell structured modified bilayer hydrogel self-healing material, comprising the following steps: S1: To prepare nano-micro wollastonite, the hydrogel matrix monomer, crosslinking agent, and initiator were added to deionized water according to the formula, and then the prepared nano-micro wollastonite powder was added and stirred to form a polymerization precursor. Subsequently, the precursor was dropped into the oil phase and polymerized completely under ultrasonic assistance, stirring and constant temperature of 65°C. After filtration, multiple washings and drying at 80°C to constant weight, the nano-micro wollastonite-modified hydrogel core layer was obtained. S2: Place the hydrogel core layer prepared in step S1 into a 0.5wt% calcium chloride solution and soak for 0.5–1.5 h; S3: The hydrogel core layer soaked in step S2 is placed in a 2wt% sodium alginate solution, and a shell hydrogel is formed by in-situ crosslinking through the slow release of calcium ions, thus obtaining a core-shell structure modified bilayer hydrogel self-healing material.

[0029] Furthermore, such as Figure 4 As shown, step S1, which involves preparing a nano- or micro-sized wollastonite-modified hydrogel core layer, specifically includes: S11. Prepare nano-micro wollastonite by mixing 75-80 parts by weight of hydrogel matrix and 20-25 parts by weight of nano-micro wollastonite. Add 25-30 parts by weight of monomer of hydrogel matrix, 0.03-0.05 parts by weight of crosslinking agent and 0.2-0.5 parts by weight of initiator to deionized water. Then add 20-25 parts by weight of the prepared nano-micro wollastonite powder to the solution obtained in step S11. Turn on the magnetic stirrer and control the stirring speed at 500 rpm. Continue stirring until a homogeneous and stable mixed solution is formed, thus obtaining the polymerization precursor. S12. Slowly drop the polymer precursor obtained in step S11 into the oil phase, while turning on the ultrasonic device and controlling the stirring speed to 800 rpm, so that the polymer precursor is dispersed in the oil phase to form a stable suspension (avoiding agglomeration); place the reaction vessel in a constant temperature environment of 65°C and keep it at a constant temperature until the polymerization is complete to form a reaction product containing wollastonite modified hydrogel particles. S13. Purification and drying of the hydrogel core layer: Filtration and separation: After the polymerization reaction is completed, the reaction product is taken out and the solid product (wollastonite modified hydrogel particles) is separated by filtration equipment to remove the oil phase and unreacted impurities; Washing and purification: Wash the filtered solid product with an appropriate amount of solvent (such as deionized water or a cleaning agent compatible with the oil phase), repeat the washing multiple times to ensure the removal of impurities such as residual oil phase, unreacted monomers, crosslinking agents and initiators; Drying treatment: The washed solid product is placed in an oven at 80°C for drying treatment (the drying time is not specified, but is based on the product reaching constant weight and complete removal of moisture). After drying, a nano-micron-sized wollastonite-modified hydrogel core layer is obtained.

[0030] Further, the preparation of nano-microscale wollastonite in step S11 includes: selecting natural wollastonite as raw material and matching it with a specific grinding media; the grinding media has a gradation of 5mm:3mm:1mm = 2:4:4 mass ratio (i.e., the grinding media is composed of grinding media with particle sizes of 5mm, 3mm, and 1mm mixed in a mass ratio of 2:4:4); controlling the mass ratio of natural wollastonite to grinding media to be 1:3.5; and performing ball milling using a dry milling method, setting the ball milling time to 1 hour, thereby pulverizing the natural wollastonite particles to the nano-microscale level, finally obtaining nano-microscale wollastonite powder, which is then stored for later use in the subsequent modification and preparation of the hydrogel core layer; Furthermore, the osmotic pressure of the calcium chloride solution in step S2 is higher than that of the sodium alginate solution in step S3. Under the action of osmotic pressure, the calcium ions adsorbed in the hydrogel core layer gradually precipitate out and undergo cross-linking polymerization reaction with the sodium alginate monomer in the sodium alginate solution and the sodium alginate in the hydrogel core layer. Example 1 (Preparation of a core-shell structure modified bilayer hydrogel self-healing material based on sodium alginate-acrylamide copolymer) The raw material ratio is as follows: Polymer monomers: Acrylamide 16wt%, sodium alginate 4wt%; Nano-micron grade wollastonite: 25wt% (prepared by ball milling process, with a mass ratio of natural wollastonite to grinding media of 1:3.5, and a grinding media gradation of 5mm:3mm:1mm = 2:4:4, dry milling for 1 hour); Crosslinking agent: N,N'-methylenebisacrylamide (BIS) 0.05 wt%; Initiator: Ammonium persulfate (APS) 0.15 wt%; Solvent: Deionized water (to bring the total to 100 wt%) Pretreatment reagent: 0.5wt% calcium chloride solution; Shell material: 2wt% sodium alginate solution; Preparation steps include: S1: Preparation of Wollastonite-modified hydrogel core S11: Gradually add the preset amounts of sodium alginate and acrylamide to deionized water and stir until completely dissolved; then add nano-micro wollastonite powder, crosslinking agent BIS, and initiator APS, turn on the magnetic stirrer, and stir continuously at a rate of 500 rpm until a homogeneous and stable mixed solution is formed to obtain the polymerization precursor; S12: Using n-hexane as the oil phase, the above polymerization precursor is slowly dripped into the oil phase while the ultrasonic equipment is turned on and the stirring speed is increased to 800 rpm to ensure that the polymerization precursor is uniformly dispersed in the oil phase and forms a stable suspension. The suspension is then placed in a constant temperature environment of 65°C and the reaction is maintained until polymerization is complete. S13: After the polymerization reaction is completed, the solid product is separated by filtration, washed repeatedly with deionized water to remove residual oil phase, unreacted monomers and additives; the washed solid product is dried in an 80℃ oven to constant weight to obtain the wollastonite-modified hydrogel core; S2: Preparation of core-shell structured bilayer hydrogel: The wollastonite-modified hydrogel core prepared in step S1 is placed in a 0.5wt% calcium chloride solution and soaked for 0.5 to 1.5 hours to allow sufficient calcium ions to be adsorbed inside the hydrogel core until the calcium ion storage reaches a stable absorption-release equilibrium state. S3: Remove the soaked hydrogel core, drain the excess solution from the surface, and place it in a 2wt% sodium alginate solution. Let it stand for a period of time. Under the action of osmotic pressure, calcium ions in the hydrogel core slowly precipitate out and undergo in-situ cross-linking polymerization with sodium alginate monomers in the solution and sodium alginate on the surface of the hydrogel core layer. Gradually, a continuous and dense shell gel is formed on the surface of the hydrogel core, and finally, a core-shell structure modified bilayer hydrogel self-healing material is obtained.

[0031] Performance test results The hydrogel self-healing material prepared in Example 1 has a tensile strength of 1.1 MPa. When used to repair a 0.2 mm wide crack in a cement-based material, the self-healing rate of the crack reached 90% after curing in a NaOH solution simulating a highly alkaline environment of cement-based materials for 28 days.

[0032] Example 2 (Preparation of a core-shell structure modified bilayer hydrogel self-healing material based on calcium alginate-acrylamide-sodium acrylate copolymer) The raw material ratio is as follows: Polymer monomers: acrylic acid (15wt%), acrylamide (5wt%) Neutralizing agent: Sodium hydroxide (5.83 wt%, used to neutralize part of the acrylic acid to form sodium acrylate) Nano-micron wollastonite: 25 wt% (preparation process same as in Example 1) Crosslinking agent: N,N'-methylenebisacrylamide (BIS) 0.05wt% Initiator: Ammonium persulfate (APS) 0.15wt% Solvent: Deionized water (to bring the total to 100 wt%) Pretreatment reagent: 0.5wt% calcium chloride solution Shell material: 2wt% sodium alginate solution Preparation steps: S1: Preparation of wollastonite-modified hydrogel core: S11: Add acrylic acid to deionized water and stir until homogeneous. Then, slowly add sodium hydroxide powder and continue stirring until completely dissolved to achieve partial neutralization of the acrylic acid, generating sodium acrylate. Subsequently, add acrylamide, nano-micron wollastonite powder, crosslinking agent BIS, and initiator APS, and stir thoroughly until a homogeneous mixed solution is formed. S12: Using the reverse suspension method, the above mixed solution is added as the aqueous phase to the preset oil phase, and a stable reverse suspension is formed under ultrasonic assistance and stirring, and the polymerization reaction is initiated at 65°C; S13: After polymerization, the solid product is filtered, washed, and dried in an 80℃ oven to constant weight to obtain a spherical wollastonite-modified hydrogel core. S2: Preparation of core-shell structured bilayer hydrogels The process is consistent with the "Preparation of Core-Shell Bilayer Hydrogel" in Example 1, namely, calcium ions are adsorbed by soaking in 0.5wt% calcium chloride solution and cross-linked in situ in 2wt% sodium alginate solution to form a shell, and finally the target product is obtained.

[0033] Performance test results The hydrogel self-healing material prepared in Example 2 has a tensile strength of 1.4 MPa; when used to repair a 0.2 mm wide crack in a cement-based material, the self-healing rate of the crack reached 85% after curing in NaOH solution for 28 days. Comparative Example 1 (Unmodified monolayer SAP hydrogel (without wollastonite modification + without shell structure)) Raw material ratio: Polymer monomers: Acrylamide (16 wt%), sodium alginate (4 wt%) (consistent with the core layer monomer ratio in Example 1) Crosslinking agent: N,N'-methylenebisacrylamide (BIS) 0.05wt% Initiator: Ammonium persulfate (APS) 0.15wt% Solvent: Deionized water (to bring the total to 100 wt%) Preparation steps: Using the same suspension polymerization method as in "Preparation of Wollastonite Modified Hydrogel Core" in Example 1, only the "nano-micron Wollastonite" component was removed, while the other operating parameters (stirring rate, polymerization temperature, drying conditions, etc.) remained the same, an unmodified single-layer superabsorbent polymer (SAP) hydrogel was prepared.

[0034] Performance test results of the hydrogel material prepared in Comparative Example 1 Mechanical stability: Under the shear force of simulated concrete mixing (stirring at 800 rpm for 30 min), the hydrogel structure has a failure rate of 60% and cannot maintain its integrity. Tensile strength: only 0.3 MPa, far lower than 1.1 MPa in Example 1; Self-healing properties: When used to repair a 0.2mm wide crack, the self-healing rate was only 45% after curing in NaOH solution for 28 days. Environmental stability: When directly exposed to NaOH solution, the hydrogel swells and disintegrates after 7 days, losing its healing function.

[0035] Comparative Example 2 (Wollastonite-modified monolayer hydrogel (Wollastonite modification + shell-less structure)) Raw material ratio: The raw material ratio is exactly the same as that of the "wollastonite-modified hydrogel core" in Example 1 (containing 25wt% nano-microscale wollastonite). Preparation steps: Only the wollastonite-modified hydrogel core is prepared, without the subsequent "calcium chloride solution soaking" and "sodium alginate solution shell polymerization" steps, to obtain a wollastonite-modified monolayer hydrogel.

[0036] Performance test results of the hydrogel material prepared in Comparative Example 2: Mechanical stability: Under simulated concrete mixing conditions, the structural failure rate was 20%, and the mechanical properties were better than those of Comparative Example 1, but there was no shell protection. Tensile strength: 0.8 MPa, lower than 1.1 MPa in Example 1; Self-healing performance: After repairing a 0.2mm wide crack and curing in NaOH solution for 28 days, the self-healing rate was 65%. Environmental stability: After soaking in NaOH solution for 14 days, the surface showed swelling and aging, and the healing activity decreased by 30% (due to the lack of a shell to shield the highly alkaline environment).

[0037] Comparative Example 3: Unmodified core-shell hydrogel (without wollastonite modification + with shell structure) Raw material ratio: Core layer raw materials: acrylamide (16wt%), sodium alginate (4wt%), crosslinking agent BIS 0.05wt%, initiator APS 0.15wt%, deionized water (to bring to 100wt%), no nano- or micro-sized wollastonite; Shell material: 0.5wt% calcium chloride solution, 2wt% sodium alginate solution (same as in Example 1); Preparation steps: The core layer preparation process involved removing nano- and micro-sized wollastonite, while the remaining core layer preparation and shell formation steps were completely consistent with those in Example 1, resulting in an unmodified core-shell structured hydrogel.

[0038] Performance test results of the hydrogel material prepared in Comparative Example 3 Mechanical stability: Under simulated concrete mixing conditions, the structural failure rate is 35% (the shell provides some protection, but the core layer is not reinforced with wollastonite). Tensile strength: 0.6 MPa, lower than 1.1 MPa in Example 1; Self-healing performance: After repairing a 0.2mm wide crack and curing in NaOH solution for 28 days, the self-healing rate was 55%. Environmental stability: The shell can delay the erosion of a highly alkaline environment, and the hydrogel maintains structural integrity after 14 days, but the core layer is not reinforced by wollastonite and has a lower healing efficiency.

[0039] Comparative Example 4: SAP hydrogel loaded with traditional chemical healing agent Raw material ratio: Conventional SAP hydrogel (without wollastonite modification, without shell structure); Loading healing agent: calcium carbonate powder (10% of the SAP mass); Preparation steps: Calcium carbonate powder was loaded into conventional SAP hydrogel using an immersion adsorption method to obtain a loaded healing hydrogel. Performance test results of the hydrogel material prepared in Comparative Example 4 Healing agent release: In the simulation of concrete mixing, 30% of calcium carbonate is released in advance, and the "ammunition is exhausted" when cracks occur; Self-healing performance: After repairing a 0.2mm wide crack and curing for 28 days, the self-healing rate is 50%. Mechanical stability: Under simulated stirring conditions, the structural failure rate reaches 50%, and the loaded healing agent is prone to leakage.

[0040] The embodiments of the present invention significantly improve the mechanical strength, environmental stability and self-healing efficiency of hydrogel through the synergistic effect of "nano-micron-level wollastonite modified core layer + calcium ion slow-release in-situ cross-linked shell layer", and its performance is superior to that of the comparative examples. Wollastonite modification is key to improving the mechanical stability of hydrogels (Comparative Example 1 and Comparative Examples 3 and 4), and the shell structure is the core to ensure chemical stability and on-demand release of healing agents in highly alkaline environments (Comparative Example 1 and Comparative Examples 1 and 2). Traditional single-layer SAPs or load-bearing SAPs suffer from defects such as poor mechanical properties, premature release of healing agents, and weak environmental adaptability. The core-shell modified structure of this invention can effectively solve the above-mentioned technical pain points.

[0041] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A core-shell structured modified bilayer hydrogel self-healing material, characterized in that, It includes a hydrogel core layer and a shell hydrogel. The hydrogel core layer is a nano-micro wollastonite-modified hydrogel, which, by weight, includes 75-80 parts of hydrogel matrix and 20-25 parts of nano-micro wollastonite, and is prepared by reverse suspension method. The shell hydrogel is formed by in-situ crosslinking with calcium ions through slow release and is coated on the surface of the hydrogel core layer.

2. The method for preparing a core-shell structure modified bilayer hydrogel self-healing material according to claim 1, characterized in that: The hydrogel matrix, by weight, comprises 25-30 parts monomer, 0.03-0.05 parts crosslinking agent, 0.2-0.5 parts initiator, and the remainder is solvent; The monomer is a combination of at least two of acrylic acid, acrylamide and sodium alginate; The crosslinking agent is N,N'-methylenebisacrylamide; The initiator is ammonium persulfate or potassium persulfate; The solvent is deionized water.

3. The method for preparing a core-shell structured modified bilayer hydrogel self-healing material according to claim 2, characterized in that: When the monomer is a combination of acrylic acid and sodium alginate, the proportions of the two in the total weight of the monomer are 80-85 parts acrylic acid and 15-20 parts sodium alginate.

4. The method for preparing a core-shell structure modified bilayer hydrogel self-healing material according to claim 3, characterized in that: The nano- and micro-sized wollastonite was prepared by ball milling. The specific preparation parameters were as follows: the mass ratio of natural wollastonite to the grinding media was 1:3.5, and the dry milling time was 1 hour; the gradation of the grinding media was 5 mm: 3 mm: 1 mm = 2: 4:

4.

5. The method for preparing a core-shell structured modified bilayer hydrogel self-healing material according to claim 4, characterized in that: The monomer of the shell hydrogel is sodium alginate; the sodium alginate cross-links and polymerizes with calcium ions seeping from the core layer of the hydrogel to form the shell hydrogel, and the carboxyl groups on the sodium alginate chain form an "egg box" structure with Ca2+.

6. The method for preparing a core-shell structure modified bilayer hydrogel self-healing material according to claim 5, characterized in that: The shell hydrogel plays a shielding and protective role in the high alkalinity and high ionic strength environment of cement-based materials, preventing premature deactivation of the hydrogel core layer. It can also rupture or swell when cracks occur or external moisture invades, triggering the swelling, filling and healing of the hydrogel core layer.

7. The method for preparing a core-shell structured modified bilayer hydrogel self-healing material according to claim 6, characterized in that: The nano- and micro-sized wollastonite has a needle-like and fibrous crystal structure, which can form a synergistic enhancement effect with the hydrogel matrix to improve the mechanical strength of the hydrogel. It can also hydrolyze in the crack environment to generate silicate products, which form a synergistic healing effect with the swelling and filling effect of the hydrogel core layer, thereby improving the density of crack repair.

8. A method for preparing a core-shell structured modified bilayer hydrogel self-healing material, characterized in that, The preparation of a core-shell structure-modified bilayer hydrogel self-healing material as described in any one of claims 1-7 comprises the following steps: S1: Preparation of nano- and micro-scale wollastonite-modified hydrogel core layers; S2: Place the hydrogel core layer prepared in step S1 into a 0.5wt% calcium chloride solution and soak for 0.5–1.5 h to allow the hydrogel core layer to adsorb sufficient calcium ions and reach an adsorption-release equilibrium. S3: The hydrogel core layer soaked in step S2 is placed in a 2wt% sodium alginate solution. Calcium ions are slowly exuded by osmotic pressure and crosslinked in situ with sodium alginate to form a shell hydrogel, ultimately obtaining a core-shell structure modified bilayer hydrogel self-healing material.

9. The method for preparing a core-shell structured modified bilayer hydrogel self-healing material according to claim 8, characterized in that, Step S1 involves preparing a nano- or micro-sized wollastonite-modified hydrogel core layer, specifically including: S11. Prepare nano-micro wollastonite: Mix 75-80 parts by weight of hydrogel matrix and 20-25 parts by weight of nano-micro wollastonite; add 25-30 parts by weight of monomer of hydrogel matrix, 0.03-0.05 parts by weight of crosslinking agent and 0.2-0.5 parts by weight of initiator to deionized water; then add 20-25 parts by weight of the pre-prepared nano-micro wollastonite powder, turn on the magnetic stirrer and stir continuously at a speed of 500 rpm until a homogeneous and stable mixed solution is formed, thus obtaining the polymerization precursor; S12. Slowly drip the polymerization precursor into the oil phase, while turning on the ultrasonic equipment and controlling the stirring rate to 800 rpm, so that the polymerization precursor is uniformly dispersed in the oil phase to form a stable suspension; place the reaction vessel in a constant temperature environment of 65℃ and keep it at a constant temperature until the polymerization is complete to form a reaction product containing wollastonite modified hydrogel particles. S13. After the polymerization reaction is completed, the solid product is separated by filtration to remove the oil phase and unreacted impurities. The solid product is repeatedly washed with an appropriate amount of solvent to remove residual impurities. Then, it is dried in an 80°C oven to constant weight to obtain a nano-microscale wollastonite-modified hydrogel core layer.

10. The method for preparing a core-shell structured modified bilayer hydrogel self-healing material according to claim 9, characterized in that, The preparation of nano-microscale wollastonite in step S11 includes: selecting natural wollastonite as raw material and matching it with a specific grinding media; the grinding media has a mass ratio of 5mm:3mm:1mm = 2:4:4; controlling the mass ratio of natural wollastonite to grinding media to be 1:3.5; and performing ball milling by dry milling for 1 hour. Through this ball milling process, the natural wollastonite particles are pulverized to the nano-microscale level, and finally nano-microscale wollastonite powder is obtained.

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