Characterisation of the barrier properties of different paint systems Part II.: Non-ideal diffusion and water uptake kinetics

Three commercial paint systems, based on water-borne acrylic, chlorinated rubber and alkyd resins, were used to evaluate the water uptake in 5% NaCl solution (wt.%) and partially deionised water. Impedance spectroscopy allows following the coating capacitance evolution and, from this, water uptake versus immersion time can be tracked. Its analysis leads to the conclusion that diffusion process seems to be an ideal Fickian one just in one type of paint. The two others have important deviations from this ideal behaviour. In those conditions, an apparent diffusion coefficient can be estimated. The study of the electrolyte flux with exposition time was analysed and fitted to an expression, which includes four exponential terms with four time constants, associated to different processes. Three time constants behave as exponential decay functions and the fourth one has an exponential growth shape. A physical meaning for the expression is suggested. The values of all the studied parameters indicate that the barrier properties of the water-borne acrylic paint are the worst. (C) 1999 Elsevier Science S.A. All rights reserved.