Robust and wear-durable coating based on halloysite nanotubes/polymer composite for passive daytime radiative cooling

Passive daytime radiative cooling (PDRC) is an efficient strategy to achieve cooling through spectrally selective emission and high solar spectrum reflection. Current preparation strategies for PDRC materials mainly involve doping inorganic micro-nanoparticles into polymers, but their solar reflectance is usually insufficient. Here, we designed a super-white inorganic coating on different substrates based on natural halloysite nanotubes (HNTs) through the doctor blade method. Polyvinyl alcohol (PVA) and water-based acrylic resin (AR) were introduced to increase the adhesion for obtaining robust inorganic coating. HNTs/PVA/AR composite coating shows excellent water resistance, wear resistance, and environmental tolerance, which is mainly due to the strong interfacial interactions between HNTs, PVA, and AR. Compared with the uncoated transparent plastic, the coated plastic exhibits a super-white appearance (whiteness of ∼95%) and maximum solar reflectance of ∼97%. The maximum temperature difference of the HNTs/PVA/AR coated and pristine copper sheet is15 °C under simulated sunlight. The PDRC property is related to the high surface roughness of the coating composed of disordered high-aspect ratio nanotubes and the high whiteness of the raw halloysite materials. The composite coating can tolerate pH from 1–13, temperature change (−60∼250 °C), and different salt environments. The composite coating increases thermal stability, flame retardancy, and giving protective effect of dye against UV degradation. In addition, the HNTs/PVA/AR coated plastic as packaging materials can maintain good freshness of different fruits under strong solar irradiation. The robust HNTs/PVA/AR composite coating with high PDRC properties shows promising application in food packaging, building energy conservation, metal anticorrosive coating, and sunscreen products.