Adhesion of epoxy to hydrotalcite conversion coatings: I. Correlation with wettability and electrokinetic measurements

Hydrotalcite (HT) based conversion coatings are being developed as an environmentally benign alternative to chromate conversion coatings. This examination investigates the relationship between the interlayer oxoanion in the HT structure and surface chemistry, and their subsequent effect on adhesion of organic coatings. The interlayer ions examined included nitrate, carbonate and persulfate. Dry and wet-state adhesion of 25–75 μm thick epoxy films were measured by the pull-off tensile strength. The results were compared with surface energy and isoelectric point measurements using the rising height method and electrophoresis of HT powders, respectively. The Lifshitz–van der Waal (LVW)/Lewis acid–base approach was employed for analysis of contact angle data. It was observed that the HTs with greater LVW bonding had better wet-state adhesion, while a reciprocal relationship was seen between dry state adhesion and the isoelectric point. The correlations with the dry and wet state adhesion are interpreted as indicating that dry state adhesion is optimized by minimizing unfavorable polar interactions between the basic epoxy and HT interfaces. Wet state adhesion, where polar interactions are disrupted, is dictated by non-polar bonding.