Structure of waterborne coatings by electrochemical impedance spectroscopyand a thermostimulated current method: influence of fillers

Electrochemical impedance spectroscopy (EIS) was coupled with a thermostimu lated current (TSC) method to investigate the effect of fillers on the barr ier properties of an epoxy-polyamido amine waterborne coating on a 2024 alu minium alloy. Four systems were studied: a clear coat and three pigmented c oatings (with and without chromates). The results obtained by TSC highlight the specific action of chromates which decrease the molecular mobility (ha rdening) of the coating by comparison with neutral fillers. This has been e xplained by the high polarity of chromates that enhances interactions with the binder. Impedance measurements carried out for free-standing films and for coated aluminium alloy have corroborated the role of chromates on the b arrier properties of the coating which remain high as a function of exposur e time in a 0.5 M NaCl solution. TSC was also used to measure the glass tra nsition temperature (T-g) of the hydrated systems. T-g was significantly de creased when the coatings were hydrated due to a strong plasticization effe ct induced by water uptake. For the different systems, we conclude that the weak barrier properties of the films (attached films) or the decrease of t he barrier properties with time (free-standing films) were linked to the va riation of the glass transition temperature by water uptake. (C) 2000 Elsev ier Science S.A. All rights reserved.