The effect of polyhedral oligomeric silsesquioxane dispersant and low surface energy additives on spectrally selective paint coatings with self-cleaning properties

Thickness-Insensitive Spectrally Selective (TISS) paint coatings were made of fluoropolymer resin binder (Lumiflon (LF), Asahi Glass Co., Ltd., Japan) and their water- and oil-repellent properties were obtained (contact angle: θwater~150°, θn−hexadecane~55°) by the addition of Polyhedral Oligomeric Silsesquioxane (POSS) characterized by amino (AP), isooctyl (IO) and perfluoro (PF) groups (i.e. AP2IO4PF2) attached on the silsesquioxane cube (–SiO3/2)8. Paint dispersions were made by modifying of black spinel pigment with trisilanol isobutyl (IB7T7(OH)3) POSS dispersant and with a single-capped silane isobutyltrimethoxysilane (IBTMS). Infrared and 29Si NMR spectra measurements were used for the identification of the structural characteristic of the corresponding POSS compounds. Surface free energy values of the pure cross-linked (no pigment added) LF binder which was determined from the measured contact angles for water, diiodomethane and formamide revealed the γtot value of the LF unpigmented resin with added AP2IO4PF2 T8 POSS was about 16.4 mN/m, which led to the θwater~110°, indicating enhancement of the hydrophobicity of the pure LF resin binder (θwater~89°).SEM micrographs, which were used for the assessment of the TISS paint coating surface morphology confirmed the beneficial dispersive effect of IB7 T7(OH)3 dispersant as compared to the IBTMS silane. The presence of large Aluminium flake pigment and finely ground black pigment led to the formation of TISS paint coating surface, which exhibited bi-hierarchical roughness, resulting in water sliding angles of less than 10°, indicating the possible self-cleaning effect (θwater>150° and θn−hexadecane~55°). The use of POSS dispersant and POSS low surface energy additive also enhanced spectral selectivity, which became the highest known so far for the black TISS paint coatings (eT~0.24 and as~0.93). The use of simple silane IBTMS dispersant even in the presence of the AP2IO4PF2 T8 POSS did not lead to the oil-repellent effect confirmed by spreading of n-hexadecane on the surface of the corresponding TISS paint coating.