Method for preparing spectrally selective polyethylene porous films using hot blade coating

Abstract

The application discloses a method for preparing a spectrum-selective polyethylene porous film by using hot blade coating. The polyethylene-paraffin mixed solution is uniformly coated on a preheated glass plate by using the hot blade coating method, and the paraffin phase in the film is replaced by n-hexane after cooling and solidification, so that a porous structure is formed. The hot blade coating is applied to the preparation of the porous aerogel by using the thermal-induced phase separation, so that the thickness of the porous film is greatly reduced, and the porous structure is retained. Since the polyethylene itself is transparent in the spectrum range and the thickness of the porous film is extremely thin due to the blade coating method, the porous film becomes a radiation refrigeration film with high visible light reflection and high infrared transmission. In addition, the production efficiency of the porous film is improved due to the simplicity of the blade coating method.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polyethylene porous membranes, and relates to a polyethylene porous membrane prepared by using a hot blade coating method. BACKGROUND

[0002] Polyethylene has the potential to be a good radiative cooling barrier material due to its almost no absorption in the atmospheric window (8-13 μm). Currently, ordinary commercial polyethylene cling film is often used to reduce the convective heat transfer of the radiative cooling device, and cannot play a role in reflecting and shielding sunlight. In recent years, the porous structure based on polymers has become the main way to optimize the solar reflectivity, and the microstructure similar to the wavelength of light wave causes interference with the wavelength of sunlight, thereby causing scattering. And by adjusting the morphology and size of the microstructure to optimize the reflectivity. Polyethylene aerogel based on thermal induced phase separation (TIPS) technology was first applied to the field of radiative cooling ([1] Leroy A, Bhatia B S, Kelsall C C, et al. High-performance subambient radiative cooling enabled by optically selective and thermally insulating polyethylene aerogel. Science Advances, 2019, 5(10): eaat9480.). In the case of 6mm thickness, it presents 92.2% net reflectivity in the entire solar spectrum, 79% transmittance in the atmospheric window and 28mW / mK thermal conductivity. And in the ideal case, it reaches a daytime cooling amplitude of 13℃. Therefore, the technologies of hot pressing, mold ([2] Zhang, J., et al. "A flexible film to block solar radiation for daytime radiative cooling." Solar Energy Materials and Solar Cells 225 (2021): 111029.) and 3D printing ([3] K Zhou, et al. "Three-Dimensional Printable Nanoporous Polymer Matrix Composites for Daytime Radiative Cooling." Nano Letters (2021).) based on thermal induced phase separation (TIPS) technology for preparing porous polyethylene materials are developed one after another, which proves the potential of polyethylene porous system radiative cooling.

[0003] Currently, polyethylene porous systems have been increasingly applied to the field of radiative cooling due to high solar reflectance and atmospheric window transmittance. However, there are still problems to be solved for this system. First of all, this porous system is mostly aerogel, and in recent years several methods for preparing thin films have been developed. However, the thickness limits the further improvement of infrared transmittance. For example, at the same mass fraction, the infrared transmittance of a 2.7 mm polyethylene aerogel is 25% higher than that of a 5.5 mm polyethylene aerogel ([4] Yang, M., et al. "A Bioinspired"Skin" with Cooperative Thermo-Optical Effect for Daytime Radiative Cooling." ACS Applied Materials & Interfaces (2020).). And, the lower thickness gives a wider range of choices for the mass fraction of polyethylene, unlike the extremely low mass fraction (~5%) required for traditional polyethylene aerogel, which can achieve very high solar reflectance and infrared transmittance. Recently, the method of mechanical mold hot pressing has reduced the thickness of polyethylene porous film to 320 μm, and achieved higher infrared transmittance (~87%) ([2] Zhang, J., et al. "A flexible film to block solar radiation for daytime radiative cooling." Solar Energy Materials and Solar Cells 225 (2021): 111029.). And by optimizing the surface morphology, it retains a very high solar reflectance. However, there is still a lot of room for improvement in its infrared transmittance, and the method based on mechanical hot pressing cannot be mass-produced continuously, which also limits the large-scale application of polyethylene porous films with spectral selectivity. SUMMARY

[0004] The present application aims to provide a method for preparing a polyethylene porous film. The present application applies the method of hot blade coating to the preparation of polyethylene aerogel, reduces the thickness, ensures the high reflectivity of the porous material, improves the atmospheric window transmittance of the porous material, and improves the production efficiency.

[0005] The technical scheme of the present application is as follows:

[0006] A method for preparing a polyethylene porous film by hot blade coating, comprising the following steps:

[0007] Step one, mix and heat high-density polyethylene and solid paraffin until completely dissolved;

[0008] Step two, transfer to a preheated heat-resistant glass plate;

[0009] Step three, the solution obtained in step two is scraped into a film and cooled to solidify;

[0010] Step four, the solidified film is peeled off and repeatedly extracted with n-hexane;

[0011] Step five, freeze-drying.

[0012] Preferably, in step one, the high-density polyethylene refers to polyethylene with a density greater than 0.95 g / cm 3 The mass percentage of high-density polyethylene and solid paraffin is 6-10:94-90, preferably 10:90.

[0013] Preferably, in step two, it is transferred to a preheated 90-150°C heat-resistant glass plate.

[0014] Preferably, in step three, the solution is scraped into a film using a wet film maker and cooled to solidify in an ice bath, with a film thickness of 1000 microns.

[0015] Preferably, in step four, n-hexane is repeatedly extracted in an ultrasonic cleaner.

[0016] Compared with the prior art, the present application has the following characteristics:

[0017] (1) The present application uses a scraping method to prepare polyethylene porous material, and adjusts the film thickness and pore structure by adjusting the ratio of polyethylene and solid paraffin and the thickness of scraping. The polyethylene porous film has excellent spectral selectivity, with a solar reflectance of 98% and the highest atmospheric window transmittance (92%) of the current radiation cooling polyethylene porous material.

[0018] (2) The present application uses solid paraffin as a polyethylene solvent, which can strengthen the wet film strength compared with using liquid paraffin as a solvent, and is convenient for later extraction into pores.

[0019] (3) The scraping method has simpler equipment requirements and better continuous manufacturing potential compared with the existing polyethylene porous material mold, hot pressing, 3D printing and other methods. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a flowchart of the present application hot scraping method for preparing polyethylene porous film.

[0021] Figure 2 is a schematic diagram of the optical microscope thickness of the polyethylene porous film prepared in Example 1.

[0022] Figure 3 is a schematic diagram of the scanning electron microscope microstructure of the polyethylene porous film prepared in Example 1.

[0023] Figure 4 is the polyethylene porous film prepared in Example 2 using liquid paraffin as solvent.

[0024] Figure 5 is the solar reflectance curve of the polyethylene porous film prepared in Example 1.

[0025] Figure 6 is the infrared transmittance curve of the polyethylene porous film prepared in Example 1.

[0026] Figure 7 is the daytime radiative cooling device diagram of the polyethylene porous film prepared in Example 1.

[0027] Figure 8 is the daytime radiative cooling curve of the polyethylene porous film prepared in Example 1. DETAILED DESCRIPTION

[0028] The present application will be further clarified by the following examples, which should not be construed as limiting the scope of the application. After reading the present application, those skilled in the art will be able to modify the application in various ways, and these modifications should be considered within the scope of the present application as defined by the appended claims.

[0029] In combination Figure 1 , the method for preparing a polyethylene porous film by hot blade coating according to the present application comprises the following steps:

[0030] Step 1: Mix and heat high-density polyethylene and solid paraffin until completely dissolved;

[0031] Step 2: Transfer the preheated heat-resistant glass plate;

[0032] Step 3: Blade coat the solution obtained in Step 2 into a film and cool and solidify;

[0033] Step 4: Peel off the solidified film and repeatedly extract with n-hexane;

[0034] Step 5: Freeze-dry.

[0035] The materials used in the following examples for preparing polyethylene porous films include: high-density polyethylene powder (HDPE Maoming Petrochemical HHM5502), solid paraffin (52-54℃), and extraction agent n-hexane. The equipment used includes: magnetic stirrer, hot plate, wet film preparation device, ultrasonic cleaner, and heat-resistant glass plate.

[0036] Example 1

[0037] The method for preparing a polyethylene porous film by hot blade coating is shown in Figure 1 , and the specific steps are as follows:

[0038] Step 1: Weigh out polyethylene powder and solid paraffin wax at mass percentages of 6:94, 8:92, and 10:90, respectively.

[0039] Step 2: After completely melting the solid paraffin wax in a magnetic stirrer at 80°C, slowly add the high-density polyethylene powder, raise the temperature to 150°C, and stir for 1.5 hours to prevent the powder from clumping and affecting the dissolution of the polyethylene.

[0040] Step 3: Preheat the heat-resistant glass plate and wet film preparation device at 150°C on the heating plate.

[0041] Step 4: Quickly transfer the completely dissolved polyethylene paraffin homogeneous phase to one end of a preheated glass plate, and use a wet film preparation device on the 1000μm side to coat it into a 1000μm thick wet film.

[0042] Step 5: Cool and solidify the wet film on the glass plate through the glass plate using an ice bath.

[0043] Step 6: After softening the solidified wet film by heating it on a heating plate at 80°C, peel it off and repeatedly extract it three times with hexane in an ultrasonic cleaner.

[0044] Step 7; Freeze-drying

[0045] The porous polyethylene membrane prepared in this embodiment significantly reduced the thickness of the porous polyethylene film material using a hot-coating method. Observation using an optical microscope revealed that its thickness was only 110 μm. Figure 2 ), and maintains a uniform porous structure, such as Figure 3 As shown. The reflection of the porous polyethylene film was measured using an integrating sphere. Figure 5 As can be seen, the polyethylene porous membrane prepared in Example 1 maintains a high reflectivity (~98%) within the solar spectrum. Furthermore, benefiting from its 110 μm thickness, its transmittance within the atmospheric window (8-13 μm) range is as high as 87-92%. Figure 6 By covering the emitter with the porous polyethylene film prepared in the example, it can be seen that at midday in midsummer with an air temperature of 35°C and a solar power of 950W, the emitter material covered with transparent polyethylene film experiences a temperature drop of nearly 10°C, which is 2°C lower than the air temperature. Figure 7 As shown in the figure. The above results indicate that the polyethylene porous membrane prepared by the hot-scraping method has excellent radiative cooling performance.

[0046] pass Figure 5 , Figure 6It can be seen that the polyethylene porous film with a mass fraction of 10% has better solar reflectivity and atmospheric window transmittance. In order to verify the radiation cooling capacity of the polyethylene porous film, a cooling test is carried out. The test device is a cavity body covered with polished aluminum plate outside the polystyrene foam to reduce the influence of solar and air convection on the cooling experiment (as shown in Figure 7 ). The emitter uses the commonly used polymer PDMS (spin-coated on the ESR reflective substrate) in the radiation cooling experiment. The thermocouple is located at the center of the back of the emitter to record temperature data, and the polyethylene porous film prepared in Example 1 is placed three centimeters above the emitter to isolate sunlight and transmit infrared radiation. This test was carried out on a six-story building in hot summer in Nanjing, and the cooling curve shown in Figure 8 was obtained.

[0047] Example 2

[0048] The preparation method of the polyethylene porous film by hot blade coating is as follows:

[0049] Step 1: weigh 1.5 g of high-density polyethylene powder and 13.5 g of liquid paraffin

[0050] Step 2: After completely melting the solid paraffin at a temperature of 80°C in a magnetic stirrer, slowly add the high-density polyethylene powder, increase the temperature to 150°C, and stir for 1.5 h to avoid powder agglomeration and affect the dissolution of polyethylene.

[0051] Step 3: Preheat the heat-resistant glass plate and wet film preparation device on the hot plate at 150°C.

[0052] Step 4: quickly transfer the completely dissolved polyethylene paraffin homogeneous phase to one end of the preheated glass plate, and use the 1000 μm side of the wet film preparation device to blade coat a 1000 μm thick wet film.

[0053] Step 5: cool the wet film on the glass plate in an ice bath through the glass plate.

[0054] Step 6: repeatedly extract the solidified wet film with n-hexane in an ultrasonic cleaning instrument for 3 times.

[0055] The polyethylene porous film prepared in this example is broken during the cleaning process and cannot form a film (as shown in Figure 4 ).

[0056] In summary, the polyethylene porous film prepared by hot blade coating method in Example 1 uses high-density polyethylene as the matrix and solid paraffin as the diluent, and the preparation of the extremely thin polyethylene porous film is completed by using a wet film coater on a preheated glass plate. As a radiation refrigeration barrier material, the polyethylene porous film prepared by the hot blade coating method of the present application realizes high reflectivity in the range of 0.3-2.5 μm and high transmittance in the range of 8-13 μm by introducing a porous structure similar in size to the wavelength of sunlight into the polyethylene matrix and ensuring an extremely thin thickness by the blade coating method. Due to the transparent nature of polyethylene itself in the entire spectral range, it has attracted widespread attention in the field of radiation refrigeration. Therefore, the hot blade coating method greatly reduces the thickness of the polyethylene porous film and reduces the requirements for the equipment for preparing the polyethylene porous material, and has greater potential for continuous large-scale manufacturing than the current traditional mold hot pressing method.

Claims

1. A method for preparing a spectrally selective polyethylene porous film by hot blade coating, characterized in that, Comprising the following steps: Step one, mix high-density polyethylene and solid paraffin and heat until completely dissolved; Step two, transfer to a preheated heat-resistant substrate; Step three, scrape the solution obtained in step two into a film and cool and solidify; Step four, peel off the solidified film and repeatedly extract with n-hexane; Step five, freeze-drying; Wherein the mass percentage of high-density polyethylene and solid paraffin is 6-10:94-90.

2. The method of claim 1, wherein, High density polyethylene means polyethylene having a density greater than 0.95 g / cm 3 3.

3. The method of claim 1, wherein, The mass percentage of high-density polyethylene and solid paraffin is 10:

90.

4. The method of claim 1, wherein, In step two, transfer to a preheated 90-150℃ heat-resistant glass plate.

5. The method of claim 1, wherein, In step three, use a wet film maker to scrape the solution into a film, and cool and solidify in an ice bath, with a film thickness of 1000 microns.

6. The method of claim 1, wherein, In step four, repeatedly extract with n-hexane in an ultrasonic cleaning instrument.

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