Improving the performance of thermochromic coatings with the use of UV and optical filters tested under accelerated aging conditions

The role of the materials used in the urban fabric is highly important for the microclimate and the energy loads of the cities. This has led to the research and development of innovative solutions with advanced thermal and optical properties. Thermochromic coatings that are reflective in summertime and absorptive in wintertime can address the demand for lower surface temperatures and energy savings. The function, however, of thermochromic materials in outdoor spaces is complex as they degrade when interacting with solar radiation. Various techniques with UV absorbers and UV filters prove that ultraviolet radiation is not the only parameter that affects thermochromism. This study aims to go one step forward and investigate the factors that destroy the reversible thermochromic circle besides UV radiation. Combinations of UV and optical filters were used on thermochromic coatings applied on concrete tiles under accelerated aging conditions of a one-month period to isolate the parts of solar spectrum that cause the photodegradation. The measurements of reflectance and color prove that the use of UV filter did not improve significantly the dark phase of thermochromic effect, while at the white phase, SR was reduced by 5% and SRvis was reduced by 9.7% compared with the uncovered sample. Covering the sample with red filter, which cuts off wavelengths below 600nm, protects most efficiently the reversible color change of the thermochromic coating as the solar reflectance at the dark phase remains unaffected during the whole experimental period.