Development of self-cleaning top-coat for cool roof

Our recent study has shown that the acrylic silicon polymer is useful to formulate self-cleaning topcoat which may maintain the thermal insulation effect of cool roof effectively. 
A 2K self-cleaning topcoat was formulated with a water-borne type acrylic silicon polymer. Its effect to maintain high solar reflectance was confirmed by outdoor exposure test in comparison with coatings having no self-cleaning function. The solar reflectance performance was well maintained regardless of installation angle, lightness of colour or pigment type. 
After outdoor exposure, drop of solar reflectance was found. It had clear correlation to change in lightness of colour for light coloured coatings. In the meantime, for dark coloured coatings, the above-mentioned correlation was relatively low. This suggests that the decline of solar reflectance was caused by dirt pick up rather than drop in the lightness of colour. 
The decline of solar reflectance, that is supposed to be caused by dirt pickup, started just after installation and it was saturated in 3 to 6 months. After a 6-months exposure, the solar reflectance of the self-cleaning topcoat dropped by about 2% while the subject decline of non-self-cleaning topcoat was about 11%. A simulation study was also done to compare the electricity consumption for air conditioning, which showed that saving of electricity consumption could be 10% more with the self-cleaning topcoat.