Progress in Organic Coatings 56 (2006) 234–239 Cathodically electro-deposited carbon-black-epoxy composite films as primer in two-coat electro-deposition Saeed Rastegar a,∗ , Zahra Ranjbar b a Farayand Rang Khodro Co. Ltd., Automotive Electro-coat Department, No. 33, 16th Avenue, Bucharest Street, Argentine Square, Tehran, Iran b Surface Coatings and Corrosion Department, Iran Color Research Center, No. 58, 196 Street, Tehranpars, Tehran, Iran Received 10 January 2006; accepted 30 May 2006 Abstract Electrochemical impedance spectroscopy was used to study the electrical properties of cathodically electro-deposited films containing conductive carbon black. Suggesting an electrochemical equivalent circuit for the deposited films, it was found that the current is transferred through the conductive paths which are extended throughout the films. The content in conductive carbon black was found to have a great impact on the electrical properties of the conductive films. Increasing the carbon black content of the films, also leads to a decrease in the amount of the dissipated electricity during the second deposition. © 2006 Elsevier B.V. All rights reserved. Keywords: Cathodic electro-deposition; Conductive carbon black; Two-coat electro-deposition 1. Introduction It is for a long time that carbon-black-polymer composites serve as electro-conductive materials in different applications [1]. By increasing the content of the conductive carbon black (CCB) in the polymeric matrix, the insulating polymer is first made anti-static, and then, at higher contents of CCB a conduc- tive composite forms. Electro-deposition (ED), as a unique application technique, has proved its advantages and has forced a lot of manufacturers to install ED lines in their paintshops [2–4]. This is mainly due to a unique characteristic of this technique, i.e. the throwing power. This property enables the coating material to penetrate into recessed area of complicated articles and to form corrosion- resistant films after baking. The main feature of this application method is its very high (nearly 95%) transfer efficiency. In an automotive paintshop, normally, the first paint layer is applied by cathodic electro-deposition (CED). The next layers are applied by air- or high-speed rotary atomizer sprays. The spraying method of application has a much less transfer effi- ciency than ED. This means that applying either of the next layers by ED would have economical benefits over the normal ∗ Corresponding author. Tel.: +98 912 1404708; fax: +98 21 88734377. E-mail address: sdrstgr@yahoo.com (S. Rastegar). spray application. Of course, it will also exert some limitations regarding color change. Anyhow, this technology is nowadays commercialized and is used industrially in a few industrial-scale lines. Regardless of the direct technological importance of the conductive electrocoats (CEC), the study of their electrical con- ductivity behavior is of fundamental importance. Obtaining a more detailed insight into the electricity transport mechanisms in these films could help us to design more efficient CECs. Sev- eral authors have studied DC and AC electrical properties of carbon-black-polymer composites at low and high fields, in wet and dry states [5–12]. But there is no systematic study relat- ing the electrochemical properties of the carbon-black-polymer films to their behavior in a second CED step. In this research, we have used, for the first time, the elec- trochemical impedance spectroscopy to investigate the elec- trochemical properties of the baked CECs and relate these properties to their characteristic behavior in the second electro- deposition step. We have also used image processing techniques to visualize the growth of the second coat as a function of time. 2. Experimental 2.1. Materials and procedures The epoxy dispersion was a commercial grade epoxy resin from DuPont Performance Coatings. Printex XE2 was used as 0300-9440/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2006.05.009