Characterization of TSSS paint coatings for solar collectors by FTIR spectroscopy

Abstract

The background to the spectral selectivity of thickness sensitive spectrally selective (TSSS) paint coatings is discussed from the chemical point of view. This means considering the solar absorptance ( a s) and thermal emittance ( e T) of the TSSS paint coatings as a function of paint film thicknesses in terms of the degree of flocculation of the pigment dispersion and from the viewpoint of the relevant intermolecular interactions which influence the ability of the pigment particles to become distributed uniformly in the paint, thus increasing its colour strength. We investigated eight TSSS paint coatings based on the standard formulation of Solariselect ® paint [Sol. Energy Mater. 18 (1989) 97], which is currently used for solar collector panels. The influence of carbon soot (C - Degussa FW2), inorganic pigment (F - Ferro 6331) and fumed silica thixotropic agent (Aerosil - Degussa) in TSSS paint coatings on the degree of their flocculation was determined from the infrared hemispherical reflectivity R ( d) λ = 2.5 which was measured from the paint film thickness. The absorption coefficient for diffuse radiation ( K) and the scattering coefficient ( S) were determined by fitting the experimentally determined R ( d) λ = 2.5 curves to a modified Kubelka-Munk function derived by Körte and Otto [Appl. Spectrosc. 43 (1988) 38]. Correlations between the type of black pigment (F and C) and elementary particle size and surface activity of fumed silica (Aerosil 380, 200 and 972) were established and compared with the experimentally determined solar absorptance values ( a s) of the corresponding TSSS paint coatings. Since fumed silica in spite of its small content (<1% by weight) in the paint dispersion, decreases the gloss of the paint, the correlation between the RMS heights of irregularities (σ) and RMS slopes ( m) of irregularities derived from the infrared reflectivity measurements according to the procedure by Bennett and Porteus [J. Opt. Soc. Am. 51 (1961) 123] were determined. This revealed that the σ values are inversely proportional to the elementary particle size of the fumed silica used (Aerosil 380, 200, 972) in the TSSS paint coatings. FTIR spectra of Solariselect ® paints, the corresponding TSSS paint coatings and model dispersions prepared from the resin binder (phenoxy resin) and various types of Aerosils were measured, and the influence of the composition of the paint dispersions on the frequency shifts of -OH vibrations and intensity changes of skeletal modes of the resin binder were established. The changes of infrared bands observed revealed that resin binder molecules become more strongly bonded when fumed silica is present in dispersions. The correlation among the strength of the interactions in a dispersion, the degree of flocculation and σ values are given and discussed.