Electrocoagulation of waterborne polymers in the presence of carbon black

Electrodeposition of paint is an electrochemical process, starting from water-borne polymers. The electrode reactions are water decomposition, leading to diffusion layers of an extreme pH. Coagulation of polymers proceeds in these layers as an acid / base reaction rather than as a direct discharge of macroions at the electrodes. The electrodeposited film behaves like a weak acid or alkaline ion exchanger. Under polarization, high electric field strength leads to charge separation and the formation of a double space charge. Current / voltage-behavior is nonlinear. The mechanism of charge transport is purely ionic. Resistivities are in the order of p = 10⁸ Ohm·cm in the wet film. The stoved film is an insulator. Electrodeposition of a second film is impossible. However, electrodeposition of the primary film in the presence of dispersed carbon black, studied in detail for the first time, leads to an appreciable electronic conductivity in the stoved film. ρ is in the order of 10⁵ - 10⁷ Ohm·cm even with a carbon black concentration of only a few wt.-%. This has been explained in terms of a stick percolation model and the preferential formation of transversal carbon black chains in the film. The relatively high resistivities found along these chains is due to the need of electron tunneling through ultrathin polymer films around the polymer wetted individual carbon black particles. Temperature behavior of resistivities, field effect and other proofs are given for this model. For practical application, the most important findung is the possibility of an electrodeposition of a coherent second layer and even of multilayers in the presence of only 1 - 5 wt.-% of an appropriate carbon black filler.