Preparation And Radiation Cooling Properties Of Porous PVDF-HFP Aqueous Coating

With the consumption of fossil energy,the environment has been seriously polluted.The greenhouse effect affects all aspects of people’s work and life.People need a lot of energy for building cooling.With the concept of energy conservation and emission reduction,new clean energy is urgently needed to meet people’s needs.Radiation cooling technology is a process in which earth surface objects emit infrared radiation to the universe through the’atmospheric window’band（8-13μm）to achieve their own cooling.This technology can reduce the demand for fossil energy and has no pollution to the environment.It is a clean technology that does not require any external energy input.At present,the excellent radiant coolers,whether photonic structures or metamaterials,have the disadvantages of complex preparation process and high cost.The preparation process of polymer materials is simple and the cost is low,but it often requires a metal substrate to improve the reflectivity and cannot be flexibly applied to various surfaces.The solvent that dissolves the polymer is harmful to the environment and human body.Waterborne coatings do not require any volatile organic compounds and have good stability,water resistance and excellent flexibility.The graded porous structure can improve the reflectivity by adjusting the pore size distribution.This paper will focus on the preparation of PVDF-HFP water-based coatings with porous structure and the improvement of radiation cooling performance.The main research contents are as follows:（1）PVDF-HFP aqueous slurry and coating were prepared by ball milling and coating process.Based on stainless steel,the effects of ball milling speed,ball milling time,ball mill size,heat treatment conditions and coating thickness on the structure and optical properties of porous coating were discussed,and the optimal preparation process was obtained.The optimum preparation conditions of the coating were as follows:the mass ratio of PVDF-HFP,SDS and deionized water was 6 g:0.06 g:15 g,the ball mill was carried out with 5 mm diameter ball mill,800 rpm ball milling speed and 1 h ball milling time.The coating was treated at 40°C for 0.5 h.When the thickness of the coating is 420μm,the optical performance is the best.The reflectivity of the coating in the solar band is 96.7%,the atmospheric window emissivity is 95.8%,and the optimal outdoor cooling result is 18.5°C.（2）In order to improve the hydrophobicity of the coating,styrene-butadiene emulsion was added as a binder on the basis of the previous test.The effect of the amount of styrene-butadiene emulsion on the hydrophobicity and radiation cooling performance of the coating was studied.It was found that when the mass ratio of acrylic resin emulsion to styrene butadiene emulsion was 1:3,the water contact angle was136°,and at 800 W/m2,the coating had a cooling effect of 12°C.The UV-heat aging resistance of the coating was tested.After 40 h UV irradiation,30 h rain and condensation for 30 h,the reflectivity of the coating decreased in the 0.3-0.6μm band.Finally,the applicability of the coating substrate was explored.It was found that the coating had good coating properties on stainless steel substrates,PE and cement substrates,and had good solar band reflectivity,but the hardness and adhesion of the coating were poor.（3）In order to improve the adhesion between the coating and the substrate and the hardness of the coating,a radiation cooling composite coating was prepared with Al₂O₃ and Ba SO4 micro-nano particles as fillers.The effects of different contents of Al₂O₃on the properties of the coating were investigated.The optimal content of Al₂O₃was 35 wt.%,and the shore hardness of the coating increased from 40 HD to 55 HD.When the thickness of the coating is 260μm,the coating can achieve a cooling effect of about 13°C when the winter irradiance is not very strong.The addition of Ba SO4particles can improve the reflectivity of the coating in the near ultraviolet region.When Al₂O₃ and Ba SO₄are used in combination,the optimal molar ratio is 1:1,and the cooling temperature is 8°C in winter.And the hardness of the coating is improved.Adding a small amount of commercial white paint to the self-made coating can improve the bonding force between the coating and the substrate,and the self-made coating has a better outdoor cooling effect than the commercial white paint on the market,which is about 4°C higher.