A method for preparing an all-weather thermal management film with mechanically adjusted reflectivity

Abstract

A method for preparing an all-weather thermal management film with mechanically adjustable reflectivity relates to the field of radiation refrigeration technology, and specifically to a method for preparing an all-weather thermal management film with mechanically adjustable reflectivity. The purpose of the present invention is to solve the problem that the reflectivity of existing thermal management materials based on radiation refrigeration is not high in refrigeration mode, the thermal management is mostly implemented in a complex system, and the thermal management ability cannot be actively and continuously adjusted in situ. Method: 1. Prepare TPU solution; 2. Control the volume of TPU solution, the injection speed of electrospinning, the receiving distance and the applied voltage to obtain an all-weather thermal management film with mechanically adjustable reflectivity. The all-weather thermal management film with mechanically adjustable reflectivity of the present invention can achieve continuous in-situ control by applying different degrees of mechanical external force, and is a thermal management capability that can be continuously switched between cooling and heating based on a wide range of reflectivity modulation capabilities.

Description

Technical Field

[0001] The present invention relates to the field of radiation cooling technology, and in particular to a method for preparing an all-weather thermal management film with mechanically adjustable reflectivity. Background Art

[0002] The growing demand for sustainable thermal management technologies is one of the most pressing challenges we need to face, especially considering that the energy demand for thermal management is expected to increase tenfold by 2050. Therefore, new functional materials that can achieve heating and cooling capabilities without requiring any electricity have become extremely attractive. Based on the radiative cooling function of materials, giving them the ability to heat using sunlight can achieve all-season thermal control needs.

[0003] Radiative cooling has been shown to be a promising energy-free cooling strategy that can emit heat from objects through an atmospheric transparent window (8-13 μm), which can alleviate global warming and urban heat island effects compared to traditional cooling methods based on vapor compression systems (refrigerators, air conditioners). For daytime radiative cooling, the high solar reflectivity of the material (>90%) is a powerful factor in determining the cooling performance. Traditional radiative cooling systems are static, but they need to use the heat of the sun for thermal heating in cold seasons. Therefore, a practical radiative cooling system is needed that can adjust its daytime reflectivity according to the thermal comfort of the human body.

[0004] Switchable technologies for achieving radiative cooling and solar heating in a single system have been initially explored. The Janus cooling / heating switching film is a multilayer structure that combines photothermal and radiative cooling layers. The film needs to be manually flipped to obtain the switch between cooling and heating during use, which complicates the preparation process and is incompatible with in-situ switching. Thermochromic poly (N-isopropylacrylamide) (PNIPAm) hydrogel can make a wide range of visible light reflectance modulation in response to changing ambient temperature, but this mechanism is passive and does not have the ability to adjust gradients. The film can be switched between the reflective and transparent states using a composite system composed of solvents that match the refractive index of the porous coating film, which is achieved by the vapor-liquid transition of the solvent caused by the external temperature difference. Although this method allows in-situ regulation, the circulation of the liquid in the cooling material is a complex process, and the solvents that match the refractive index of the polymer are mostly organic or even toxic solvents, which are not suitable for practical applications.

[0005] Currently, there are relatively few patents related to thermal management of cooling-heating polymers. The patent application document with publication number CN115323626A only reports on the radiant cooling performance produced by polymers doped with inorganic fillers, but does not reflect the heating aspect. Summary of the invention

[0006] The purpose of the present invention is to solve the problems that the reflectivity of existing thermal management materials based on radiation cooling is not high in cooling mode, the implementation of thermal management is mostly a complex system and its thermal management ability cannot be actively and continuously adjusted in situ, and to provide a method for preparing an all-weather thermal management film with mechanically adjustable reflectivity.

[0007] A method for preparing an all-weather thermal management film with mechanically adjustable reflectivity is specifically completed in the following steps:

[0008] 1. Preparation of TPU solution:

[0009] Dissolving thermoplastic polyurethane in an organic solvent to obtain a TPU solution;

[0010] 2. Use TPU solution as spinning solution, add a certain volume of spinning solution into a 5mL disposable syringe, fix the NO.22 blunt metal needle, connect the positive electrode to the metal needle and apply a certain high voltage, control the injection speed, receiving distance and applied voltage for electrospinning, and finally dry the film prepared by electrospinning to obtain an all-weather thermal management film with mechanically adjusted reflectivity.

[0011] Technical effects of the present invention:

[0012] 1. The present invention prepares an all-weather thermal management film with mechanically adjustable reflectivity by electrospinning technology. The film can reversibly adjust the reflectivity by mechanical stretching, thereby realizing in-situ solvent-free switching between radiative cooling and solar heating;

[0013] 2. The all-weather thermal management film with mechanically adjustable reflectivity prepared by the present invention has radiation cooling capability and is a radiation cooling film integrating high reflectivity and high emissivity; the film can undergo dynamic hot and cold cycles and can continuously adjust the temperature; the film is a thermoplastic polyurethane film nanofiber with a diameter of 0.5-1 μm and has a strong scattering ability in the air;

[0014] 3. The all-weather thermal management film with mechanically adjustable reflectivity of the present invention can realize continuous in-situ control by applying mechanical external forces of different degrees. It is a thermal management capability that can continuously switch between cooling and heating based on the wide range of reflectivity modulation capability. It is a material that can switch between solar heating and radiation cooling without energy consumption.

[0015] 4. In cooling mode, the all-weather thermal management film with mechanically adjustable reflectivity prepared by the present invention exhibits a strong reflectivity of 95.6% at a wavelength of 0.25-2.5 μm and an infrared emissivity of 92.6% at a wavelength of 8-13 μm; it achieves a maximum temperature reduction of 10°C in outdoor temperature measurement, and the cooling power is 118.25 W m -2When the film is deformed by mechanical tension, the reflectivity drops to 61.4%, achieving a maximum temperature rise of 9.5°C in outdoor temperature measurement, and the solar heating power can reach 220.34W m -2 ;

[0016] 5. An all-weather thermal management film with mechanically adjustable reflectivity prepared by the present invention exhibits excellent cyclic stability (10,000 stretching cycles). The high haze exhibited in the stretched state can achieve uniform indoor lighting in practical applications. Its hydrophobicity and excellent aging resistance, acid rain corrosion resistance, and wear resistance give this simple thermal management material a broader application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Spectra of reflection and emissivity of the all-weather thermal management film with mechanically adjustable reflectivity prepared in Example 1 under specific strains generated by different mechanical external forces;

[0018] Figure 2 The scanning electron microscope images of the all-weather thermal management film with mechanically adjustable reflectivity prepared in Example 1 under specific strains generated by different mechanical external forces, ε-0% is uniaxial strain of 0%, ε-20% is uniaxial strain of 20%, ε-50% is uniaxial strain of 50%, and ε-80% is uniaxial strain of 80%;

[0019] Figure 3 The measured temperature difference curve of the outdoor cooling capacity of the all-weather thermal management film with mechanically adjustable reflectivity prepared in Example 1 under specific strains generated by different mechanical external forces, in the figure, ε-0% is unidirectional strain of 0%, ε-20% is unidirectional strain of 20%, ε-50% is unidirectional strain of 50%, and ε-80% is unidirectional strain of 80%. DETAILED DESCRIPTION

[0020] Specific implementation method 1: This implementation method is a method for preparing an all-weather thermal management film with mechanically adjusted reflectivity, which is specifically completed in the following steps:

[0021] 1. Preparation of TPU solution:

[0022] Dissolving thermoplastic polyurethane in an organic solvent to obtain a TPU solution;

[0023] 2. Use TPU solution as spinning solution, add a certain volume of spinning solution into a 5mL disposable syringe, fix the NO.22 blunt metal needle, connect the positive electrode to the metal needle and apply a certain high voltage, control the injection speed, receiving distance and applied voltage for electrospinning, and finally dry the film prepared by electrospinning to obtain an all-weather thermal management film with mechanically adjusted reflectivity.

[0024] Specific embodiment 2: This embodiment differs from specific embodiment 1 in that: the organic solvent in step 1 is N,N-dimethylformamide or a mixture of N,N-dimethylformamide and solvent I, wherein the volume ratio of N,N-dimethylformamide to solvent I is 1:1; the solvent I is acetone, ethyl acetate or tetrahydrofuran. The other steps are the same as those in specific embodiment 1.

[0025] Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that: the mass fraction of the TPU solution in step 1 is 12% to 20%; the volume of the spinning solution in step 2 is 1 mL to 4 mL. The other steps are the same as those in specific implementation method 1 or 2.

[0026] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that the mass fraction of the TPU solution in step 1 is 18%; the volume of the spinning solution in step 2 is 3 mL, and the reflectivity of the all-weather thermal management film with mechanically adjusted reflectivity reaches 96%. The other steps are the same as those in specific embodiments 1 to 3.

[0027] Specific implementation method 5: This implementation method is different from specific implementation methods 1 to 4 in that the injection speed in step 2 is 0.1 mm / min to 0.4 mm / min and the receiving distance in step 2 is 10 cm to 25 cm. The other steps are the same as those in specific implementation methods 1 to 4.

[0028] Specific embodiment 6: This embodiment differs from specific embodiments 1 to 5 in that the injection speed in step 2 is 0.2 mm / min, the fiber diameter of the all-weather thermal management film with mechanically adjusted reflectivity is 0.5 to 1 μm, and the receiving distance in step 2 is 20 cm. The other steps are the same as those in specific embodiments 1 to 5.

[0029] Specific embodiment 7: This embodiment differs from specific embodiments 1 to 6 in that the applied voltage in step 2 is 10 kV to 16 kV. The other steps are the same as those in specific embodiments 1 to 6.

[0030] Specific embodiment 8: This embodiment differs from specific embodiments 1 to 7 in that the applied voltage in step 2 is 12 kV, the drying temperature in step 2 is 100° C., and the drying time is 24 hours. The other steps are the same as those in specific embodiments 1 to 7.

[0031] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that the ambient temperature of the electrospinning in step 2 is 40° C. and the humidity is 25%. The other steps are the same as those in specific embodiments 1 to 8.

[0032] Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that: a unidirectional external force is applied to the all-weather thermal management film with mechanically adjustable reflectivity, and the wrinkled part of the film is unfolded to achieve a gradient change in its reflectivity on the same film; the unidirectional strain generated by the unidirectional external force is 0 to 80%. The other steps are the same as those of specific embodiments 1 to 9.

[0033] The following examples are used to verify the beneficial effects of the present invention:

[0034] Embodiment 1: A method for preparing an all-weather thermal management film with mechanically adjustable reflectivity is specifically completed by the following steps:

[0035] 1. Preparation of TPU solution:

[0036] Thermoplastic polyurethane was dissolved in an organic solvent and mechanically stirred in an oil bath at 40°C for 12 hours to obtain a TPU solution;

[0037] The organic solvent described in step 1 is a mixture of N,N-dimethylformamide and acetone, wherein the volume ratio of N,N-dimethylformamide to acetone is 1:1;

[0038] The mass fraction of the TPU solution described in step 1 is 18%;

[0039] Second, 3 mL TPU solution was used as the spinning solution, a certain volume of spinning solution was added into a 5 mL disposable syringe, a NO.22 blunt metal needle was fixed, the positive electrode was connected to the metal needle and a certain high voltage was applied, the injection speed, receiving distance and applied voltage were controlled for electrospinning, and finally the film prepared by electrospinning was dried at 100 ° C for 24 h to obtain an all-weather thermal management film with mechanically adjusted reflectivity;

[0040] The injection speed described in step 2 is 0.2 mm / min;

[0041] The receiving distance described in step 2 is 20cm;

[0042] The applied voltage described in step 2 is 12 kV;

[0043] The environment temperature of the electrospinning in step 2 is 40° C. and the humidity is 25%.

[0044] 3. Apply a unidirectional external force to the all-weather thermal management film with mechanically adjustable reflectivity, wherein the unidirectional strain generated by the unidirectional external force is 80% strain, and test its reflectivity, emissivity and cooling capacity.

[0045] Example 2: The difference between this example and Example 1 is that in step three, a unidirectional external force is applied to the all-weather thermal management film with mechanically adjusted reflectivity, and the unidirectional strain generated by the unidirectional external force is 70% strain. The other steps and parameters are the same as those in Example 1.

[0046] Example 3: The difference between this example and Example 1 is that in step three, a unidirectional external force is applied to the all-weather thermal management film with mechanically adjusted reflectivity, and the unidirectional strain generated by the unidirectional external force is 60%; the other steps and parameters are the same as those in Example 1.

[0047] Example 4: The difference between this example and Example 1 is that in step three, a unidirectional external force is applied to the all-weather thermal management film with mechanically adjusted reflectivity, and the unidirectional strain generated by the unidirectional external force is 50% strain. The other steps and parameters are the same as those in Example 1.

[0048] Example 5: The difference between this example and Example 1 is that in step three, a unidirectional external force is applied to the all-weather thermal management film with mechanically adjusted reflectivity, and the unidirectional strain generated by the unidirectional external force is 40%; the other steps and parameters are the same as those in Example 1.

[0049] Example 6: The difference between this example and Example 1 is that in step three, a unidirectional external force is applied to the all-weather thermal management film with mechanically adjusted reflectivity, and the unidirectional strain generated by the unidirectional external force is 30%; the other steps and parameters are the same as those in Example 1.

[0050] Example 7: The difference between this example and Example 1 is that in step three, a unidirectional external force is applied to the all-weather thermal management film with mechanically adjusted reflectivity, and the unidirectional strain generated by the unidirectional external force is 20%; the other steps and parameters are the same as those in Example 1.

[0051] Example 8: The difference between this example and Example 1 is that in step three, a unidirectional external force is applied to the all-weather thermal management film with mechanically adjusted reflectivity, and the unidirectional strain generated by the unidirectional external force is a strain of 10%. The other steps and parameters are the same as those in Example 1.

[0052] Example 9: The difference between this example and Example 1 is that in step three, a unidirectional external force is applied to the all-weather thermal management film with mechanically adjusted reflectivity, and the unidirectional strain generated by the unidirectional external force is 0% strain. The other steps and parameters are the same as those in Example 1.

[0053] The test data of the all-weather thermal management films with mechanically adjusted reflectivity prepared in Examples 1 to 9 are shown in Table 1;

[0054] Table 1

[0055]

[0056] From the results shown in Table 1, it can be seen that the present invention can obtain a radiation cooling-based fiber film with adjustable reflectivity under simple mechanical external force by selecting a suitable polymer elastomer through electrospinning technology. In the cooling mode, the fiber film exhibits a strong reflectivity of 95.60% at a wavelength of 0.25-2.5μm and an infrared emissivity of 92.6% at a wavelength of 8-13μm; the cooling power is 118.25W m -2 When the film is deformed by mechanical tension, its reflectivity drops to 61.4%. Due to the intrinsic infrared absorption capacity of TPU material, its emissivity drops to a smaller extent (78.6%). At this time, the solar heating power can reach 220.34W m -2 .

[0057] Figure 1 Spectra of reflection and emissivity of the all-weather thermal management film with mechanically adjustable reflectivity prepared in Example 1 under specific strains generated by different mechanical external forces;

[0058] from Figure 1 are the reflectance and emissivity spectra measured at different strain levels, from Figure 1 It can be seen that between 0-20% strain levels, the reflectivity decreases slowly and remains above 91%. For larger strains, the reflectivity decreases more or less linearly at 80% strain to 61%. In contrast, due to the inherent high infrared absorption of TPU, the infrared emissivity of the Es-TPU film in the atmospheric window decreases more slowly and remains around 80% throughout the applied strain range.

[0059] Figure 2 The scanning electron microscope images of the all-weather thermal management film with mechanically adjustable reflectivity prepared in Example 1 under specific strains generated by different mechanical external forces, ε-0% is uniaxial strain of 0%, ε-20% is uniaxial strain of 20%, ε-50% is uniaxial strain of 50%, and ε-80% is uniaxial strain of 80%;

[0060] from Figure 2 It can be seen that the strain applied to the membrane causes the nanofibers to straighten along the deformation direction and bend in the perpendicular direction; at the same time, the deformation is associated with a reduction in membrane thickness and an increase in pore size, which leads to a lower area density of nanofibers, which reduces the available scattering interface, resulting in greater transmittance. Above the 50% strain level, large pores with sizes close to the thermal infrared wavelength begin to appear, which also explains the gradual decrease in thermal emissivity observed above this strain.

[0061] Figure 3The measured temperature difference curve of the outdoor cooling capacity of the all-weather thermal management film with mechanically adjustable reflectivity prepared in Example 1 under specific strains generated by different mechanical external forces, in the figure, ε-0% is unidirectional strain of 0%, ε-20% is unidirectional strain of 20%, ε-50% is unidirectional strain of 50%, and ε-80% is unidirectional strain of 80%.

[0062] from Figure 3 It can be seen that: when the strain of the film is 0%, the temperature drops by 10°C from 12:00 to 13:00, showing good cooling performance; after 15:00, the irradiance decreases and the relative humidity increases, resulting in a weakened cooling capacity; as the degree of deformation increases, the cooling capacity of the TPU film gradually decreases, and the light transmitted by the solar energy will be converted into heat; when the strain is 50%, the temperature of the substrate covered by the film is almost the same as the ambient temperature, that is, the cooling capacity of Es-TPU-50% is offset by the transmitted sunlight. At higher strain levels, it was finally observed that the maximum temperature of the film with a strain of 80% increased by 9.5°C.

Claims

1. A method for preparing an all-weather thermal management film with mechanically adjustable reflectivity, characterized in that A unidirectional external force is applied to the all-weather thermal management film with mechanically adjustable reflectivity prepared by the preparation method, and the wrinkled part of the film is unfolded, so as to realize a gradient change of its reflectivity on the same film; the unidirectional strain generated by the unidirectional external force is 0-80%; continuous in-situ control is realized by applying different degrees of mechanical external force, and the thermal management capability of cooling-heating continuous switch is realized based on the wide range of reflectivity modulation capability; the all-weather thermal management film with mechanically adjustable reflectivity has radiation cooling capability, and is a radiation cooling film integrating high reflectivity and high emissivity; the film can perform dynamic hot and cold cycles, and can continuously adjust the temperature, and exhibits a strong reflectivity of 95.6% at a wavelength of 0.25-2.5μm, and an infrared emissivity of 92.6% at a wavelength of 8-13μm; a cooling of up to 10°C is achieved in outdoor temperature measurement, and the cooling power is 118.25W m -2 When the film is deformed by mechanical tension, the reflectivity drops to 61.4%, achieving a maximum temperature rise of 9.5°C in outdoor temperature measurement, and the solar heating power can reach 220.34W m -2 ; The preparation method is specifically completed according to the following steps:

1. Preparation of TPU solution: Thermoplastic polyurethane was dissolved in an organic solvent and mechanically stirred in an oil bath at 40°C for 12 hours to obtain a TPU solution; The organic solvent described in step 1 is a mixture of N,N-dimethylformamide and acetone, wherein the volume ratio of N,N-dimethylformamide to acetone is 1:1; The mass fraction of the TPU solution described in step 1 is 18%; Second, 3 mL TPU solution was used as the spinning solution, a certain volume of spinning solution was added into a 5 mL disposable syringe, a NO.22 blunt metal needle was fixed, the positive electrode was connected to the metal needle and a certain high voltage was applied, the injection speed, receiving distance and applied voltage were controlled for electrospinning, and finally the film prepared by electrospinning was dried at 100 ° C for 24 h to obtain an all-weather thermal management film with mechanically adjusted reflectivity; The injection speed described in step 2 is 0.2 mm / min; The receiving distance described in step 2 is 20cm; The applied voltage described in step 2 is 12 kV; The environment temperature of the electrospinning described in step 2 is 40°C and the humidity is 25%; 3. Apply unidirectional external force to the all-weather thermal management film with mechanically adjustable reflectivity, reversibly adjust the reflectivity through mechanical stretching, and thus achieve in-situ solvent-free switching between radiative cooling and solar heating.

Images