Cathodic electro-deposition of low-temperature curable water-soluble polyepoxide coatings

Epoxidized phenol-novolac (EPN) type of polyepoxide resin was used for developing water-soluble cathodically electro-depositable coatings which could be self-curable without using any external/additional cross-linking agent. The self-curing of cathodically deposited films could be effected at a low temperature of 80 °C in 30 min, as compared to 150-170 °C in 30 min practised presently in many commercial cathodic electro-deposition (CED) installations. In this work, effect of varying molar ratio of secondary amines such as diethanol amine (DEtOA) and diethyl amine to epoxidized phenol-novolac (EPN) on the solubility of EPN-secondary amine adducts prepared at 80, 60 and 30 °C for varying reaction times was investigated. Self-curing characteristics of EPN-DEtOA (1:2 moles) adduct at 30-160 °C were studied. Kinetics of film growth during CED was investigated by using aqueous solutions of acetic acid neutralized EPN-DEtOA (1:2 moles) adduct. Variables for kinetic studies were deposition time (30-600 s), applied voltage (5-250 V) and solution concentration (1-20% (w/w)). By carrying out CED of polyepoxide films, values of electro-deposition (ED) characteristics found were: ED yield as 5.89 mg/cm², Coulombic yield as 19.78 mg/C and dry film thickness (DFT) as 52.12 μm. Plots of DFT vs. deposition time (t), DFT vs. t^1/2 and current density vs. field strength were drawn to calculate the values of Coulombic efficiency of the process and film conductivity. Cathodically electro-deposited films were finally characterized for physical properties, chemical resistance and corrosion protection.