A novel perspective for reflective cooling composites: Influence of the difference between the effective refractive index of polymeric matrix and inorganic functional particles

Reflective cooling composites have been successfully developed and widely utilized in common life and production. Most of the previous works focused on the effect of reflective fillers with different crystal structures or sizes on the reflective performance of composites, whereas relatively few studies have studied the influence of polymeric matrix on the reflective performance. Hence, this study, applying theoretical analysis and experimental verification, investigated the impact of the difference between the refractive index (n) of the polymeric matrix and rutile titanium dioxide (TiO₂, n = 2.74) on the reflective cooling property. The theoretical analysis indicated that the n of single material is positively correlative with the reflective performance, as well as the biggish difference between the refractive index of the polymeric matrix and reflective fillers can significantly improve the reflectance of composites. Herein, the polymeric matrix was acquired by blending high-density polyethylene (HDPE, n = 1.59) and ethylene-co-octene elastomer (POE, n = 1.48), yet the addition of rutile TiO₂ remained the same content of 3 wt%. The experimental results show that the total solar reflectance of neat HDPE is three times higher than that of neat POE, while the total solar reflectance of the POE/TiO₂ composite increases by 3.85% in comparison of the HDPE/TiO₂ composite. Besides, the HDPE/POE samples without TiO₂ also have a higher reflectance compared with the neat HDPE and POE, and the reflectance of HDPE/POE increases with increasing POE content. The results of reflective performance verify the conclusions of theoretical analysis. Moreover, the reflectance of HDPE/POE/TiO₂ ternary system increases with the increase of POE content. Finally, the results of outdoor and indoor temperature tests are consistent with the results of reflective performance.