A constant-temperature self-heating body, a preparation method and application thereof

Abstract

This invention relates to the field of self-heating product technology, specifically disclosing a constant-temperature self-heating body, its preparation method, and its application. The Fe@CuO microparticles used can rapidly heat up in the initial stage of the reaction, and the electrolyte hydrogel microspheres regulate the release of electrolytes and water according to temperature. The biochar aerogel, with its abundant hierarchical pores, provides an efficient diffusion path for oxygen, enabling it to transport oxygen into the heating body, resulting in a uniform temperature distribution and preventing localized overheating. In addition, the biochar aerogel is loaded with the phase change material paraffin, which prolongs the time for the user to go from comfortable to excessively cold or hot, thus improving the thermal comfort of the constant-temperature self-heating body.

Description

Technical Field

[0001] This invention relates to the field of self-heating product technology, specifically to a constant-temperature self-heating element, its preparation method, and its application. Background Technology

[0002] The self-heating element is the core functional unit of self-heating products. Its principle is to convert chemical energy into heat energy through a redox reaction. This technology mainly relies on metal powders (such as iron, zinc, and aluminum) reacting with oxygen in the presence of an electrolyte to generate corresponding metal oxides and release heat; essentially, it is an electrochemical corrosion process. Currently, the commonly used self-heating system is the iron-oxygen system. Its preparation process involves uniformly mixing powdered raw materials such as iron powder, activated carbon, and salt, and then wrapping them with non-woven fabric to form the self-heating element.

[0003] Currently, self-heating elements generally suffer from several problems. For example, iron powder in the heating element may clump, lack oxygen, or have poor contact, leading to incomplete reaction, resulting in low heating efficiency and material waste. Insufficient control of moisture and oxygen during the heating process also results in issues such as heating peaks, poor heating duration, and instability. Therefore, designing a constant-temperature self-heating element capable of continuously and stably releasing heat is crucial for promoting the development of self-heating products towards higher safety and reliability. Summary of the Invention

[0004] The purpose of this invention is to provide a constant-temperature self-heating element, its preparation method, and its application, which has a long heating time, good constant temperature effect, and stable heat release, thus solving the problems of fast heating rate, poor heating time, and poor stability of heating elements.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A method for preparing a constant-temperature self-heating element, specifically: Step 1: Add reduced iron powder to copper sulfate solution to coat the surface of reduced iron powder with copper oxide layer to form Fe@CuO particles; Step 2: Using sodium chloride and sodium acetate as electrolytes, add isopropylacrylamide monomer for cross-linking to form electrolyte hydrogel microspheres; Step 3: Mix straw with ammonium bicarbonate and pyrolyze to form multi-level pore biochar, then mix with cellulose nanofiber suspension, freeze dry to form biochar aerogel, pulverize to obtain biochar aerogel particles; Step 4: Using negative pressure adsorption, paraffin is adsorbed into biochar aerogel to form paraffin-loaded aerogel particles. Step 5: Mix 200-mesh reduced iron powder, Fe@CuO microparticles, electrolyte hydrogel microspheres, biochar aerogel microparticles, loaded paraffin aerogel microparticles, vermiculite and activated carbon to obtain an iron-based heating agent. Place the iron-based heating agent between two layers of non-woven fabric, and finally seal the edges and package it to obtain a constant temperature self-heating body.

[0006] As a limitation of the present invention, the preparation method of the Fe@CuO particles is as follows: Reduced iron powder was added to copper sulfate solution and stirred until homogeneous. Then, sodium dodecylbenzenesulfonate solution was added and magnetically stirred at 200-300 rpm for 5-10 min. After heating in a water bath at 60-70℃, sodium hydroxide solution was added and stirred at 200-300 rpm for 30-60 min. After the reaction was completed, ammonium persulfate solution was added and reacted at 60-70℃ and 200-300 rpm for 1-1.5 h. After the reaction was completed, the mixture was cooled, filtered, washed with deionized water, and vacuum dried at 60-70℃ for 4-6 h to obtain Fe@CuO particles.

[0007] As a limitation of the present invention, the mass ratio of the reduced iron powder, copper sulfate, ammonium persulfate, and sodium dodecylbenzenesulfonate is (8-10):(10-13):(2.5-3):(0.03-0.05); the sodium dodecylbenzenesulfonate solution contains 1.5-2 wt% sodium dodecylbenzenesulfonate; the copper sulfate solution contains 4-6 wt% copper sulfate; the sodium hydroxide solution contains 4-6 wt% sodium hydroxide; and the ammonium persulfate solution contains 4-6 wt% ammonium persulfate.

[0008] When reduced iron powder is added to a copper sulfate solution, under alkaline conditions, copper ions form copper hydroxide precipitate on the surface of the iron powder particles. Then, under the action of ammonium persulfate, the copper hydroxide precipitate on the surface of the iron powder dehydrates to form more stable copper oxide, forming Fe@CuO core-shell particles. In the electrolyte solution, the Fe@CuO core-shell particles form a tiny galvanic cell system, which accelerates the electrochemical corrosion rate of the iron powder, enabling the self-heating body to heat up rapidly and increasing the heating rate of the self-heating body. In addition, the Fe@CuO core-shell particles can prevent the oxides generated during the oxidation of iron powder from covering the surface and forming a passivation layer, which would lead to incomplete oxidation of iron powder and low heating efficiency of the self-heating body.

[0009] As a limitation of the present invention, the preparation method of the electrolyte hydrogel microspheres is as follows: Sodium chloride and sodium acetate were added to deionized water and magnetically stirred at 200-300 rpm for 20-30 min at 40-50℃ to form an electrolyte solution. Isopropylacrylamide monomer and methylenebisacrylamide crosslinking agent were added to the electrolyte solution and magnetically stirred at 200-300 rpm for 30-40 min at 50-60℃. Ammonium persulfate initiator was added and the reaction was carried out at 200-300 rpm for 3-4 h under nitrogen protection at 70-80℃. After the reaction was completed, the mixture was filtered and vacuum dried at 40-50℃ for 6-8 h to obtain electrolyte hydrogel microspheres.

[0010] As a limitation of the present invention, the mass ratio of sodium chloride to sodium acetate is (10-14):(5-9); the mass ratio of isopropylacrylamide monomer, methylenebisacrylamide crosslinking agent and ammonium persulfate initiator is (45-55):(0.4-0.6):(0.4-0.6).

[0011] Isopropylacrylamide monomers crosslink in an electrolyte solution to form hydrogel microspheres, transforming liquid water into "solid water." This maintains the porosity inside the self-heating element, ensuring oxygen diffusion within the element. The microspheres contain electrolytes, which diffuse to the outside of the hydrogel microspheres during the initial heating phase. After the self-heating element has reacted for a period of time, and the temperature rises above the LCST temperature of the hydrogel, the polymer chains on the surface of the hydrogel microspheres dehydrate and entangle to form an outer shell. This shell hinders the diffusion of internal electrolytes and water outward, thereby reducing the heating rate and prolonging the heating time of the self-heating element. It also prevents overheating of the element, which could cause safety issues.

[0012] As a limitation of the present invention, the preparation method of the biochar aerogel particles is as follows: After crushing the straw, mix it with ammonium bicarbonate and ball mill it thoroughly at 200-300 rpm for 20-30 min until it is uniformly mixed. Under nitrogen protection, heat it to 600-620℃ at a heating rate of 10-15℃ / min and keep it at that temperature for 1-3 h. After the temperature is maintained, cool it to room temperature, wash it with deionized water, and vacuum dry it at 60-70℃ for 6-8 h to obtain multi-level pore biochar. Add cellulose nanofibers to deionized water, stir magnetically at 400-500 rpm for 40-60 min, and ultrasonically disperse it for 20-30 min. Add multi-level pore biochar and stir magnetically at 200-300 rpm for 90-120 min to form a biochar slurry. Then freeze-dry it at (-20)-(-30)℃ for 24-30 h to obtain biochar aerogel. Crush the biochar aerogel and sieve it to obtain biochar aerogel microparticles.

[0013] As a limitation of the present invention, the mass ratio of straw to ammonium bicarbonate is 1:(1.2-1.5), and the biochar slurry contains 1.5-3.5 wt% multi-level pore size biochar and 1.5-3.5 wt% cellulose nanofibers.

[0014] Under nitrogen protection, straw decomposes upon heating to form biochar. Ammonium bicarbonate acts as a pore-forming agent, and the gases produced during decomposition (ammonia, carbon dioxide, etc.) etch and scour the biochar framework, creating a rich, hierarchical porous structure within the biochar. This structure possesses a high specific surface area and well-developed channels, providing an environment for the redox reaction of iron powder. The hierarchical biochar is then added to a cellulose nanofiber liquid, where the cellulose nanofibers form a three-dimensional porous framework, creating an aerogel structure. Biochar itself has a good oxygen adsorption capacity; further processing into aerogel enhances this adsorption capacity due to its large specific surface area, providing more oxygen support for the heating reaction. Additionally, the low thermal conductivity of aerogel allows it to act as a barrier in self-heating bodies, reducing heat loss from the environment and improving the thermal efficiency of the self-heating body.

[0015] As a limitation of the present invention, in the paraffin-loaded biochar aerogel particles, the mass ratio of biochar aerogel to paraffin is (7-8):(8-9), the pressure during negative pressure adsorption is (-0.05)-(-0.07) MPa, and the adsorption time is 20-30 min.

[0016] By utilizing negative pressure adsorption, the phase change material paraffin is stored in an aerogel structure. When the heating temperature reaches the melting point of the paraffin, the paraffin melts and absorbs latent heat, preventing the heating element from overheating and posing a safety risk to the user. At the end of the heating process, the melted paraffin solidifies and releases the absorbed latent heat, delaying the temperature drop of the heating element and extending the heating time. This macroscopically controls the entire heating process of the heating element. The complex structure of the biochar aerogel particles can effectively reduce the outflow of molten paraffin, preventing the molten paraffin from blocking the oxygen transport channels and causing a decrease in the heating efficiency of the self-heating element.

[0017] As a limitation of the present invention, the iron-based heating agent comprises, by weight, 42-77 parts of 200-mesh reduced iron powder, 3-4 parts of Fe@CuO microparticles, 3-10 parts of electrolyte hydrogel microspheres, 3-8 parts of biochar aerogel microparticles, 2-6 parts of paraffin-loaded aerogel microparticles, 2-10 parts of vermiculite, and 10-20 parts of activated carbon.

[0018] A constant-temperature self-heating body prepared by any of the above preparation methods is used in the preparation of self-heating eye masks and self-heating moxibustion patches.

[0019] Compared with the prior art, the beneficial effects of the present invention are: This invention incorporates Fe@CuO core-shell microparticles into the self-heating element as an initiator, which reacts rapidly, causing the element to heat up quickly. Reduced iron powder, as the main heating material, continuously undergoes redox reactions, releasing heat. Electrolyte hydrogel microspheres slowly release electrolytes, extending the heating time of the self-heating element while preventing overheating and safety issues. Biochar aerogel, with its abundant hierarchical pores, provides an efficient diffusion path for oxygen, delivering it into the heating element and ensuring uniform temperature distribution, preventing localized overheating. Furthermore, the aerogel is also a good insulation material; when loaded with paraffin, the phase change of the paraffin regulates the temperature of the self-heating element, extending its heating time and providing a more comfortable experience for the user. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. The terminology used in the embodiments is for describing specific implementation schemes, not for limiting the scope of protection of the present invention. The dosages in the embodiments are laboratory-scale tests and can be scaled up proportionally. 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.

[0021] Reduced iron powder (particle size: 80 mesh, 200 mesh, reduction degree > 97%), non-woven fabric (material: PET, thickness: 0.3mm, weight: 45g / m²) 2 Vermiculite (specific surface area: 1.5m³) 2 / g), activated carbon (particle size: 200 mesh, ash content ≤5%).

[0022] Example 1: A method for preparing a constant-temperature self-heating element, specifically as follows: Step 1: Add sodium dodecylbenzenesulfonate to deionized water and stir until homogeneous to form a 1.6 wt% sodium dodecylbenzenesulfonate solution. Add 10 g of 80 mesh reduced iron powder to 200 mL of 5 wt% copper sulfate solution and stir until homogeneous. Then add 2.5 mL of the 1.6 wt% sodium dodecylbenzenesulfonate solution and stir magnetically at 300 rpm for 5 min. Then, under a 60°C water bath heating condition, add 50 mL of 5 wt% sodium hydroxide solution and stir at 300 rpm for 30 min. After the reaction is complete, add 50 mL of 5 wt% ammonium persulfate solution and react at 60°C and 300 rpm for 1 h. After the reaction is complete, cool, filter, wash with deionized water, and vacuum dry at 60°C for 4 h to obtain Fe@CuO particles. Step 2: Add 10g of sodium chloride and 5g of sodium acetate to 100mL of deionized water. Stir magnetically at 40℃ and 200rpm for 30min to form an electrolyte solution. Add 5g of isopropylacrylamide monomer and 0.05g of methylenebisacrylamide crosslinking agent to the electrolyte solution. Stir magnetically at 50℃ and 200rpm for 30min. Add 0.05g of ammonium persulfate initiator. React at 70℃ and 300rpm for 4h under nitrogen protection. After the reaction is complete, filter and vacuum dry at 40℃ for 8h to obtain electrolyte hydrogel microspheres. Step 3: Crush 100g of straw to 40 mesh, mix with 150g of ammonium bicarbonate, and ball mill at 300rpm for 20min until uniform. Under nitrogen protection, heat to 600℃ at a rate of 10℃ / min and hold at 600℃ for 2h. After holding, cool to room temperature, wash with deionized water, and vacuum dry at 60℃ for 8h to obtain multi-level pore biochar. Add 10g of cellulose nanofibers to 490mL of deionized water, stir magnetically at 500rpm for 60min, and ultrasonically disperse for 30min. Add 10g of multi-level pore biochar and stir magnetically at 300rpm for 120min to form a biochar slurry. Then freeze-dry at -20℃ for 24h to obtain biochar aerogel. Crush the biochar aerogel and pass it through an 80-mesh sieve to obtain biochar aerogel microparticles. Step 4: Heat 80g of paraffin to 60℃, add 80g of biochar aerogel, adsorb under -0.07MPa negative pressure for 30min, cool to room temperature, pulverize, and pass through an 80-mesh sieve to obtain paraffin-loaded aerogel microparticles. Step 5: Add 420g of 200-mesh reduced iron powder, 40g of Fe@CuO microparticles, 100g of electrolyte hydrogel microspheres, 80g of biochar aerogel microparticles, 60g of loaded paraffin aerogel microparticles, 100g of vermiculite, and 200g of activated carbon to a mixer in sequence, mix at 150 rpm for 10 minutes to obtain an iron-based heating agent. Discharge the iron-based heating agent through a hopper into the middle of two layers of non-woven fabric, and finally seal the edges and package it to obtain a constant temperature self-heating body.

[0023] Example 2: A method for preparing a constant-temperature self-heating element, specifically as follows: Step 1: Add sodium dodecylbenzenesulfonate to deionized water and stir until homogeneous to form a 1.6 wt% sodium dodecylbenzenesulfonate solution. Add 10 g of 80 mesh reduced iron powder to 200 mL of 5 wt% copper sulfate solution and stir until homogeneous. Then add 2.5 mL of the 1.6 wt% sodium dodecylbenzenesulfonate solution and stir magnetically at 300 rpm for 5 min. Then, under a 60°C water bath heating condition, add 50 mL of 5 wt% sodium hydroxide solution and stir at 300 rpm for 30 min. After the reaction is complete, add 50 mL of 5.5 wt% ammonium persulfate solution and react at 60°C and 300 rpm for 1 h. After the reaction is complete, cool, filter, wash with deionized water, and vacuum dry at 60°C for 4 h to obtain Fe@CuO particles. Step 2: Add 12g of sodium chloride and 7g of sodium acetate to 100mL of deionized water. Stir magnetically at 40℃ and 200rpm for 30min to form an electrolyte solution. Add 5g of isopropylacrylamide monomer and 0.05g of methylenebisacrylamide crosslinking agent to the electrolyte solution. Stir magnetically at 50℃ and 200rpm for 30min. Add 0.05g of ammonium persulfate initiator. React at 70℃ and 300rpm for 4h under nitrogen protection. After the reaction is complete, filter and vacuum dry at 40℃ for 8h to obtain electrolyte hydrogel microspheres. Step 3: Crush 100g of straw to 40 mesh, mix with 150g of ammonium bicarbonate, and ball mill at 300rpm for 20min until uniform. Under nitrogen protection, heat to 600℃ at a rate of 10℃ / min and hold at 600℃ for 2h. After holding, cool to room temperature, wash with deionized water, and vacuum dry at 60℃ for 8h to obtain multi-level pore biochar. Add 10g of cellulose nanofibers to 490mL of deionized water, stir magnetically at 500rpm for 60min, and ultrasonically disperse for 30min. Add 12g of multi-level pore biochar and stir magnetically at 300rpm for 120min to form a biochar slurry. Then freeze-dry at -20℃ for 24h to obtain biochar aerogel. Crush the biochar aerogel and pass it through an 80-mesh sieve to obtain biochar aerogel microparticles. Step 4: Heat 80g of paraffin to 60℃, add 70g of biochar aerogel, adsorb under -0.07MPa negative pressure for 30min, cool to room temperature, pulverize, and pass through an 80-mesh sieve to obtain paraffin-loaded aerogel microparticles. Step 5: Add 450g of 200-mesh reduced iron powder, 40g of Fe@CuO microparticles, 90g of electrolyte hydrogel microspheres, 70g of biochar aerogel microparticles, 50g of paraffin-loaded aerogel microparticles, 100g of vermiculite, and 200g of activated carbon to a mixer in sequence, mix at 150 rpm for 10 minutes to obtain an iron-based heating agent. Discharge the iron-based heating agent through a hopper between two layers of non-woven fabric, and finally seal the edges and package it to obtain a constant temperature self-heating body.

[0024] Example 3: A method for preparing a constant-temperature self-heating element, specifically as follows: Step 1: Add sodium dodecylbenzenesulfonate to deionized water and stir until homogeneous to form a 1.6 wt% sodium dodecylbenzenesulfonate solution. Add 10 g of 80 mesh reduced iron powder to 200 mL of 5 wt% copper sulfate solution and stir until homogeneous. Then add 2.5 mL of the 1.6 wt% sodium dodecylbenzenesulfonate solution and stir magnetically at 300 rpm for 5 min. Then, under a 60°C water bath heating condition, add 50 mL of 6 wt% sodium hydroxide solution and stir at 300 rpm for 30 min. After the reaction is complete, add 50 mL of 6 wt% ammonium persulfate solution and react at 60°C and 300 rpm for 1 h. After the reaction is complete, cool, filter, wash with deionized water, and vacuum dry at 60°C for 4 h to obtain Fe@CuO particles. Step 2: Add 14g of sodium chloride and 9g of sodium acetate to 100mL of deionized water. Stir magnetically at 40℃ and 200rpm for 30min to form an electrolyte solution. Add 5g of isopropylacrylamide monomer and 0.05g of methylenebisacrylamide crosslinking agent to the electrolyte solution. Stir magnetically at 50℃ and 200rpm for 30min. Add 0.05g of ammonium persulfate initiator. React at 70℃ and 300rpm for 4h under nitrogen protection. After the reaction is complete, filter and vacuum dry at 40℃ for 8h to obtain electrolyte hydrogel microspheres. Step 3: Crush 100g of straw to 40 mesh, mix with 150g of ammonium bicarbonate, and ball mill at 300rpm for 20min until uniform. Under nitrogen protection, heat to 600℃ at a rate of 10℃ / min and hold at 600℃ for 2h. After holding, cool to room temperature, wash with deionized water, and vacuum dry at 60℃ for 8h to obtain multi-level pore biochar. Add 10g of cellulose nanofibers to 490mL of deionized water, stir magnetically at 500rpm for 60min, and ultrasonically disperse for 30min. Add 15g of multi-level pore biochar and stir magnetically at 300rpm for 120min to form a biochar slurry. Then freeze-dry at -20℃ for 24h to obtain biochar aerogel. Crush the biochar aerogel and pass it through an 80-mesh sieve to obtain biochar aerogel microparticles. Step 4: Heat 90g of paraffin to 60℃, add 70g of biochar aerogel, adsorb under -0.07MPa negative pressure for 30min, cool to room temperature, pulverize, and pass through an 80-mesh sieve to obtain paraffin-loaded aerogel microparticles. Step 5: Add 480g of 200-mesh reduced iron powder, 40g of Fe@CuO microparticles, 80g of electrolyte hydrogel microspheres, 60g of biochar aerogel microparticles, 40g of loaded paraffin aerogel microparticles, 100g of vermiculite, and 200g of activated carbon to a mixer in sequence, mix at 150 rpm for 10 minutes to obtain an iron-based heating agent. Discharge the iron-based heating agent through a hopper into the middle of two layers of non-woven fabric, and finally seal the edges and package it to obtain a constant temperature self-heating body.

[0025] Based on Example 1, the following comparative experiments were conducted, specifically Comparative Example 1, Comparative Example 2, and Comparative Example 3, as described below: Comparative Example 1: This comparative example relates to a method for preparing a constant-temperature self-heating element. The difference from Example 1 is that the reduced iron powder in the iron-based heating agent is not modified. Specifically: Step 1: Add 10g of sodium chloride and 5g of sodium acetate to 100mL of deionized water. Stir magnetically at 40℃ and 200rpm for 30min to form an electrolyte solution. Add 5g of isopropylacrylamide monomer and 0.05g of methylenebisacrylamide crosslinking agent to the electrolyte solution. Stir magnetically at 50℃ and 200rpm for 30min. Add 0.05g of ammonium persulfate initiator. React at 70℃ and 300rpm for 4h under nitrogen protection. After the reaction is complete, filter and vacuum dry at 40℃ for 8h to obtain electrolyte hydrogel microspheres. Step 2: Crush 100g of straw to 40 mesh, mix with 150g of ammonium bicarbonate, and ball mill at 300rpm for 20min until uniform. Under nitrogen protection, heat to 600℃ at a rate of 10℃ / min and hold at 600℃ for 2h. After holding, cool to room temperature, wash with deionized water, and vacuum dry at 60℃ for 8h to obtain multi-level pore biochar. Add 10g of cellulose nanofibers to 490mL of deionized water, stir magnetically at 500rpm for 60min, and ultrasonically disperse for 30min. Add 10g of multi-level pore biochar and stir magnetically at 300rpm for 120min to form a biochar slurry. Then freeze-dry at -20℃ for 24h to obtain biochar aerogel. Crush the biochar aerogel and pass it through an 80-mesh sieve to obtain biochar aerogel microparticles. Step 3: Heat 80g of paraffin to 60℃, add 80g of biochar aerogel, adsorb under -0.07MPa negative pressure for 30min, cool to room temperature, pulverize, and pass through an 80-mesh sieve to obtain paraffin-loaded aerogel microparticles. Step 4: Add 420g of 200-mesh reduced iron powder, 40g of 80-mesh reduced iron powder, 100g of electrolyte hydrogel microspheres, 80g of biochar aerogel microparticles, 60g of paraffin-loaded aerogel microparticles, 100g of vermiculite, and 200g of activated carbon to a mixer in sequence, mix at 150 rpm for 10 minutes to obtain an iron-based heating agent. Discharge the iron-based heating agent through a hopper between two layers of non-woven fabric, and finally seal the edges and package it to obtain a constant temperature self-heating body.

[0026] Comparative Example 2: This comparative example relates to a method for preparing a constant-temperature self-heating body. The difference from Example 1 is that no paraffin-loaded aerogel particles were added. Specifically: Step 1: Add sodium dodecylbenzenesulfonate to deionized water and stir until homogeneous to form a 1.6 wt% sodium dodecylbenzenesulfonate solution. Add 10 g of 80 mesh reduced iron powder to 200 mL of 5 wt% copper sulfate solution and stir until homogeneous. Then add 2.5 mL of the 1.6 wt% sodium dodecylbenzenesulfonate solution and stir magnetically at 300 rpm for 5 min. Then, under a 60°C water bath heating condition, add 50 mL of 5 wt% sodium hydroxide solution and stir at 300 rpm for 30 min. After the reaction is complete, add 50 mL of 5 wt% ammonium persulfate solution and react at 60°C and 300 rpm for 1 h. After the reaction is complete, cool, filter, wash with deionized water, and vacuum dry at 60°C for 4 h to obtain Fe@CuO particles. Step 2: Add 10g of sodium chloride and 5g of sodium acetate to 100mL of deionized water. Stir magnetically at 40℃ and 200rpm for 30min to form an electrolyte solution. Add 5g of isopropylacrylamide monomer and 0.05g of methylenebisacrylamide crosslinking agent to the electrolyte solution. Stir magnetically at 50℃ and 200rpm for 30min. Add 0.05g of ammonium persulfate initiator. React at 70℃ and 300rpm for 4h under nitrogen protection. After the reaction is complete, filter and vacuum dry at 40℃ for 8h to obtain electrolyte hydrogel microspheres. Step 3: Crush 100g of straw to 40 mesh, mix with 150g of ammonium bicarbonate, and ball mill at 300rpm for 20min until uniform. Under nitrogen protection, heat to 600℃ at a rate of 10℃ / min and hold at 600℃ for 2h. After holding, cool to room temperature, wash with deionized water, and vacuum dry at 60℃ for 8h to obtain multi-level pore biochar. Add 10g of cellulose nanofibers to 490mL of deionized water, stir magnetically at 500rpm for 60min, and ultrasonically disperse for 30min. Add 10g of multi-level pore biochar and stir magnetically at 300rpm for 120min to form a biochar slurry. Then freeze-dry at -20℃ for 24h to obtain biochar aerogel. Crush the biochar aerogel and pass it through an 80-mesh sieve to obtain biochar aerogel microparticles. Step 4: Add 420g of 200-mesh reduced iron powder, 40g of Fe@CuO microparticles, 100g of electrolyte hydrogel microspheres, 80g of biochar aerogel microparticles, 100g of vermiculite, and 200g of activated carbon to a mixer in sequence, mix at 150rpm for 10min to obtain an iron-based heating agent. Discharge the iron-based heating agent through a hopper into the middle of two layers of non-woven fabric, and finally seal the edges and package it to obtain a constant temperature self-heating body.

[0027] Comparative Example 3: This comparative example relates to a method for preparing a constant-temperature self-heating body. The difference from Example 1 is that the electrolyte is not coated with hydrogel microspheres. Specifically: Step 1: Add sodium dodecylbenzenesulfonate to deionized water and stir until homogeneous to form a 1.6 wt% sodium dodecylbenzenesulfonate solution. Add 10 g of 80 mesh reduced iron powder to 200 mL of 5 wt% copper sulfate solution and stir until homogeneous. Then add 2.5 mL of the 1.6 wt% sodium dodecylbenzenesulfonate solution and stir magnetically at 300 rpm for 5 min. Then, under a 60°C water bath heating condition, add 50 mL of 5 wt% sodium hydroxide solution and stir at 300 rpm for 30 min. After the reaction is complete, add 50 mL of 5 wt% ammonium persulfate solution and react at 60°C and 300 rpm for 1 h. After the reaction is complete, cool, filter, wash with deionized water, and vacuum dry at 60°C for 4 h to obtain Fe@CuO particles. Step 2: Crush 100g of straw to 40 mesh, mix with 150g of ammonium bicarbonate, and ball mill at 300rpm for 20min until uniform. Under nitrogen protection, heat to 600℃ at a rate of 10℃ / min and hold at 600℃ for 2h. After holding, cool to room temperature, wash with deionized water, and vacuum dry at 60℃ for 8h to obtain multi-level pore biochar. Add 10g of cellulose nanofibers to 490mL of deionized water, stir magnetically at 500rpm for 60min, and ultrasonically disperse for 30min. Add 10g of multi-level pore biochar and stir magnetically at 300rpm for 120min to form a biochar slurry. Then freeze-dry at -20℃ for 24h to obtain biochar aerogel. Crush the biochar aerogel and pass it through an 80-mesh sieve to obtain biochar aerogel microparticles. Step 3: Heat 80g of paraffin to 60℃, add 80g of biochar aerogel, adsorb under -0.07MPa negative pressure for 30min, cool to room temperature, pulverize, and pass through an 80-mesh sieve to obtain paraffin-loaded aerogel microparticles. Step 4: Add 420g of 200-mesh reduced iron powder, 40g of Fe@CuO microparticles, 65g of sodium chloride, 35g of sodium acetate, 80g of biochar aerogel microparticles, 60g of paraffin-loaded aerogel microparticles, 100g of vermiculite, and 200g of activated carbon to a mixer in sequence, mix at 150 rpm for 10 minutes to obtain an iron-based heating agent. Discharge the iron-based heating agent through a hopper between two layers of non-woven fabric, and finally seal the edges and package it to obtain a constant temperature self-heating body.

[0028] Testing experiment: Heating performance test: Test samples of constant temperature self-heating body were prepared according to each embodiment and comparative example. Three thermocouple temperature measuring points were fixed with 3M standard tape at the center point of the non-woven fabric of the test sample and two symmetrical points one-third of the distance from the edge of the sample. The samples were placed in a 25℃ constant temperature chamber. Temperature-time curves were recorded using thermocouple temperature measuring points. The average heating rate (rate of heating to 40℃), maximum temperature, effective heating time (time ≥ 40℃), and temperature fluctuation (standard deviation of temperature after the plateau period above 40℃) of the test sample were calculated. Three samples were tested for each type of sample, and the average value of the results was taken.

[0029]

[0030] Conclusion: The test data shows that the heating performance of the constant-temperature self-heating body prepared in the examples is better than that of the comparative examples. Compared with Example 1, the constant-temperature self-heating body of Comparative Example 1 did not modify the reduced iron powder, and its average heating rate, maximum temperature, and effective heating time were lower than those of Example 1. Its temperature fluctuation was also greater than that of Example 1. The constant-temperature self-heating body of Comparative Example 1 had a slow heating rate and incomplete heat release from the iron powder. The constant-temperature self-heating body of Comparative Example 2 did not add loaded paraffin aerogel, and its maximum temperature was much higher than that of Example 1. Its temperature fluctuation was also relatively large, and its temperature-time curve showed obvious peaks during the test. The constant-temperature self-heating body of Comparative Example 3 did not coat the electrolyte with hydrogel microspheres, and its effective heating time was shorter than that of Example 1, and its temperature fluctuation was also higher. In summary, the constant-temperature self-heating body provided by this invention has a long heating time, good temperature control, stable heat release, and high product safety and reliability.

[0031] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing a constant-temperature self-heating element, characterized in that: Specifically: Step 1: Add reduced iron powder to copper sulfate solution to coat the surface of reduced iron powder with copper oxide layer to form Fe@CuO particles; Step 2: Using sodium chloride and sodium acetate as electrolytes, add isopropylacrylamide monomer for cross-linking to form electrolyte hydrogel microspheres; Step 3: Mix straw with ammonium bicarbonate and pyrolyze to form multi-level pore biochar, then mix with cellulose nanofiber suspension, freeze dry to form biochar aerogel, pulverize to obtain biochar aerogel particles; Step 4: Using negative pressure adsorption, paraffin is adsorbed into biochar aerogel to form paraffin-loaded aerogel particles. Step 5: Mix 200-mesh reduced iron powder, Fe@CuO microparticles, electrolyte hydrogel microspheres, biochar aerogel microparticles, loaded paraffin aerogel microparticles, vermiculite and activated carbon to obtain an iron-based heating agent. Place the iron-based heating agent between two layers of non-woven fabric, and finally seal the edges and package it to obtain a constant temperature self-heating body.

2. The method for preparing a constant-temperature self-heating element according to claim 1, characterized in that: The preparation method of Fe@CuO particles is as follows: Reduced iron powder was added to copper sulfate solution and stirred until homogeneous. Then, sodium dodecylbenzenesulfonate solution was added and magnetically stirred at 200-300 rpm for 5-10 min. After heating in a water bath at 60-70℃, sodium hydroxide solution was added and stirred at 200-300 rpm for 30-60 min. After the reaction was completed, ammonium persulfate solution was added and reacted at 60-70℃ and 200-300 rpm for 1-1.5 h. After the reaction was completed, the mixture was cooled, filtered, washed with deionized water, and vacuum dried at 60-70℃ for 4-6 h to obtain Fe@CuO particles.

3. The method for preparing a constant-temperature self-heating element according to claim 2, characterized in that: The mass ratio of reduced iron powder, copper sulfate, ammonium persulfate, and sodium dodecylbenzenesulfonate is (8-10):(10-13):(2.5-3):(0.03-0.05); the sodium dodecylbenzenesulfonate solution contains 1.5-2 wt% sodium dodecylbenzenesulfonate; the copper sulfate solution contains 4-6 wt% copper sulfate; the sodium hydroxide solution contains 4-6 wt% sodium hydroxide; and the ammonium persulfate solution contains 4-6 wt% ammonium persulfate.

4. The method for preparing a constant-temperature self-heating element according to claim 1, characterized in that: The preparation method of electrolyte hydrogel microspheres is as follows: Sodium chloride and sodium acetate were added to deionized water and magnetically stirred at 200-300 rpm for 20-30 min at 40-50℃ to form an electrolyte solution. Isopropylacrylamide monomer and methylenebisacrylamide crosslinking agent were added to the electrolyte solution and magnetically stirred at 200-300 rpm for 30-40 min at 50-60℃. Ammonium persulfate initiator was added and the reaction was carried out at 200-300 rpm for 3-4 h under nitrogen protection at 70-80℃. After the reaction was completed, the mixture was filtered and vacuum dried at 40-50℃ for 6-8 h to obtain electrolyte hydrogel microspheres.

5. The method for preparing a constant-temperature self-heating element according to claim 4, characterized in that: The mass ratio of sodium chloride to sodium acetate is (10-14):(5-9); the mass ratio of isopropylacrylamide monomer, methylenebisacrylamide crosslinking agent and ammonium persulfate initiator is (45-55):(0.4-0.6):(0.4-0.6).

6. The method for preparing a constant-temperature self-heating element according to claim 1, characterized in that: The preparation method of biochar aerogel particles is as follows: After crushing the straw, mix it with ammonium bicarbonate and ball mill it thoroughly at 200-300 rpm for 20-30 min until it is uniformly mixed. Under nitrogen protection, heat it to 600-620℃ at a heating rate of 10-15℃ / min and keep it at that temperature for 1-3 h. After the temperature is maintained, cool it to room temperature, wash it with deionized water, and vacuum dry it at 60-70℃ for 6-8 h to obtain multi-level pore biochar. Add cellulose nanofibers to deionized water, stir magnetically at 400-500 rpm for 40-60 min, and ultrasonically disperse it for 20-30 min. Add multi-level pore biochar and stir magnetically at 200-300 rpm for 90-120 min to form a biochar slurry. Then freeze-dry it at (-20)-(-30)℃ for 24-30 h to obtain biochar aerogel. Crush the biochar aerogel and sieve it to obtain biochar aerogel microparticles.

7. The method for preparing a constant-temperature self-heating element according to claim 6, characterized in that: The mass ratio of straw to ammonium bicarbonate is 1:(1.2-1.5). The biochar slurry contains 1.5-3.5 wt% multi-level pore size biochar and 1.5-3.5 wt% cellulose nanofibers.

8. The method for preparing a constant-temperature self-heating element according to claim 1, characterized in that: In the paraffin-loaded biochar aerogel particles, the mass ratio of biochar aerogel to paraffin is (7-8):(8-9). Under negative pressure adsorption, the pressure is (-0.05)-(-0.07) MPa and the adsorption time is 20-30 min.

9. The method for preparing a constant-temperature self-heating element according to claim 1, characterized in that: By weight, the iron-based heating agent comprises: 42-77 parts of 200-mesh reduced iron powder, 3-4 parts of Fe@CuO microparticles, 3-10 parts of electrolyte hydrogel microspheres, 3-8 parts of biochar aerogel microparticles, 2-6 parts of paraffin-loaded aerogel microparticles, 2-10 parts of vermiculite, and 10-20 parts of activated carbon.

10. The constant-temperature self-heating element prepared by the preparation method according to any one of claims 1-9, characterized in that: The constant-temperature self-heating element is used in the preparation of self-heating eye masks and self-heating moxibustion patches.

Images