Daytime radiative cooling semi-transparent roof tile coating from eco-friendly limestone to bio-based hydroxyapatite for tropical environment.

Daytime passive radiative cooling (DPRC) has attracted considerable interest as an energy-efficient strategy to achieve temperature reduction by reflecting incoming solar radiation while emitting thermal radiation. In this work, we developed a semi-transparent (light transmittance = 71.4% ± 0.8%) waterborne polyurethane/nano-micro calcium carbonate composite (WPU/CaCO3) with 60 wt% loading via ball milling for heat reflection. Later, CaCO3 was replaced with a bio-based hydroxyapatite (HAp) with 50 wt% loading as a durable and sustainable pigment for roof tile coating. The optimized coatings with CaCO3 and HAp were applied on the fiber cement roof tiles, thereby achieving a temperature reduction compared with an uncoated roof tile of approximately 5.3°C and 9.6°C under direct sunlight, respectively, without yellowing appearances after sun exposure for both cases. Additionally, coating on other outdoor materials has been demonstrated. Both WPU/CaCO3 and WPU/HAp show strong potential as a sustainable solution for radiative cooling semi-transparent paints.