Water Transport in Organic Coatings

The electrochemical impedance spectroscopy (EIS) technique was used to evaluate the water transport (diffusion and equilibrium water uptake) and the dielectric properties of freestanding polyimide (Kapton^® and PMDA-ODA) and polyethyleneterephthalate (PET) membranes at 25 and 40°C, respectively, and in supported PMDA-ODA-coated metals. Permeability and diffusion coefficients of freestanding films were also obtained by using the Payne cup method and the MacBain quartz spring balance in order to assess the reliability of the EIS method when compared to other techniques. Results from this work show that the diffusivity of water in polyimide films varies from 1.42 x 10^–9 to 3.53 x 10^–9 cm² s^–1 for thickness between 2.4 and 125 μm, while the equilibrium water uptake varies from 2.31 to 4.63% by weight for the same range of thickness. The average calculated dielectric constant of the freestanding Kapton^® films is 4.5. Water diffusion coefficient in PET varies from 2.11 x 10^–9 to 12.61 x 10^–9 cm² s^–1 for thicknesses between 22 and 205 μm, while the equilibrium water uptake varies from 0.50 to 0.95% by weight for the same range of thickness. The average calculated dielectric constant of the freestanding PET films is 3.6. Transport properties’ results obtained through the electrochemical technique are in reasonable agreement with those obtained with the classical gravimetric method. Capacitance data obtained with the EIS technique were interpreted in terms of a model in which the electrical analog of the film is composed of either a parallel RC or of a series of parallel RC circuits. Thick films are better described in terms of a series of parallel RC circuits. Theoretical analysis presented in this paper indicates that transport properties of water through freestanding and applied films (paints) can be obtained from EIS (capacitance data) only when specific requirements are met.