A novel semi-empirical method to determine the internal reflectance of the polymeric coating layer in the mid-IR region

Abstract

This study presents a semi-empirical method to determine the internal reflectances of the polymeric coating layer in the IR waveband. An optical system including IR spectroradiometer, integrating sphere and blackbody source was also designed as the measurement system. The internal reflectance can be determined by solving the simultaneous equations of transmittance and reflectance derived by the multi-reflection model for a free standing coating film. The transmittance and reflectance of the free standing coating layer can be obtained by measuring the transmittance and reflectance of the coating/substrate (composite layers) system and then resolving it via the plane parallel plate model. Experimental results indicate that the internal reflectance of the free standing polymer film is negligibly small. However, the internal reflectance of the polymeric coating backed with an aluminum substrate is higher than that of the standing polymer film and corresponds to the internal transmittance. Observations involving polymer binders with different IR transparencies confirm the above events. The correlation between internal reflectances and internal transmittances can be used to predict the reflectances of polymeric coating systems accurately.