Heterogeneous aerogel based on waste paperboard as well as preparation method and application of heterogeneous aerogel

Abstract

The invention discloses heterogeneous aerogel based on waste paperboards as well as a preparation method and application of the heterogeneous aerogel. The heterogeneous aerogel based on the waste paperboard is composed of aerogel and a phase change material, and the aerogel is prepared by crushing and soaking the waste paperboard and then using a freezing casting method; the phase change material is an organic phase change material. Only water is used as a solvent, so that the preparation process is more environment-friendly. Compared with aerogel prepared from cellulose, the cost is lower. The aerogel and the phase change material have good energy absorption characteristics after being compounded, so that the aerogel composite material has buffering characteristics. Meanwhile, when the heterogeneous aerogel is prepared, a heterogeneous structure with any layer number can be formed through a staged laminated freezing technology, and the multifunctional heterogeneous aerogel composite material with multiple functions of photo-thermal, electric heating, catalysis, adsorption and the like is formed according to different additives in each layer of aerogel.

Description

Technical Field

[0001] The present invention belongs to the field of new building composite materials, and particularly relates to a heterogeneous aerogel based on waste cardboard, a preparation method thereof, and an application thereof. Background Art

[0002] With the increasing demand for building energy conservation and emission reduction as well as residential comfort, the demand for green, low-carbon, and multifunctional environmental protection building materials has also increased rapidly. Aerogel is a common thermal insulation material, which has the advantages of light weight and good thermal insulation performance. However, the traditional aerogel preparation technology is complex, the materials are expensive, and the functions are single, which seriously hinders the popularization and application of aerogel materials.

[0003] The main raw materials for traditional aerogels are graphene, polyimide, nanocellulose, etc., and their prices are relatively expensive. At present, some patents and literatures ("Research on the Preparation of Cellulose Aerogel from Waste Materials and Its Application in Water Treatment", CN 112591728 A, CN108686628A) have published the manufacture of aerogels using raw materials such as waste newspapers, straws, and natural plants. However, the preparation process generally requires the prior use of strong alkalis, strong acids, or high-temperature calcination to remove lignin and other impurities in the raw materials. The preparation process is complex and prone to generating highly polluted wastewater. Optimizing the preparation process of waste cellulose and reducing or even avoiding the use of excessive chemical reagents meet the current requirements of green and sustainable development.

[0004] Composite aerogels with latent heat type phase change that can actively regulate temperature can be prepared by combining aerogels with phase change materials. However, due to the hydrophobic properties of conventional aerogels, the composite aerogel structure is unstable and the phase change material is extremely prone to leakage. At the same time, the traditional homogeneous aerogel has a simple structure and single function, and cannot meet the needs of people for the multifunctionalization of building materials. There is an urgent need to develop a new aerogel manufacturing process to modularize the aerogel structure and diversify the functions, in order to obtain a stable and multifunctional aerogel composite material. Summary of the Invention

[0005] To solve the above problems, a first aspect of the present invention provides a heterogeneous aerogel based on waste cardboard, which is composed of an aerogel and a phase change material. The aerogel is prepared by using the freeze-casting method after waste cardboard is crushed and soaked; the phase change material is an organic phase change material.

[0006] According to another aspect of the present invention, a preparation method of a heterogeneous aerogel based on waste cardboard is provided, including the following steps: Step 1, use a crusher and an ultrasonic vibrator to crush waste cardboard, and then put the cardboard powder into clean water. After soaking for a period of time, the waste paper fibers are softened to obtain pulp; Step 2: Add long-chain polymers such as sodium alginate and polyvinyl alcohol to the pulp. The addition amount is about 0.1-4% of the pulp weight, which can further improve the structural strength of the aerogel. Step 3: Pour the prepared pulp into a mold, and then apply a low temperature of -40°C on one side of the mold until the pulp is completely solidified. Step 4: Put the frozen pulp into a vacuum freeze-drying oven to remove moisture and obtain a heterogeneous aerogel. Step 5: Place the aerogel in the melted phase change material solution and in a vacuum environment to promote the filling of the phase change material in the pores of the aerogel. The phase change material is filled inside the porous structure of the aerogel by vacuum adsorption. Step 6: Remove the excess phase change material on the surface of the aerogel to obtain a heterogeneous aerogel. During the preparation process of the aerogel, there is no need to use chemical reagents such as sodium hydroxide to remove lignin. Only the cellulose-rich waste needs to be crushed and soaked, and the only solvent required for preparation is water. The main components of the final product of the multifunctional aerogel are aerogel and phase change material, and the mass ratio of aerogel to phase change material is 1:9-9:1; the pore size inside the aerogel is <300 microns, and the density of the aerogel is ≤0.04 g / cm 3 ; After the phase change material is filled in the pores of the aerogel, when the phase change material melts, due to the high energy absorption of the porous structure and the liquid phase change material of the aerogel, it has certain buffering performance.

[0007] Furthermore, the crushing process of waste cardboard needs to go through a cyclic process of crushing and ultrasonic dispersion, and the number of cycles of crushing and ultrasonic dispersion does not exceed 5 times. Thus, it is ensured that the crushed paper fibers have an aspect ratio greater than 20. The sources of waste cardboard are mainly cellulose-rich solid wastes such as express boxes, paper packaging boxes, newspapers, and books.

[0008] Furthermore, the soaking time of the waste paper fibers after crushing is 24-48 h.

[0009] Furthermore, the phase change material is one of paraffin, linear alkanes or fatty acids.

[0010] Furthermore, after the upper layer of pulp is completely solidified in Step 3, repeat the steps of pouring the pulp into the mold and low-temperature solidification at least once to obtain a multi-layer heterogeneous aerogel. For the multi-layer heterogeneous multifunctional aerogel, its multifunctionality mainly comes from functional modification in each structural layer. The aerogel forms a heterogeneous system through a laminated freezing process, and the number of heterogeneous structural layers can be any number such as one layer, two layers, three layers, etc. according to the number of liquid addition times during the freeze-casting process.

[0011] Furthermore, carbon nanotubes or carbon black are added to the repeatedly added pulp. According to the functional design requirements in each layer structure, adding different modified materials can obtain a multifunctional heterogeneous aerogel composite material with various functions such as photothermal, electrothermal, catalytic, and adsorption.

[0012] According to another aspect of the present invention, there is provided an application of a heterogeneous aerogel based on waste cardboard in the fields of phase change intelligent fabrics, phase change energy-saving buildings, phase change heat accumulators, phase change electronic components, and phase change battery thermal management.

[0013] The above technical solutions of the present invention have the following beneficial technical effects: Using waste cardboard or paper directly as the raw material for manufacturing aerogels, there is no need to perform alkali washing on the paper to remove impurities, which greatly reduces the manufacturing cost and environmental harm; The novel preparation process of multi-layer heterogeneous aerogels can add different functions to the aerogels according to requirements, the preparation method is simple, and the functions can be modularly customized; The heterogeneous aerogels with a specific structure design have a better adsorption effect on oily phase change materials, which can significantly improve the heat storage performance and stability of the materials. The block structure composed of multi-level porous aerogels and organic phase change materials can significantly improve the impact energy absorption performance and endow the material with good buffering performance. Brief Description of the Drawings

[0014] Figure 1 It is a flowchart of the preparation method of the heterogeneous aerogel based on waste cardboard; Figure 2 It is a picture of the waste paper aerogel of Example 1 of the present invention; Figure 3 It is a SEM photograph of the waste paper aerogel of Example 1 of the present invention; Figure 4 It is a SEM photograph of the waste paper aerogel of Example 1 of the present invention after adsorbing the phase change material; Figure 5 It is a DSC test curve of the heterogeneous aerogel and pure eicosane of Example 1 of the present invention; Figure 6 It is a test comparison diagram of the heterogeneous aerogel of Example 1 of the present invention for simulated building thermal management; Figure 7 It is a buffer test picture of the heterogeneous aerogel of Example 1 of the present invention for a dropped egg; Figure 8 It is a picture of the double-layer heterogeneous aerogel of Example 2 of the present invention; Figure 9 It is a SEM picture of the aerogel prepared in the comparative example of the present invention.

[0015] Figure 10 It is a microstructural diagram of the aerogel obtained in Comparative Example 1. Detailed implementation manners

[0016] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in combination with specific implementation manners and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the present invention.

[0017] The first aspect of the present invention provides a heterogeneous aerogel based on waste cardboard, which is composed of an aerogel and a phase change material. The aerogel is prepared by using a freeze-casting method after waste cardboard is crushed and soaked; the phase change material is an organic phase change material.

[0018] As Figure 1 shown, another aspect of the present invention provides a method for preparing a heterogeneous aerogel based on waste cardboard, including the following steps: Step 1, use a crusher and an ultrasonic vibrator to crush waste cardboard, and then put the cardboard powder into clear water. After soaking and stirring for a period of time, the waste paper fibers are softened to obtain a uniform pulp; Step 2, add long-chain polymers such as sodium alginate and polyvinyl alcohol to the pulp, and the addition amount is about 0.1-4% of the weight of the pulp, which can further improve the structural strength of the aerogel; Step 3, pour the prepared pulp into a mold, and then apply a low temperature of -40 °C to one side of the mold until the pulp is completely solidified; Step 4, put the frozen pulp into a vacuum freeze-drying oven to remove moisture to obtain a heterogeneous aerogel; Step 5, put the aerogel into the melted phase change material solution and place it in a vacuum environment to promote the phase change material to fill the voids in the aerogel. The phase change material is filled inside the porous structure of the aerogel by vacuum adsorption; Step 6, remove the excess phase change material on the surface of the aerogel to obtain a heterogeneous aerogel. No chemical reagents such as sodium hydroxide are required to remove lignin during the preparation of the aerogel. Only the waste rich in cellulose needs to be crushed and soaked, and the only solvent required for preparation is water. The final finished product of the multifunctional aerogel mainly consists of an aerogel and a phase change material, and the mass ratio of the aerogel to the phase change material is 1:9-9:1; the pore size inside the aerogel is <300 microns, and the density of the aerogel is ≤0.04 g / cm 3 ; after the voids inside the aerogel are filled with the phase change material, when the phase change material melts, the aerogel has a certain buffering performance due to the high energy absorption of the porous structure and the liquid phase change material.

[0019] Preferably, the crushing process of waste cardboard needs to go through a cycle of crushing and ultrasonic dispersion, and the number of cycles of crushing and ultrasonic dispersion does not exceed 5 times. This ensures that the crushed paper fibers have an aspect ratio greater than 20. The sources of waste cardboard are mainly solid wastes rich in cellulose such as express boxes, paper packaging boxes, newspapers, and books.

[0020] Preferably, the soaking time of waste paper fibers after crushing is 24 - 48 h.

[0021] Preferably, the phase change material is one of paraffin, linear alkanes, or fatty acids.

[0022] Preferably, after the upper layer of pulp is completely solidified in step 3, the steps of pouring the pulp into the mold and solidifying it at a low temperature are repeated at least once to obtain a multi-layered heterogeneous aerogel. The multi-layered heterogeneous multifunctional aerogel, its multifunctionality mainly comes from functional modification in each structural layer. The aerogel forms a heterogeneous system through a laminated freezing process, and the number of heterogeneous structural layers can be any number such as one layer, two layers, three layers, etc. according to the number of liquid addition times in the freeze-casting process.

[0023] Preferably, carbon nanotubes or carbon black are added to the repeatedly added pulp. According to the functional design requirements in each layer structure, adding different modification materials can obtain a multifunctional heterogeneous aerogel composite material with functions such as photothermal, electrothermal, catalytic, and adsorption.

[0024] Another aspect of the present invention provides an application of a heterogeneous aerogel based on waste cardboard in the fields of phase change intelligent fabrics, phase change energy-saving buildings, phase change heat accumulators, phase change electronic components, and phase change battery thermal management.

[0025] Example 1: A preparation method of a heterogeneous aerogel based on waste cardboard, comprising the following steps: Step 1, take 2 g of corrugated paper, use a crusher and an ultrasonic vibrator to crush it in a cycle of 3 times, then mix the paper fibers with 97 g of deionized water, and use a stirrer to continuously soak and stir for 24 h to obtain a uniform paper fiber slurry; Step 2, add 1 g of sodium alginate powder to the paper fiber slurry, heat the slurry temperature to 50 °C, and continue to stir for 3 h to ensure that the sodium alginate is completely dissolved in the slurry; Step 3, pour the slurry mixture obtained in step 2 into a Teflon mold with a side length of 2 cm, and use a circulating coolant at -40 °C to freeze the pulp from the bottom of the mold until the pulp in the mold is completely solidified; Step 4, put the solidified pulp and the mold together into a vacuum freeze dryer and dry for 12 h to remove the moisture in the material to obtain a heterogeneous aerogel; Step 5: Immerse the prepared aerogel into melted eicosane, and then conduct vacuum treatment on it. Under the action of the internal and external pressure difference, eicosane is adsorbed into the pores of the aerogel. Finally, wipe off the unadsorbed eicosane on the surface with filter paper to obtain a monolayer heterogeneous multifunctional aerogel.

[0026] The paper fiber aerogel prepared in this example is as Figure 2 shown. The prepared aerogel has the characteristics of light weight and stable structure. It is detected that the density of the obtained aerogel is lower than 0.04 g / cm 3 ; The SEM image of the aerogel is as Figure 3 shown. The finally obtained monolayer heterogeneous multifunctional aerogel is as Figure 3 shown. With the assistance of the directional freezing casting technology, the paper fibers successfully form a multi-layered porous structure with a pore width of less than 50 μm.

[0027] The SEM image of the monolayer heterogeneous multifunctional aerogel prepared in this example is as Figure 4 shown. It can be seen from Figure 4 that the pores of the multifunctional aerogel are all filled with the phase change material eicosane, and only a small amount of pores are exposed. Figure 5 shows that the phase change temperatures of the aerogel and pure eicosane are basically the same, but the enthalpy value of the aerogel is slightly lower than that of pure eicosane. The phase change enthalpy value of the aerogel is about 188.64 J / g, and the phase change enthalpy value of pure eicosane is about 215.65 J / g, and the actual loading rate is about 87.48%.

[0028] The multifunctional aerogel prepared in this example has good temperature regulation performance. Figure 6 Figure a in Figure 6Figure b shows the test results after using the multifunctional aerogel prepared in this example to adjust the indoor temperature. In the control model of the aerogel, the indoor temperature rises rapidly after the xenon lamp is turned on. At the 1225th second, the temperature at point A on the roof reaches 46.8 °C, and the temperature recorded at point B on the floor is 42.9 °C. After that, the temperature fluctuates within a certain range. After turning off the light source after 3600 seconds of illumination, the indoor temperature drops rapidly. After about 1815 seconds, the temperatures at points A and B are almost equal, reaching thermal equilibrium with the external environment. In contrast, the experimental results of the model house equipped with the multifunctional aerogel show different patterns. Initially, under the illumination of the light source, the indoor temperature rises rapidly. However, when the temperature at point A reaches about 36.2 °C, the heating rates at points A and B slow down significantly. This may be because the eicosane in the aerogel melts, absorbing the excess heat and converting it into latent heat, thus alleviating the rapid rise in indoor temperature. After 3600 seconds of illumination, the temperature at point A is 45.4 °C, and the temperature at point B is 38.1 °C, which are 1.4 °C and 4.8 °C lower than those of the control group, respectively.

[0029] The multifunctional aerogel prepared in this example has good buffering performance. To evaluate this buffering capacity, a control experiment was conducted, as Figure 7 shown: A cooked egg was dropped from a height of 25 cm to impact the glass surface and the multifunctional aerogel with the eicosane in a molten state, respectively. As Figure 7 shown, when the egg came into contact with the glass, it received a large impact, Figure 7 and a large area of the eggshell cracked in c. In contrast, when the egg was dropped from the same height onto the molten aerogel, the material only underwent slight deformation during the impact, the egg did not bounce back, and there were no obvious cracks on the eggshell. This performance indicates that the phase change material in the pores of the aerogel exhibits shear thickening characteristics, effectively absorbing the impact energy. The experimental results confirm that the multifunctional aerogel has excellent energy absorption performance in the molten state, indicating its potential as a buffer material.

[0030] Example 2: A preparation method of a heterogeneous aerogel based on waste cardboard, comprising the following steps: Step 1: Take 2 g of waste paper, crush it 4 times using a crusher and an ultrasonic vibrator, and then mix it with 97 g of deionized water. Continuously soak and stir it with a stirrer for 48 h to obtain a uniform paper fiber slurry; Step 2: Add 1 g of sodium alginate powder to the paper fiber slurry, heat the slurry temperature to 50 °C, and continue stirring for 3 h to ensure that the sodium alginate is completely dissolved in the slurry; Step 3: Pour the slurry mixture obtained in Step 2 into a cylindrical Teflon mold. The poured slurry should not exceed three-fourths of the maximum height of the mold. Then, use a circulating coolant at -40°C to freeze the pulp starting from the bottom of the mold until the pulp in the mold is completely solidified. Step 4: Take 10 g of the slurry obtained in Step 2, add 0.1 g of carbon nanotubes, and then use ultrasonic waves to oscillate it so that the carbon nanotubes are evenly dispersed in the slurry. Step 5: Pour the slurry prepared in Step 4 into the mold of Step 3, and then continue to freeze until the newly added slurry is completely frozen. Step 6: Put the solidified pulp and the mold together into a vacuum freeze dryer and dry for 12 h to remove the moisture in the material, obtaining an aerogel with a double-layer structure. Step 7: Immerse the prepared aerogel in melted eicosane, and then perform a vacuum treatment on it. Under the action of the internal and external pressure difference, eicosane is adsorbed into the pores of the aerogel. Finally, wipe off the unadsorbed eicosane on the surface with filter paper to obtain a double-layer heterogeneous multifunctional aerogel.

[0031] The double-layer heterogeneous multifunctional aerogel prepared in this example has structural characteristics different from those of Example 1. Due to the addition of carbon nanotubes, part of the aerogel appears black and half appears light yellow in appearance, as Figure 8 shown. The carbon nanotubes at the top have good photothermal conversion performance, enabling this example to not only possess the functions described in Example 1 but also have photothermal conversion performance, capable of converting sunlight into heat for storage.

[0032] Example 3: A method for preparing a heterogeneous aerogel based on waste cardboard, comprising the following steps: Step 1: Take 2 g of waste paper, use a crusher and an ultrasonic vibrator to crush it 4 times in a cycle, and then mix it with 97 g of deionized water. Use a stirrer to continuously soak and stir for 48 h to obtain a uniform paper fiber slurry. Step 2: Add 1 g of sodium alginate powder to the paper fiber slurry, heat the slurry temperature to 50°C, and continue to stir for 3 h to ensure that the sodium alginate is completely dissolved in the slurry. Step 3: Take 10 g of the slurry obtained in Step 2, add 0.1 g of carbon black powder, and then use ultrasonic waves to oscillate it so that the carbon black powder is evenly dispersed in the slurry. Then, pour the obtained black slurry mixture into a square Teflon mold. The poured slurry should not exceed three-fourths of the maximum height of the mold. Use a circulating coolant at -40°C to freeze the pulp starting from the bottom of the mold until the pulp in the mold is completely solidified. Step 4: Pour the slurry prepared in Step 3 into the mold of Step 3, and then continue to freeze until the newly added slurry is completely frozen; Step 5: Put the solidified pulp and the mold together into a vacuum freeze dryer and dry for 12 h to remove the moisture in the material, obtaining an aerogel with a double-layer structure; Step 6: Immerse the prepared aerogel into the melted paraffin wax for sectioning, and then perform a vacuum treatment on it. Under the action of the internal and external pressure difference, the paraffin liquid is adsorbed into the pores of the aerogel. Finally, wipe off the unadsorbed eicosane on the surface with filter paper to obtain a double-layer heterogeneous multifunctional aerogel.

[0033] The double-layer heterogeneous multifunctional aerogel prepared in this example has structural characteristics different from those of Examples 1 and 2. Due to the addition of carbon black, the composite aerogel is black as a whole, as Figure 9 shown. Carbon black has the advantage of being cheaper than other photothermal materials, making the manufacturing cost of this example significantly reduced in addition to possessing the functions described in Example 1. And because the paper fibers are overall loaded with carbon black, the thermal conductivity of the composite aerogel is improved.

[0034] Comparative Example 1: A heterogeneous aerogel based on waste cardboard proposed by the present invention needs to strictly control the aspect ratio of paper fibers during implementation. Too long or too short paper fibers will cause changes in the microstructure of the aerogel during implementation, resulting in the instability of the aerogel structure. The following is the comparative example of the present invention.

[0035] Step 1: Take 2 g of corrugated paper, break it up by using a crusher and an ultrasonic vibrator in a cycle of 6 times, and then mix the paper fibers with 97 g of deionized water. Use a stirrer to continuously soak and stir for 24 h to obtain a uniform paper fiber slurry; Step 2: Add 1 g of sodium alginate powder to the paper fiber slurry, heat the slurry temperature to 50 °C, and continue to stir for 3 h to ensure that the sodium alginate is completely dissolved in the slurry; Step 3: Pour the slurry mixture obtained in Step 2 into a Teflon mold with a side length of 2 cm, and use a circulating coolant at -40 °C to freeze the pulp from the bottom of the mold until the pulp in the mold is completely solidified; Step 4: Put the solidified pulp and the mold together into a vacuum freeze dryer and dry for 12 h to remove the moisture in the material, obtaining an aerogel; The microstructural diagram of the aerogel obtained in this comparative example is as Figure 10 shown. Due to too many crushing cycles, the aspect ratio of the paper fibers becomes smaller, making the ice crystals in the freeze-casting process unable to effectively separate the paper fibers. Eventually, a large number of paper fibers are cross-stacked and an effective multi-layer aerogel structure cannot be formed.

[0036] It should be understood that the above specific embodiments of the present invention are only used for exemplary illustration or explanation of the principles of the present invention, and do not constitute a limitation to the present invention. Therefore, any modifications, equivalent replacements, improvements, etc. made without departing from the spirit and scope of the present invention shall be included within the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all variations and modifications that fall within the scope and boundaries of the appended claims, or equivalent forms of such scope and boundaries.

Claims

1. A heterogeneous aerogel based on waste cardboard, characterized in that: The invention is composed of aerogel and phase change material. The aerogel is prepared by crushing and soaking discarded cardboard and then using a freezing casting method. The phase change material is an organic phase change material.

2. A method for preparing a heterogeneous aerogel based on waste cardboard, characterized in that: The following steps are involved: Step 1: Use a crusher and an ultrasonic vibrator to crush the waste cardboard, then put the cardboard powder into clean water, soak and stir for a period of time to soften the waste paper fibers to obtain pulp; Step 2, adding sodium alginate, polyvinyl alcohol and other long-chain polymers to the pulp, the added amount being about 0.1-4% of the weight of the pulp; Step 3, pour the prepared pulp into the mold, and then apply a low temperature of -40°C to one side of the mold until the pulp is completely solidified; Step 4, placing the frozen pulp in a vacuum freeze drying chamber to remove moisture to obtain a heterogeneous aerogel; Step 5, placing the aerogel into the melted phase change material solution and placing it in a vacuum environment to enable the phase change material to fill the gaps in the aerogel; Step 6, removing excess phase change material on the surface of the aerogel to obtain a heterogeneous aerogel.

3. The method for preparing a heterogeneous aerogel based on waste cardboard according to claim 2, characterized in that: The waste cardboard crushing process needs to go through a cycle of crushing and ultrasonic dispersion, and the number of crushing and ultrasonic dispersion cycles does not exceed 5 times.

4. The method for preparing a heterogeneous aerogel based on waste cardboard according to claim 2, characterized in that: The soaking and stirring time of the waste paper fibers after crushing is 24-48 hours.

5. The method for preparing a heterogeneous aerogel based on waste cardboard according to claim 2, characterized in that: The phase change material is one of paraffin, straight-chain alkanes or fatty acids.

6. The method for preparing a heterogeneous aerogel based on waste cardboard according to claim 2, characterized in that: In step 3, after the upper layer of pulp is completely solidified, the steps of pouring the pulp into a mold and solidifying at low temperature are repeated at least once to obtain a multi-layer heterogeneous aerogel.

7. The method for preparing a heterogeneous aerogel based on waste cardboard according to claim 6, characterized in that: Repeatedly add carbon nanotubes or carbon black to the pulp.

8. Application of a waste cardboard-based heterogeneous aerogel in the fields of phase change smart fabrics, phase change energy-saving buildings, phase change heat storage devices, phase change electronic components, and phase change battery thermal management.

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