Influence of chemical composition on solar reflectance and thermal emitance of cool materials

Abstract

Cool materials have higher values of solar reflectance and lower, although high, thermal emittance as compared with conventional coatings, thus heat less their surfaces. However, manufacturers mistakenly denominate them insulating, because despite low thermal conductivity of paint constituents, their thickness is very thin, which does not exceed a few millimeters, and is not enough to prevent heat transference by conduction. Therefore, physical phenomena responsible for surface temperature reduction in these materials are the superficial radiative properties (solar reflectance and thermal emittance), which are not available by manufacturers. Therefore, it aims to identify chemical elements present on cool surfaces that influence solar reflectance and thermal emittance. For this, spectral reflectance and thermal emittance were measured with methods and equipment standardized by Cool Roof Rating Council (CRRC) of 23 cool elastomeric materials selected from Brazilian market and surfaces’ micrographs were analyzed with EDS (dispersive energy x-ray detector) coupled to field emission scanning electron microscope. Results indicated that surfaces with matte surface finish are more reflective than semi-gloss ones and these more than high-gloss ones because of lower amounts of carbon and higher of oxygen, which was demonstrated by strong statistical correlation at 0.1% probability and visualized micrographs differences. Therefore, higher concentrations of titanium dioxide pigment, more reflective to infrared spectrum, did not reach higher reflectance. Furthermore, metallic element (titanium) presence in greater amounts on surfaces was not determinant for lower thermal emittance values.