The design and calculation of the infrared properties for low emissivity coatings dependent on the structure and distribution of flake Al pigments

Flake Al pigment is the main component of infrared low emissivity coatings, which plays an important role in controlling the coating‘s infrared optical properties. To clarify this influence mechanism, the flake Al powder with controllable thickness and particle size were prepared by the method of physical vapor deposition combined with ultrasonic crushing. The effects of structure parameters for flake Al powder, such as thickness and particle size, on the surface morphology and infrared emissivity of the coating was systematically studied. The results show that the smaller particle size leads to poorer directional arrangement degree of flake Al powder and increased edge scattering. The thinner thickness results in more absorption probability from resin to infrared radiation, thereby decreasing the infrared reflectivity and increasing the infrared emissivity of coating. These experimental phenomena are effectively elucidated by the calculation of bidirectional reflectance distribution function based on the Comsol software. Moreover, the Kubelka layer model optimized by the rough surface model was used to explain and predict the correlation between the infrared emissivity of the coating and the particle size and thickness of the flake Al powder. Based on the measured surface roughness of the coating and the complex refractive index of the flake Al powder, the error between the experimental and calculated results is only 1.2 %. The calculated model predicts that the coating exhibits lowest infrared emissivity when the thickness of flake Al powder is 400 nm. This work holds a significant guidance for the subsequent preparation of infrared low-emissivity coatings.