Theory of Dielectric Properties of Heterogeneous Substances Applied to Water in a Paint Film

Abstract

Abstract The water content of 12 paints on steel substrate, exposed to 0.1M sodium chloride at 25 C for 24 h, was calculated from impedance measurements and compared with gravimetric determinations. Brasher's empirical expression was compared with five mixture formulas derived from the theory of dielectric properties of heterogeneous substances by Boettcher, Bruggeman, Looyenga, Wagner, and Rayleigh. The 12 paints were based on two grades of red lead and iron oxide in epoxy, chlorinated rubber, alkyd, and linseed oil binders. The significance of the results for cyclic wet/dry paint tests according to (1) the difference between the water absorption and desorption rates of a single paint and (2) the testing of different types of paints in the same cycle test program are exemplified. A possible explanation is given for the poor correlation often found between the results of cyclic wet/dry tests and those obtained in practical applications.