Preparation Of Polyurethane-Based Low-Infrared Emissivity Multifunctional Composite Coating And Its Performance

To ensure the normal operation of aircraft radome,domestic and foreign aircraft researchers often coat the airborne radome with multifunctional protective coatings with antistatic,wave-transparent,and rain-erosion resistant properties,so the preparation of multifunctional integrated airborne radome composite coatings has become a hot research topic at home and abroad.With the development of various stealth technologies in the direction of omnidirectional full spectrum,infrared stealth research for radomes is also receiving more and more attention from researchers at home and abroad.Generally,infrared stealth materials require high reflection and low absorption,while radomes often use wave-transparent materials with low reflection and high transmission properties,and the two performance requirements are mutually constrained,so a good infrared stealth material will harm the wave-transparent performance of the radome protective coating,and it is difficult to achieve compatibility between the two properties.Based on the lack of research related to infrared stealth in the radome part of aircraft at home and abroad,the preparation of composite coatings with wavetransparent and antistatic properties,good mechanical properties,and infrared stealth function is of great significance to the development of infrared stealth composite protective coatings for aircraft radome.Acrylic polyurethane(PU)has good chemical and physical properties,good weather resistance,high surface gloss and hardness,strong adhesion,and good mechanical properties.In composite coating materials,the adhesive is one of the important factors affecting the mechanical properties,infrared stealth performance,and dielectric properties of the coating.However,in the 8-14 μm infrared wavelength range,PU has an infrared absorption peak and a high infrared emissivity.The key problem to be solved in this article is how to reduce the infrared emissivity of the coating while maintaining compatibility with wave transmission and antistatic properties,as well as good mechanical and environmental resistance properties.The selection of fillers plays a leading role in achieving multiple functions of composite coating material.This study prepared composite coatings using acrylic PU as the matrix and floating aluminum powder,modified aluminum powder,short-cut antistatic carbon fiber(CF),and hollow glass microspheres(HGMS)as fillers.The effect of various fillers on the infrared stealth performance,wave transmission performance,antistatic performance,and mechanical performance of the composite coating was explored by adjusting the amount of filler added.This provides a reliable theoretical basis for preparing PU-based multifunctional composite coating materials and finding the most suitable filler ratio.Coatings with better comprehensive performance were selected for environmental resistance testing,and the experimental results showed that:(1)In the infrared stealth and wave-transparent properties,the floating aluminum powder plays a dominant role in improving the infrared stealth performance of the coating,while the opposite is true for the wave-transparent performance,between 10 wt.%～15 wt.%and around 12.5 wt.%,the infrared emissivity of the coating appears to exceed the hyperosmotic threshold with the decrease of the aluminum powder content,and the infrared emissivity of the coating is about 0.55 at this time;the antistatic carbon fiber will make the infrared stealth performance of the composite coating.When the amount of antistatic carbon fiber increases from 0 to 4 wt.%,the IR emissivity of the composite coating increases by no more than 0.05,and the changes in dielectric constant and loss angle tangent values do not exceed 0.5 and 0.01.Respectively,SiO2coated modified aluminum powder,although it will lead to a certain degree of increase in the IR emissivity of the coating,the addition of ortho Ethylene silicate(TEOS)added during coating increases the IR emissivity of the coating by about 0.05 per 1 ml,but under the condition that the amount of SiO2 coating is not too much(TEOS addition<4 ml),the dielectric constant and loss angle tangent of the coating decrease,which is very favorable to improve the wavetransparent performance of the coating;adding a small amount of HGMS has little effect on the IR stealth and wave-transparent performance of the coating,and adding 4 wt.%hollow microbeads,the IR emissivity of the coating only increased about 0.02,the coating dielectric constant decreased about 1,and the loss angle tangent value decreased about 0.1.(2)In terms of mechanical properties,among the fillers studied in this paper,all fillers harmed the mechanical properties of the coatings except for the Al powder with a small amount of SiO2 encapsulated on the surface,which had some enhancing effect on the mechanical properties of the coatings.When the content of Al powder increased from 0 to 20 wt.%,the tensile strength and elongation at break decreased from about 25 MPa and 350%to 19.68 MPa and 190.89%,respectively.For aluminum powder coated with a small amount of SiO2(TEOS addition≤2 ml),the tensile strength and elongation at the break of the coating showed a small increase.The effect of short-cut antistatic carbon fibers on the elongation at the break of the coating was particularly obvious,as the elongation at the break of the coating was about 200%without the addition of antistatic carbon fibers,and less than 50%with the addition of 4 wt.%antistatic carbon fibers;meanwhile,the tensile strength also decreased to some extent,from 19.68 MPa to 12.63 MPa.With the addition of HGMS in an amount≤4 wt.%,the tensile strength and the elongation at the break of the coating were reduced.The tensile strength and elongation at the break of the coatings showed a slight decrease of less than 0.5 MPa and 5%,respectively.(3)In terms of antistatic performance,antistatic carbon fiber is the main filler to improve the antistatic performance of the coating.When other conditions remain unchanged and the addition of short-cut antistatic carbon fiber increases from 0 to 4 wt.%,the surface resistivity of the composite coating decreases from>500 MΩ/□ to 5.2 MΩ/□.The floating aluminum powder can help the antistatic carbon fibers to build a conductive network,thus enhancing the antistatic performance of the coating.When other conditions remain unchanged and the aluminum powder content is increased from ≤5 wt.%to 20 wt.%,the surface resistivity of the coating decreases from>500 MΩ/□ to 5.2 MΩ/□.HGMS also contributes to the improvement of the antistatic performance of the composite coating,and its addition facilitates the short-cut antistatic When the content of aluminum powder is 20 wt.%and CF content is 3 wt.%,the difference of surface resistivity of the coating before and after the addition of HGMS is more than 150 MΩ/□.(4)To achieve the best overall performance,the most suitable filler ratios are 20 wt.%modified aluminum powder,4 ml of TEOS added for modification,3 wt.%short-cuts antistatic carbon fiber,and 4 wt.%HGMS.The IR emissivity of the prepared coating can be controlled at about 0.5,the ε at about 12,the tanδ at about 0.09,the surface resistivity of the coating can reach 7MΩ/□,and the mechanical properties of the coating will not be significantly reduced.(5)The polyurethane-based multifunctional composite coating prepared in this paper has good environmental resistance.Including good resistance to high and low-temperature alternation,water resistance,heat resistance,salt spray corrosion resistance,and resistance to hot and humid environment performance,in a variety of harsh environments without blistering or peeling phenomenon,and still have good to maintain a low infrared emissivity(rise of less than 0.05)and surface resistivity(rise of less than 1 MΩ/□).