Investigation of the protective ultraviolet absorbers in a space environment. I. Rate of evaporation and vapor pressure studies

Compounds which are effective in protecting materials from terrestial ultraviolet radiation may, in a high vacuum environment, suffer loss both by evaporation and by photochemical degradation from the shorter wavelength ultraviolet radiation. The rates of evaporation from a free surface in a high vacuum of the commercially available protective ultraviolet absorbers have been measured as a function of temperature. Vapor pressures of the solid absorbers were calculated from the rate of evaporation data. The equilibrium vapor pressures of the liquid absorbers were determined by a direct vapor spectrophotometric method. Absorbers containing more than one hydroxy group were found to be considerably less volatile than the monohydroxy compounds, probably because of intermolecular hydrogen bonding of the former.