Long-acting gradient self-heating material, preparation method and application thereof
By using gradient structure design and specific material combinations, the problems of uneven temperature and short heating time in self-heating materials have been solved, achieving a long-lasting and uniform heating effect, which is suitable for products such as hand warmers.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- HUBEI TUOYING NEW MATERIAL CO LTD
- Filing Date
- 2026-02-06
- Publication Date
- 2026-06-05
Smart Images
Figure CN122143444A_ABST
Abstract
Description
Technical Field
[0001] This application relates to the field of self-heating materials, and in particular to a long-lasting gradient self-heating material, its preparation method, and its application. Background Technology
[0002] Self-heating products that utilize the energy released by the oxidation reaction of iron powder to generate heat are widely used in daily life, including hand warmers and foot warmers. The reaction principle is based on the corrosion of iron by oxygen absorption in humid air. Existing self-heating materials are made from iron powder, activated carbon, water, superabsorbent resin, and sodium chloride. Their effective heating time (not lower than 40℃) generally does not exceed 10 hours. This short effective heating time makes it difficult to meet the needs of prolonged outdoor activities in cold environments or applications requiring longer effective heating, such as medical heat therapy.
[0003] In recent years, improved types of hand warmers have appeared on the market, such as those using a multi-layered structure design to extend the effective heating time. While multi-layered hand warmers can alleviate the problem of rapid temperature decay in traditional hand warmers to some extent by stacking different materials to extend the heating time, the poor compatibility between the materials and the susceptibility to electrolytic corrosion between the layers lead to localized temperature increases while other areas experience temperature decreases, resulting in uneven overall temperature and large temperature fluctuations. This negatively impacts the user experience and offers only a limited extension on the heating time. Summary of the Invention
[0004] The main purpose of this application is to propose a long-lasting gradient self-heating material and its preparation method, which aims to solve the problems of uneven effective heating temperature and short effective heating time of existing self-heating materials such as hand warmers.
[0005] In a first aspect, this application provides a long-lasting gradient self-heating material, comprising a bottom layer, a first molecular sieve spacer layer, an intermediate layer, a second molecular sieve spacer layer and a surface layer arranged sequentially. The surface layer comprises the following raw materials in parts by weight: 10-15 parts iron powder, 4-5 parts activated carbon, 0.5-1.0 parts sodium chloride, and 4.3-4.9 parts first water-absorbing resin hydrogel. The intermediate layer comprises the following raw materials in parts by weight: 0.1-0.4 parts calcium chloride, 3.0-3.5 parts vermiculite, and 4.3-4.9 parts second water-absorbing resin hydrogel. The bottom layer comprises the following raw materials in parts by weight: 0.5-1.0 parts PMMA-coated phase change wax, 2.9-3.5 parts diatomaceous earth, and 4.3-4.9 parts third water-absorbing resin hydrogel. The raw materials for preparing the PMMA-coated phase change wax include: phase change wax, methyl methacrylate, crosslinking agent, initiator, and maleic anhydride-modified polyvinyl alcohol.
[0006] By adopting the above technical solution, the surface layer, made of a mixture of iron powder, activated carbon, sodium chloride, and a first water-absorbing resin hydrogel, serves as the starting layer, which can rapidly heat up to the operating temperature. The intermediate layer, made of a mixture of calcium chloride, vermiculite, and a second water-absorbing resin hydrogel, serves as the transition layer, which has the function of heat preservation and heat storage, can conduct heat to the bottom layer more evenly, and can also extend the heating time to a certain extent. The bottom layer, made of a mixture of PMMA-coated phase change wax, diatomaceous earth, and a third water-absorbing resin hydrogel, serves as the heat storage layer, which can further improve the effective heating time. Adjacent layers are separated by molecular sieve spacers to achieve interface isolation. The molecular sieve spacers can adsorb free water molecules, avoid interlayer electrolytic corrosion, and improve the uniform heat conduction effect between layers. In the technical solution of this application, the multi-layer structure of the bottom layer, the first molecular sieve spacer layer, the intermediate layer, the second molecular sieve spacer layer, and the surface layer arranged in sequence can realize the step release of heat, significantly improve the effective heating time of the material, and the temperature fluctuation range during effective heating is small, and the heating temperature is more uniform.
[0007] For the middle layer, vermiculite, with its layered porous structure, can impede rapid heat diffusion, achieving heat storage and conducting heat more evenly to the bottom layer, resulting in a more uniform heating temperature. Calcium chloride is highly hygroscopic and easily deliquescent, dynamically maintaining stable humidity in the system to promote stable heat storage and conduction by vermiculite. Calcium chloride and vermiculite construct a stable heat buffer framework, achieving the effects of heat preservation, heat storage, and uniform heat conduction.
[0008] For the bottom layer, the PMMA-coated phase change wax, prepared from phase change wax, methyl methacrylate, crosslinking agent, initiator, and maleic anhydride-modified polyvinyl alcohol, exhibits relatively uniform particle size, dense and smooth particle surface, and suitable wall strength, ensuring uniform dispersion and stable phase change heat storage capacity of the PMMA-coated phase change wax in the bottom layer. Diatomaceous earth, with its highly porous microstructure, forms a porous insulating shell, effectively promoting the PMMA-coated phase change wax material to undergo sufficient phase change heat storage, effectively increasing the heating time, and also contributing to a more uniform heating temperature to some extent.
[0009] Understandably, during actual use, the bottom layer is close to human skin or the area to be treated with heat, while the surface layer is in contact with the outside air.
[0010] Preferably, the surface layer comprises the following raw materials in parts by weight: 12 parts iron powder, 4.8 parts activated carbon, 0.75 parts sodium chloride, and 4.67 parts first water-absorbing resin hydrogel.
[0011] Preferably, in the intermediate layer, the second absorbent resin hydrogel material comprises 4.67 parts by weight; and in the bottom layer, the third absorbent resin hydrogel material comprises 4.67 parts by weight.
[0012] Optionally, the method for preparing the PMMA-coated phase change wax includes the following steps: (1) Polyvinyl alcohol is mixed with toluene and swollen to obtain a polyvinyl alcohol swollen solution; maleic anhydride is mixed with acetone to obtain a maleic anhydride solution; the maleic anhydride solution is added to the polyvinyl alcohol swollen solution and reacted at 95~110℃ for 2.5~3.5h, cooled to room temperature, filtered, washed and dried to obtain maleic anhydride modified polyvinyl alcohol; (2) Heat the water to 80~90℃, add the maleic anhydride modified polyvinyl alcohol obtained in step (1) to the water, stir, and obtain an aqueous phase; (3) Heat and melt the phase change wax, add methyl methacrylate, crosslinking agent and initiator, stir to obtain oil phase; (4) Add the oil phase obtained in step (3) to the aqueous phase obtained in step (2), stir and react at 80~90℃, cool to room temperature, filter, wash and dry to obtain the PMMA-coated phase change wax.
[0013] By adopting the above technical solution, the aqueous phase formed by mixing maleic anhydride-modified polyvinyl alcohol with water can fully and stably exert its emulsifying and dispersing properties, promote the complete emulsification of the core material phase change wax, and ensure that the methyl methacrylate wall material can smoothly coat the phase change wax. The resulting PMMA-coated phase change wax has a more uniform particle size, a dense and smooth particle surface, and suitable wall material strength, which ensures the uniform dispersion of PMMA-coated phase change wax in the bottom layer and stable phase change heat storage capacity.
[0014] Optionally, in step (1), the weight ratio of polyvinyl alcohol to maleic anhydride is 50:(3.75~6.25). In step (2), the weight ratio of maleic anhydride modified polyvinyl alcohol to water is 12:(250~300). In step (3), the weight ratio of phase change wax, methyl methacrylate, crosslinking agent and initiator is 20:(15~17):(0.7~1.0):(0.3~0.5). In step (4), the weight ratio of the aqueous phase to the oil phase is (1.3~1.6):1.
[0015] By adopting the above technical solution and controlling the amount of raw materials used in the preparation of PMMA-coated phase change wax, the particle size of the obtained MMA-coated phase change wax is made more uniform, the particle surface is dense and smooth, and the wall material strength is improved.
[0016] Optionally, in the bottom layer, the weight ratio of PMMA-coated phase change wax to diatomaceous earth is 0.75:3.2.
[0017] By adopting the above technical solution and further optimizing the ratio of PMMA-coated phase change wax to diatomaceous earth, the heat storage and insulation performance of the bottom layer can be further improved, the heating time can be effectively increased, and the uniformity of the heating temperature can be effectively promoted.
[0018] Optionally, in the intermediate layer, the weight ratio of calcium chloride to vermiculite is 0.25:3.2.
[0019] By adopting the above technical solution and further optimizing the ratio of calcium chloride to vermiculite, the insulation and uniform thermal conductivity of the intermediate layer can be further improved.
[0020] Optionally, the raw material for preparing the first molecular sieve spacer layer and the second molecular sieve spacer layer is ZSM-5 molecular sieve, and the amount of ZSM-5 molecular sieve in the first molecular sieve spacer layer and the second molecular sieve spacer layer is the same. The weight ratio of the ZSM-5 molecular sieve in the first molecular sieve spacer layer to the weight of the diatomaceous earth is (1.5~1.7):3.2.
[0021] By adopting the above technical solution, the adjacent two layers are separated by a molecular sieve spacer layer made of ZSM-5 molecular sieve, which effectively improves the interlayer heat conduction efficiency and makes the interlayer heat transfer more uniform, thus ensuring the uniformity of the material's heating temperature.
[0022] Optionally, the raw materials for preparing the first, second, and third water-absorbing resin hydrogels include water-absorbing resin and water, and the weight ratio of the water-absorbing resin to water is (5~7):50.
[0023] Secondly, this application provides a method for preparing a long-lasting gradient self-heating material as described in any of the above claims, comprising the following steps: S1. Mix iron powder, activated carbon and sodium chloride, add the first water-absorbing resin hydrogel material, stir, and compress into tablets to obtain the surface layer. Seal and store for later use. S2. Mix calcium chloride, vermiculite, and second water-absorbing resin hydrogel, stir, compress into tablets to obtain the intermediate layer, seal and store for later use; S3. Mix PMMA-coated phase change wax, diatomaceous earth and third water-absorbing resin hydrogel, stir, compress into tablets to obtain the bottom layer, seal and store for later use. S4. Provide the bottom layer obtained in step S3, and spread molecular sieves on one surface of the bottom layer to obtain a first molecular sieve spacer layer; provide the intermediate layer obtained in step S2, and cover the first molecular sieve spacer layer with the intermediate layer, and spread molecular sieves on the surface of the intermediate layer to obtain a second molecular sieve spacer layer; provide the top layer obtained in step S1, and cover the second molecular sieve spacer layer with the top layer, and press them together to obtain the long-lasting gradient self-heating material.
[0024] By adopting the above technical solution, heat is released in stages through gradient structure design. The surface layer can be heated to the operating temperature quickly, the middle layer achieves heat preservation and uniform heat conduction, and the bottom layer achieves sufficient heat storage and insulation. Adjacent layers are isolated by molecular sieve spacers to avoid material deterioration, effectively improve heat conduction efficiency, and ensure that the overall heating time of the material is greatly improved and the heating temperature is more uniform.
[0025] Optionally, when performing tableting in steps S1 to S4, the pressure is 0.1 to 0.3 MPa and the tableting time is 5 to 10 seconds.
[0026] Thirdly, this application also provides an application of the self-heating material described above in a hand warmer.
[0027] In summary, this application includes at least one of the following beneficial technical effects: 1. In the technical solution of this application, heat is released in a stepwise manner through a gradient structure design. The surface layer can be heated to the operating temperature quickly, the middle layer achieves heat preservation and uniform heat conduction, the bottom layer achieves sufficient heat storage and heat preservation, and the adjacent layers are isolated by molecular sieve spacers to avoid material deterioration. This effectively improves the uniform heat conduction effect between layers, ensures that the overall heating time of the material is greatly improved, and the temperature fluctuation range during effective heating is small, and the heating temperature is relatively uniform.
[0028] 2. Calcium chloride and vermiculite are mixed in a specific ratio. Vermiculite, with its layered porous structure, can impede rapid heat diffusion, achieving heat storage and conducting heat more evenly to the bottom layer, resulting in a more uniform heating temperature. Calcium chloride is highly hygroscopic and easily deliquescent, dynamically maintaining stable humidity in the system, which can promote stable heat storage and conduction by vermiculite. Calcium chloride and vermiculite construct a stable heat buffer framework, achieving the effects of heat preservation, heat storage, and uniform heat conduction.
[0029] 3. The PMMA-coated phase change wax, prepared using phase change wax, methyl methacrylate, crosslinking agent, initiator, and maleic anhydride-modified polyvinyl alcohol, exhibits relatively uniform particle size, dense and smooth particle surface, and suitable wall strength, ensuring uniform dispersion of the PMMA-coated phase change wax in the underlying layer and its phase change heat storage capacity. Diatomaceous earth, with its highly porous microstructure, can form a porous insulating shell, effectively promoting the PMMA-coated phase change wax material to undergo sufficient phase change heat storage, effectively increasing the heating time, and also contributing to a more uniform heating temperature to some extent. Attached Figure Description
[0030] Figure 1 This is a schematic diagram of the structure of the long-lasting gradient self-heating material provided in Embodiment 1 of this application.
[0031] The structure consists of: 1. bottom layer; 2. first molecular sieve spacer layer; 3. middle layer; 4. second molecular sieve spacer layer; and 5. surface layer. Detailed Implementation
[0032] The present application will be further described in detail below with reference to embodiments. All raw materials involved in this application are commercially available, wherein... Polyvinyl alcohol: Polyvinyl alcohol 1750, Shanghai Chuangsai Technology Co., Ltd., brand name: Bofeimeike; Phase change wax: Sinopec Nanyang Energy & Chemical Co., Ltd., NYXC-47 phase change wax; Double-terminated allyl polyether: Double-terminated allyl polyether DMS-4000, Wuhan Aoke Special Chemical Co., Ltd.; Superabsorbent polymer: Superabsorbent polymer, Fuhe New Material Technology (Shanghai) Co., Ltd., Item No. 190622; Reduced iron powder: 600 mesh particle size, Lingshou County Xuyang Mining Co., Ltd.; Activated carbon: Shanghai Xinyuda Energy Chemical Co., Ltd.; Vermiculite: Guangdong Yuanfeng Chemical Reagent Co., Ltd.; Diatomaceous earth: Shanghai Yuanye Biotechnology Co., Ltd.; ZSM-5 molecular sieve: Tianjin Nanhua Catalyst Co., Ltd. Preparation Example 1
[0033] A method for preparing a PMMA-coated phase change wax includes the following steps: (1) Add 50g of polyvinyl alcohol to 200mL of toluene and stir at 100rpm for 30min to obtain polyvinyl alcohol swelling solution; dissolve 5g of maleic anhydride in 70mL of acetone to obtain maleic anhydride solution; add maleic anhydride solution to polyvinyl alcohol swelling solution, heat and react in an oil bath at 95℃ for 3h, cool to room temperature (25℃), filter to remove solvent, wash with acetone and anhydrous ethanol in sequence to complete the first wash, and then perform the second and third washes in the same manner as the first wash. After completing the three washes, dry under vacuum at 70℃ to constant weight to obtain maleic anhydride modified polyvinyl alcohol.
[0034] (2) Provide 285 mL of deionized water, heat the deionized water to 85 °C, add 12 g of maleic anhydride modified polyvinyl alcohol obtained in step (1) to the heated deionized water, control the temperature of the system to 90 °C, stir at a stirring speed of 200 rpm for 50 min to obtain the aqueous phase.
[0035] (3) Provide 20g of phase change wax, heat the phase change wax to 60℃, keep it at this temperature, stir at a stirring rate of 60rpm, add 16g of methyl methacrylate, 0.8g of crosslinking agent double-terminated allyl polyether and 0.4g of initiator azobisisobutyronitrile, and continue stirring at a stirring rate of 60rpm for 10min at 60℃ to obtain the oil phase.
[0036] (4) Add the oil phase obtained in step (3) to the aqueous phase obtained in step (2), wherein the mass ratio of the aqueous phase to the oil phase is 1.4:1; stir at a stirring rate of 200 rpm for 10 min, heat the water bath to 85°C, stir at a stirring rate of 250 rpm for 5 h, filter to remove the solvent, wash with hot water at 65°C and anhydrous ethanol at 25°C in sequence to complete the first washing, and perform the second and third washings in the same manner as the first washing. After completing the three washings, dry under vacuum at 70°C to constant weight to obtain PMMA-coated phase change wax. Preparation Example 2
[0037] This preparation example is based on preparation example 1, the difference being that in step (1), the mass of maleic anhydride used is 3.75 g, and the other steps are the same as in preparation example 1. Preparation Example 3
[0038] This preparation example is based on preparation example 1, the difference being that in step (1), the mass of maleic anhydride used is 6.25 g, and the other steps are the same as in preparation example 1. Example 1
[0039] A method for preparing a long-lasting gradient self-heating hand warmer includes the following steps: S1. Mix 6g of water-absorbing resin and 50g of deionized water, and stir at 100rpm for 15min to obtain water-absorbing resin hydrogel.
[0040] S2. Take 4.67g of the water-absorbing resin hydrogel obtained in step S1 as the first water-absorbing resin hydrogel; mix 12g of reduced iron powder, 4.8g of activated carbon and 0.75g of sodium chloride, stir at 100rpm for 2min, add the first water-absorbing resin hydrogel, and continue stirring at 100rpm for 5min to obtain the first mixture; pour the first mixture into a tablet mold with a length of 13cm, a width of 10cm and a depth of 12mm, spread the first mixture evenly, press with 0.3MPa pressure for 10s to obtain a surface layer with a length of 13cm and a width of 10cm, seal and store for later use.
[0041] S3. Take 4.67g of the absorbent resin hydrogel obtained in step S1 as the second absorbent resin hydrogel; mix 0.25g of calcium chloride, 3.2g of vermiculite and the second absorbent resin hydrogel, and stir at 100rpm for 5min to obtain the second mixture; pour the second mixture into a tablet mold with a length of 13cm, a width of 10cm and a depth of 12mm, spread the second mixture evenly, and press it with 0.2MPa pressure for 8s to obtain an intermediate layer with a length of 13cm and a width of 10cm, seal and store for later use.
[0042] S4. Take 4.67g of the water-absorbing resin hydrogel material obtained in step S1 as the third water-absorbing resin hydrogel material; mix 0.75g of PMMA-coated phase change wax prepared in Preparation Example 1, 3.2g of diatomaceous earth and the third water-absorbing resin hydrogel material to obtain the third mixture; pour the third mixture into a tablet mold with a length of 13cm, a width of 10cm and a depth of 12mm, spread the third mixture evenly, press it with a pressure of 0.15MPa for 6s to obtain a bottom layer with a length of 13cm and a width of 10cm, seal and store for later use.
[0043] S5. Provide the bottom layer 1 obtained in step S4 and place it horizontally. Evenly spread 1.67g of ZSM-5 molecular sieve on the upper surface of the bottom layer and scrape it evenly with a scraper so that the molecular sieve completely covers the upper surface of the bottom layer and the thickness of the molecular sieve is relatively uniform, so as to form the first molecular sieve spacer layer 2 on the bottom layer. Provide the intermediate layer 3 obtained in step S3 and cover the first molecular sieve spacer layer with the intermediate layer. Press it with a pressure of 0.15MPa for 8s to press the 1.67g of ZSM-5 molecular sieve into the bottom layer. ZSM-5 molecular sieves are evenly distributed on the upper surface of the intermediate layer and scraped evenly with a scraper to ensure that the molecular sieves completely cover the upper surface of the intermediate layer and that the thickness of the molecular sieves is relatively uniform, so as to form a second molecular sieve spacer layer 4 on the intermediate layer; the surface layer 5 obtained in step S2 is provided and covered on the second molecular sieve spacer layer, and pressed with a pressure of 0.2MPa for 10s to obtain a long-lasting gradient self-heating material. The obtained long-lasting gradient self-heating material is put into a single-sided non-woven bag (with a pressure difference of 1.21KPa and an air permeability of 45000±50 s / 100mL; one side is covered with film and the other side is covered with adhesive, with release paper on the adhesive; the bottom layer is close to the adhesive) and the non-woven bag is put into an oxygen-barrier sealed plastic bag to make a hand warmer. Examples 2-3
[0044] Examples 2 and 3 are based on Example 1, the difference being that in step S4, the total mass of PMMA-coated phase change wax and diatomaceous earth is kept different from 3.95g, but the ratio of their amounts is adjusted; the other steps remain the same as in Example 1. Specifically, In Example 2, the amount of PMMA-coated phase change wax used was 0.5g, and the amount of diatomaceous earth used was 3.45g.
[0045] In Example 3, the amount of PMMA-coated phase change wax used was 1g, and the amount of diatomaceous earth used was 2.95g. Examples 4-5
[0046] Examples 4 and 5 are based on Example 1, the difference being that in step S4, the source of the PMMA-coated phase change wax is different, while the other steps remain the same as in Example 1. Specifically, In Example 4, the PMMA-coated phase change wax was obtained from Preparation Example 2.
[0047] In Example 5, the PMMA-coated phase change wax was obtained from Preparation Example 3. Examples 6-7
[0048] Examples 6 and 7 are based on Example 1, except that in step S3, the total mass of calcium chloride and vermiculite remains constant at 3.45g, but the ratio of their amounts is adjusted; the other steps are the same as in Example 1. Specifically, In Example 6, the amount of calcium chloride used was 0.1g, and the amount of vermiculite used was 3.35g.
[0049] In Example 7, the amount of calcium chloride used was 0.4g and the amount of vermiculite used was 3.05g. Comparative Example 1
[0050] This comparative example is based on Example 1, except that in step S4, PMMA-coated phase change wax of equal mass is used to replace diatomaceous earth, while the other steps are the same as in Example 1. Comparative Example 2
[0051] This comparative example is based on Example 1, except that in step S4, an equal mass of phase change wax is used to replace PMMA coating of the phase change wax, while the other steps remain the same as in Example 1. Comparative Example 3
[0052] This comparative example is based on Example 1, except that in step S3, an equal mass of sodium chloride is used to replace calcium chloride, while the other steps remain the same as in Example 1. Comparative Example 4
[0053] This comparative example is based on Example 1, except that in step S3, calcium chloride of equal mass is used to replace vermiculite, while the other steps are the same as in Example 1. Performance testing
[0054] The oxygen-barrier sealed plastic bags of the hand warmers prepared in Examples 1-7 and Comparative Examples 1-4 were removed, and the resulting nonwoven bags containing long-lasting gradient self-heating materials were used as samples. The heating temperature of the samples was tested. The test results are shown in Table 1 below.
[0055] Heating temperature detection: The sample is placed in an environment of 25±2℃ and relative humidity of 50±5%, and the temperature of the sample is measured. Specific test indicators include: (1) Record the total time from when the temperature of the middle part of the upper surface (near the surface layer) of the sample rises from room temperature to 40°C until it drops below 40°C, which is the effective heating time; (2) Record the heating temperature of three parts (left end, middle and right end) on the lower surface of the sample (near the bottom layer) at 1 hour of the test (t=0 at the beginning of the test); during the test, peel off the release paper, place the temperature probe in the corresponding test position, and then put the release paper back on before measuring the temperature.
[0056] Table 1 Sample Heating Temperature
[0057] As shown in Table 1, the test results of this application realize the step-by-step release of heat through gradient structure design. The surface layer can be heated to the operating temperature quickly, the middle layer achieves heat preservation and uniform heat conduction, and the bottom layer achieves sufficient heat storage and heat preservation. Adjacent layers are isolated by molecular sieve spacer layers to avoid material deterioration, effectively improve heat conduction efficiency, and the effective heating time is more than 13.9 hours. Moreover, the temperature fluctuation range during heating is small and the heating temperature is relatively uniform.
[0058] Examples 1-3 investigated the effect of the ratio of PMMA-coated phase change wax and diatomaceous earth on the thermal performance of the prepared materials; Examples 4-5, based on Example 1, investigated the effect of different PMMA-coated phase change waxes on the thermal performance of the prepared materials; Examples 6-7, based on Example 1, investigated the effect of the ratio of calcium chloride and vermiculite on the thermal performance of the prepared materials. Comparative Examples 1-4 investigated the effects of diatomaceous earth, PMMA-coated phase change wax, calcium chloride, and vermiculite on the thermal performance of the prepared materials, respectively.
[0059] The embodiments described in this specific implementation are preferred embodiments of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the principles of this application should be covered within the scope of protection of this application.
Claims
1. A long-lasting gradient self-heating material, characterized in that, It includes a bottom layer, a first molecular sieve spacer layer, an intermediate layer, a second molecular sieve spacer layer and a surface layer arranged in sequence. The surface layer includes the following raw materials in parts by weight: 10-15 parts iron powder, 4-5 parts activated carbon, 0.5-1.0 parts sodium chloride, and 4.3-4.9 parts first water-absorbing resin hydrogel. The intermediate layer comprises the following raw materials in parts by weight: 0.1-0.4 parts calcium chloride, 3.0-3.5 parts vermiculite, and 4.3-4.9 parts second water-absorbing resin hydrogel. The bottom layer comprises the following raw materials in parts by weight: 0.5-1.0 parts PMMA-coated phase change wax, 2.9-3.5 parts diatomaceous earth, and 4.3-4.9 parts third water-absorbing resin hydrogel. The raw materials for preparing the PMMA-coated phase change wax include: phase change wax, methyl methacrylate, crosslinking agent, initiator, and maleic anhydride-modified polyvinyl alcohol.
2. The long-lasting gradient self-heating material according to claim 1, characterized in that, The method for preparing the PMMA-coated phase change wax includes the following steps: (1) Polyvinyl alcohol is mixed with toluene and swollen to obtain a polyvinyl alcohol swollen solution; maleic anhydride is mixed with acetone to obtain a maleic anhydride solution; the maleic anhydride solution is added to the polyvinyl alcohol swollen solution and reacted at 95~110℃ for 2.5~3.5h, cooled to room temperature, filtered, washed and dried to obtain maleic anhydride modified polyvinyl alcohol; (2) Heat the water to 80~90℃, add the maleic anhydride modified polyvinyl alcohol obtained in step (1) to the water, stir, and obtain an aqueous phase; (3) Heat and melt the phase change wax, add methyl methacrylate, crosslinking agent and initiator, stir to obtain oil phase; (4) Add the oil phase obtained in step (3) to the aqueous phase obtained in step (2), stir and react at 80~90℃, cool to room temperature, filter, wash and dry to obtain the PMMA-coated phase change wax.
3. The long-lasting gradient self-heating material according to claim 2, characterized in that, In step (1), the weight ratio of polyvinyl alcohol to maleic anhydride is 50:(3.75~6.25). In step (2), the weight ratio of maleic anhydride modified polyvinyl alcohol to water is 12:(250~300). In step (3), the weight ratio of phase change wax, methyl methacrylate, crosslinking agent and initiator is 20:(15~17):(0.7~1.0):(0.3~0.5). In step (4), the weight ratio of the aqueous phase to the oil phase is (1.3~1.6):
1.
4. The long-lasting gradient self-heating material according to claim 1, characterized in that, In the bottom layer, the weight ratio of PMMA-coated phase change wax to diatomaceous earth is 0.75:3.
2.
5. The long-lasting gradient self-heating material according to claim 1, characterized in that, In the intermediate layer, the weight ratio of calcium chloride to vermiculite is 0.25:3.
2.
6. The long-lasting gradient self-heating material according to claim 1, characterized in that, The raw material for preparing the first molecular sieve spacer layer and the second molecular sieve spacer layer is ZSM-5 molecular sieve, and the amount of ZSM-5 molecular sieve used in the first molecular sieve spacer layer and the second molecular sieve spacer layer is the same. The weight ratio of the ZSM-5 molecular sieve in the first molecular sieve spacer layer to the weight of the diatomaceous earth is (1.5~1.7):3.
2.
7. The long-lasting gradient self-heating material according to claim 1, characterized in that, The raw materials for preparing the first, second, and third water-absorbing resin hydrogels include water-absorbing resin and water, and the weight ratio of the water-absorbing resin to the water is (5~7):
50.
8. A method for preparing a long-lasting gradient self-heating material as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1. Mix iron powder, activated carbon and sodium chloride, add the first water-absorbing resin hydrogel material, stir, and compress into tablets to obtain the surface layer. Seal and store for later use. S2. Mix calcium chloride, vermiculite, and second water-absorbing resin hydrogel, stir, compress into tablets to obtain the intermediate layer, seal and store for later use; S3. Mix PMMA-coated phase change wax, diatomaceous earth and third water-absorbing resin hydrogel, stir, compress into tablets to obtain the bottom layer, seal and store for later use. S4. Provide the bottom layer obtained in step S3, and spread the molecular sieve on one surface of the bottom layer to obtain the first molecular sieve spacer layer; Provide the intermediate layer obtained in step S2, cover the first molecular sieve spacer layer with the intermediate layer, and sprinkle the molecular sieve on the surface of the intermediate layer to obtain the second molecular sieve spacer layer; The surface layer obtained in step S1 is provided, and the surface layer is covered on the second molecular sieve spacer layer and pressed together to obtain the long-lasting gradient self-heating material.
9. The method for preparing the long-lasting gradient self-heating material according to claim 8, characterized in that, During the tableting process in steps S1 to S4, the pressure is 0.1 to 0.3 MPa and the tableting time is 5 to 10 seconds.
10. The application of a long-lasting gradient self-heating material as described in any one of claims 1 to 7 in a hand warmer.