Experimental Measurement of the Transportation Properties of Automotive Cataphoretic Paints

Total automotive paint systems generally include three different coating layers: a cataphoretic primer, a sealer, and a top coat. The top coat gives the final aesthetic to the system, and the cataphoretic primer is supposed to be the ultimate protective layer against corrosion as well as the adhesion promoter, being directly in contact with the metal (or with the surface-treated metal). For this reason, it is very important to control the diffusion rate, through the cataphoretic primer, of poisonous species such as chloride or cathodic delamination initiators; i. e. oxygen and water. Although numerous studies have been reported on the corrosion mechanisms affecting automotive body panels, (either primer coated or fully painted), little has been done with regard to the transportation properties of the different paint layers concerning species such as oxygen and water vapour that are usually considered to be rate determining in the degradation of those systems. We have attempted, in a first stage, to measure experimentally the transportation kinetics of such species in cataphoretic primers. To this aim, free films have been made with different compositions in terms of pigment/filler ratio. The influence of the curing temperature has also been investigated. The measurements of water vapour and oxygen permeabilities, together with Electrochemical Impedance Spectroscopy of the free films yields a very sensitive discrimination between the different systems and shows a good correlation with the results of other quality control testing carried out by the paint manufacturers.