Durability of microporous hybrid silica coatings: Optical and wetting properties

Abstract

Bifunctional hybrid silica sol-gel coatings were synthesized, which showed both increased light transmittance and water-repellence. The as-prepared coatings of variable thickness (100–175 nm) were found to have low refractive index (1.250–1.275), high porosity (36%–42%), and maximum light transmittance up to 99.8%. Water-repellence of the surface was due to high water contact angle (90°) and low contact angle hysteresis (1–2°). Ellipsometric porosimetry and transmission electron microscopy measurements confirmed the microporous structure of the coatings. A systematic investigation for studying the durability and the deterioration of optical and surface properties was studied during aqueous soaking tests at different pH. During these experiments atomic force microscopy showed that the morphology of the surface did not change. The changes in the wetting properties were also confirmed by surface free energy measurements and an increase of the polar component was observed. The optical properties could be regenerated completely or partially after the soaking via heat treatment at 400°C. The light transmittance of the samples remained high after the soaking, most likely due to the swelling of the solid matrix. The light transmittance and the water-repellent properties of the coatings showed long-term stability up to a year of storage.