Evaluating thermal and durability performance of acrylic cool colored coatings containing NIR-inorganic reflective pigments for roof coating application

Abstract

Cool coatings applied on top of roof panels have been studied for decades as an approach to mitigate the urban heat island effect. Prior research has mostly focused on analyzing the aesthetical and thermal properties of the coating material at initial state without considering how these parameters are affected by exposure to the outdoor environment. This thesis proposes an experimental approach to create and estimate thermal, aesthetical and durability performance of cool coatings. Three different so-called near infrared (NIR) inorganic reflective pigments were added to an acrylic based transparent paint and applied on metal substrates to give rise to a cooling and color sensation (black, yellow and a semitransparent color). In addition, different concentration of the NIR inorganic reflective pigments were added to the paint formulations to see its effect on the mentioned parameters. The cool coatings were subjected to accelerated UVB radiation (for a total of 480 h, 96h/cycle) to evaluate its effect on the mentioned parameters. A characterization study of the pigments and the formed cool coatings in terms of morphology, chemical composition, phase structure and dispersion quality were conducted using powder X-ray diffraction (PXRD) and scanning electron microscopy (SEM) assisted with electron X-ray spectroscopy (SEMEDS). The aesthetical properties investigated for the cool coatings were limited to colour, in accordance with CIELAB colour coordination standard, and specular gloss measurements. The thermal behavior of the cool coatings was assessed in the near infrared region of light by subjecting the coating substrates to a 150 W IR-emitted lamp. The chemical degradation process of the coating material was assessed and monitored using FTIR-ATR. The results show that after 480 h of accelerated UVB exposure no significant alteration of the mentioned parameters was detected. No significant thermal variation of the cool coatings was observed regardless of NIR inorganic pigment type or concentration. The gloss decay and initial glossiness showed slight dependence on the type and concentration of pigment added to the paint formulation. Furthermore, no significant color alteration was observed for each cool coating sample. Lastly, there was no direct correlation for each cool coating sample that indicated that minor aesthetical alteration of coatings had a strong effect on the thermal properties of the coatings. Furthermore, the results also suggest that the total amount of time the cool coatings were exposed to UVB-light was insufficient to properly analyze how UVB degradation mechanism affected the mentioned parameters since only minor alteration of the surface chemistry and roughness of the cool coatings were observed.