Effect of titanium dioxide (TiO2) with different crystal forms and surface modifications on cooling property and surface wettability of cool roofing materials

- The cool roofing materials show high NIR and solar reflectance.
- The cool roofing materials perform excellent heat-insulation property.
- The fabricated ASA/TiO2 hybrid materials have excellent weather resistance.
- A hydrophobic surface was achieved by adding hydrophobic modified TiO2.
- The materials all perform good mechanical properties to some extent.

The cool roofing materials were fabricated to solve overheating concerns from absorbed solar energy (Ultraviolet-Visible-Near infrared) and infrared thermal energy absorbed from the ambient. In this study, four types of titanium dioxide (TiO2), namely hydrophobic rutile nano-TiO2, hydrophilic anatase nano-TiO2, unmodified rutile TiO2 and unmodified anatase TiO2, were chosen to fabricate cool roofing materials due to their high solar reflectance and excellent actual heat-insulation properties. Matrix is poly (acrylonitrile-styrene-acrylate) (ASA) resin for its excellent weather resistance. The reflectance was measured by an Ultraviolet-Visible-Near infrared (UV-vis-NIR) spectrophotometer and actual heat-insulation properties by a self-designed device. Contact angle analysis was conducted by a contact angle meter. When hydrophobic rutile nano-TiO2 particles (5 wt%) introduced, the ASA/TiO2 hybrid material possesses a reflectance of 45.2% throughout NIR and 59.4% throughout the whole solar spectrum. The thermal emissivity in the region of 8-13 µm is 0.87, as well as 0.86 in the region of 2.5-15 µm. The high solar reflectance and high thermal emissivity lead to cool materials with excellent cooling property. Simultaneously, significant drop in temperature shows excellent cooling property in comparison with unloaded ASA resin. Specifically, a maximum decrease of 34 °C can be observed in the indoor temperature test using a solar simulator, and a decrease of 10 °C can be achieved when tested outdoors under natural solar radiation. In particular, the contact angle of the sample added with hydrophobic rutile nano-TiO2 particles is 103°, which forms a hydrophobic surface. In addition, the cool roofing materials fabricated in this study demonstrate excellent weather resistance, meeting the strict requirements for outdoor use.