Cathodic electrodeposition of self-curable polyepoxide resins based on cardanol

Abstract

Cathodic electrodeposition (CED) behavior and film properties of coating binders as modified polyepoxide resins based on cardanol, a constituent of Cashew Nut Shell Liquid (CNSL), have been studied. These coating binders were synthesized by epoxidation of cardanol–formaldehyde novolacs (CNs) with epichlorohydrin and subsequently modified with secondary amine to make them water-thinnable by neutralization with acetic acid, and suitable for CED. Laboratory synthesized coating binders, modified, epoxidized cardanol–formaldehyde novolacs (MECNs), were cathodically electrodeposited on pretreated MS panels to investigate their electrodeposition behavior and film properties of deposited coating binders. Among the nine MECNs prepared by varying molar ratios of cardanol to formaldehyde as well as ECN to DEtOA, only seven MECNs (MECN1, MECN2, MECN3, MECN6, MECN7, MECN8, and MECN9) were water-soluble and electrodepositable, whereas MECN4 and MECN5 were not water soluble, and hence not considered for further studies. These seven MECNs were used for the study of their electrodeposition characteristics, such as electrodeposition yield (ED yield), coulombic yield (CY), dry film thickness (DFT), and properties of their deposited films. The most suitable molar ratio of ECN:DEtOA for the preparation of MECNs was found to be 1:1. The CN prepared by using cardanol and formaldehyde in the molar ratio of 1:0.7 was used for the preparation of ECN2 and MECN2 prepared from this was found to be the most suitable in terms of deposition behavior and overall film properties. The film of MECN2 electrodeposited at constant voltage (100 V) was found to be the best in terms of film properties along with corrosion resistance, as it passed 800-h exposure to salt spray atmosphere. MECN2 was optimized for its electrodeposition characteristics like ED yield as 3.62 mg/cm2, CY as 35.87 mg/C, and DFT as 25.26 μm. Through a wide cure window, the films of MECN2 were found to be self-curable at an optimum cure schedule of 160°C/30 min. The electrodeposited films of MECNs had good physical, chemical, and corrosion resistance properties, but demonstrated low resistance to xylene, in particular. The study emphasized the electrodeposition behavior and film properties of the prepared MECN resins as binders for CED coating formulations, which were self-curable without using any external crosslinker. The self-curing of the deposited films was achieved via a novel self-curing mechanism, i.e., one molecule chemistry through anionic polymerization. The prepared epoxide resins as MECNs could be cathodically electrodeposited as primer coat for the protection of metallic substrates against corrosion.