Effect of IR transparency of the top polymer layer on low thermal IR emittance paint coatings

A two-layer model was developed to accurately represent low thermal IR emittance paint coatings. The coatings were formed by mixing aluminum flakes with polymeric binders. Because of the different densities of these two components, a two-layer structure was constructed with a pure polymeric layer on top and an inhomogeneous layer underneath. The absorption properties of the top polymeric layer hinder the LR reflectance of the coating. This two-layer model was used to quantitatively determine how IR transparency of the top polymer layer influences the reflectance of the coating. The bottom layer was considered to be a substrate with a specific diffuse reflectance, and the ray tracing method was used to calculate the reflectance of this polymer-coated, diffusing substrate system. The reflectance of the coating with a low absorption coefficient (0.01 mu m(-1)) top polymer (3 mu m thick) was reduced 50% compared with when a high absorption coefficient (0.1 mu m(-1)) top polymer was used. Measurements of coatings of chlorinated rubber and alkyd rubber on a diffuse gold substrate confirmed the reliability of the model predictions.