Effect of Nano Silica on Moisture Absorption of Polyurethane Clear Coats as Studied by EIS and Gravimetric Methods

Electrochemical impedance spectroscopy and gravimetric methods were carried out to analyze water uptake of thin polyurethane films separately filled with either hydrophobic or hydrophilic nano-silica. Correspondingly the impedance spectra and gravimetric data were interpreted in terms of movement of water within the coating and/or to the coating/metal interface. Although the obtained results from gravimetric and EIS (Electrochemical Impedance Spectroscopy) methods followed generally a same trend, the water uptake values were not exactly identical. Type and content of nano silica had significant effects on the capability of water absorption of thin filled coatings. Indeed, it was found that rate and maximum level of moisture absorption of polyurethane coatings strongly depended on the type and loads of nano-silica within the clear coat. To the other words, nano-particles with different nature affects on hydrophilicity and hydrophobicity of the organic matrix resulting in increasing or decreasing of water absorption of the resulted coating. Prog. Color Colorants Coat. 3(2010), 92-100. © Institute for Color Science and Technology. 1. Introduction Electrochemical impedance spectroscopy has been used by several authors for the determination of water content in organic coatings [1-3]. The reason for this interest lies on the fact that the capacitance of a coating is affected by penetration of water [4]. Many factors influence water absorption of polymeric coatings, such as temperature, geometry of exposed surface, polymer nature (permeable or impermeable) and diffusivity [5-7]. One of the most important mechanisms of water absorption in polymeric