Ultraviolet, water, and thermal aging studies of a waterborne polyurethane elastomer-based high reflectivity coating

Abstract

• High reflectance polyurethane coating was studied for optical reflectance purposes. • The coating was aged independently under UV, water, and thermal cycling conditions. • Both chemical and physical changes were monitored to identify the aging mechanism. • Steady drop in diffused reflectivity was observed during the aging. • Chemical and physical aging correlated to the reflectance drops during the aging. A waterborne aliphatic polyurethane-based coating was studied for accelerated ultra-violet (UV), water (WT), and thermal (TH) aging for a period of 1000 h. To monitor the coating durability, samples were tested every 200 h. ATR-FTIR spectroscopy was used to monitor the chemical changes occurring during the aging process. UV–vis with integrating sphere was used to track the change in diffused reflectance, while the optical microscope and the scanning white light interferometry (SWLI) were used for surface characterization. FTIR studies of coatings subjected to UV exposure indicated a decrease in functional groups such as CO NH, C H, C O, and C O C. The appearance of functional groups such as N H is attributed to chain scission of the polyurethane binder in the coating. Investigation of the degradation mechanism in water and thermal aging showed physical effects through water penetration and the mismatch in the coefficient of thermal expansion as the primary causes of degradation. In all aging scenarios, the reduction of reflectivity was largely due to physical defects caused by the different aging mechanisms.