Effect of titanium dioxide (TiO₂) on largely improving solar reflectance and cooling property of high density polyethylene (HDPE) by influencing its crystallization behavior

In this study, the different crystal forms of titanium dioxide (TiO ₂) were added into high density polyethylene (HDPE) to fabricate cool material. Crystal structure, crystallization behavior, crystal morphology were investigated by wide angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC) and polarized optical microscope (POM). Scanning electron microscope (SEM) was applied to observe dispersion of TiO₂ particles in the HDPE matrix and the cross section morphology. The solar reflectance and actual cooling property were evaluated by UV-Vis-NIR spectrometer and a self-designed device. By adding TiO₂ particles into HDPE matrix, the polymer chain could crystallize into more perfect and thermal stable lamella. The presence of TiO₂ particles dramatically increased the number of nucleation site therefore decreased the crystal size. The subsequent solar reflectance was related to the degree of crystallinity, the spherulite size of HDPE, refractive index, and distribution of TiO₂ particles in HDPE matrix. It was found the rutile TiO₂ could largely improve the total solar reflectance from 28.2% to 51.1%. Finally, the temperature test showed that the composites had excellent cooling property, which was in accordance with solar reflectance result.