Adhesion of epoxy to hydrotalcite conversion coatings: II. Surface modification with ionic surfactants

Hydrotalcites (HT) are naturally occurring compounds being developed as environmentally benign conversion coatings for aluminum aerospace alloys. A successful conversion coating should provide corrosion resistance and act as a base for adhesion with subsequent organic layers. In the first paper of this series, the effect of interlayer anion substitution on the adhesion of epoxy primer was investigated. Interfacial tension and electrostatic interactions were found to influence adhesion between epoxy and conversion coatings. It was believed that increased adhesion could be attained by modification of these properties. This investigation examines the effect of ionic surfactants on the corrosion resistance and epoxy adhesion for HT-based conversion coatings. Post-treatment of a NO3HT conversion coating with a trialkyl ammonium surfactant (Adogen®-464) increased the pull-off tensile strength (POTS) of an epoxy coating by 55%. However, treatment with a linear anionic surfactant, sodium lauryl sulfate (SLS); a linear cationic surfactant, cetyltrimethylammonium bromide (CTAB); or a divalent cation (Ca2+) had no effect on the epoxy POTS. Electrophoresis and the rising height method were employed to investigate the effect of the Adogen on electrostatic interactions and interfacial tension, respectively. Adogen decreased the isoelectric point (IEP) of NO3HT powders from 11.3 to 10.5. This was attributed to a shift in the Outer Helmholtz Plane (OHP) by the Adogen. A model epoxy molecule, N-methylethanolamine (NMEA), was used for contact angle measurements on conversion coating powders by the rising height method. The contact angle data was found to correlate with the epoxy dry adhesion rankings reported in Part 1 of this series. Adogen adsorption on NO3HT powders decreased the NMEA contact angle from 67.9 to 24.1°. Fourier Transform Infrared (FTIR) spectroscopy did not indicate primary bonding between NO3HT and Adogen. However, covalent bonding between NO3HT and NMEA (and, thus, epoxy) was suggested by FTIR. The improvement of epoxy adhesion on NO3HT by Adogen is believed to be due to the increased wettability of the HT.