Plasma-polymerized organosilanes as protective coatings for solar front-surface mirrors

Abstract

A plasma-polymerization technique has been investigated as a method to apply uniform, thin (micrometers) protective coatings to front surface mirrors employed in various solar applications. These coatings were also found to offer anti-static qualities and weather resistance. Specifically, the coatings were organosilicone polymers based on precursors such as vinyltrimethylsilane (VTMS), methyltrimethoxysilane (MTMOS), and vinyltrimethoxysilane (VTMOS). We found negligible changes in hemispherical and specular reflectance when these coatings were applied. Both silver and aluminum front surface mirrors were evaluated. Surface electrostatic charge decay measured following corona charging mirror surfaces suggest that coatings containing oxygen (MTMOS, VTMOS) had improved anti-static behavior compared to the nonoxygen containing polymer (VTMS). The initial charge decay on the oxygenated materials occurred in less than 1 min versus greater than half an hour for the VTMS coating. Weatherability tests conducted in a temperature-humidity cycling environmental chamber showed that the plasma-polymerized coatings provided excellent protection for aluminized mirrors (6 month test). Silvered mirrors were observed to degrade after 2 months, however, unprotected silver mirrors degrade in a matter of hours in this environment. Improved abrasion resistance was achieved for MTMOS coatings synthesized in an oxygen rich plasma when compared to the uncoated glass in a falling sand abrasion test (ASTM D968-51).