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

Electrochemical impedance spectroscopy (EIS) was coupled with a thermostimulated current (TSC) method to investigate the effect of fillers on the barrier properties of an epoxy-polyamido amine waterborne coating on a 2024 aluminium alloy. Four systems were studied: a clear coat and three pigmented coatings (with and without chromates). The results obtained by TSC highlight the specific action of chromates which decrease the molecular mobility (hardening) of the coating by comparison with neutral fillers. This has been explained 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 barrier properties of the coating which remain high as a function of exposure time in a 0.5 M NaCl solution. TSC was also used to measure the glass transition temperature (T_g) of the hydrated systems. T_g was significantly decreased when the coatings were hydrated due to a strong plasticization effect induced by water uptake. For the different systems, we conclude that the weak barrier properties of the films (attached films) or the decrease of the barrier properties with time (free-standing films) were linked to the variation of the glass transition temperature by water uptake.