Water permeability of organic/inorganic hybrid coatings prepared by sol-gel method: a comparison between gravimetric and capacitance measurements and evaluation of non-Fickian sorption models

Acrylic-TiO2 coatings prepared from alkoxide precursors were deposited on mild steel. The coatings were transparent and continuous at thicknesses up to 200 μm. The water uptake characteristics were investigated by EIS measurements in 3 wt% NaCl solutions. The non-ideal diffusion is analysed considering a superposition of Case II swelling and Fickian sorption. The physical meaning of the coefficient (SCc) that is linear in time is discussed. A comparison with the relaxation model of Berens and Hopfenberg notably shows that SCc can numerically account for relaxation phenomena in the case when the rearrangements in the polymer structure are so slow that they exceed the measuring time. EIS results are compared with gravimetric measurements on free films. The influence of titania content on the water uptake properties (solubility, diffusion coefficient and long-term behaviour) of the hybrids has also been studied.