A study of adhesion on stainless steel in an epoxy/dicyandiamide coating system: Influence of glass transition temperature on wet adhesion

• The adhesion strength of an epoxy/dicyandiamide coating after immersion in water was remarkably improved by control of its wet Tg.
• The highest wet Tg was achieved using a combination of bis-phenol A and phenol novolac epoxy, and imidazole curing accelerator.
• An interaction accompanied by electron transfer between the amino group and the hydroxyl groups was confirmed by X-ray photoelectron spectroscopy (XPS).

A coating was developed by controlling its glass transition temperature (Tg). This coating attained its highest Tg value in wet conditions (wet Tg) of higher than 100 °C, and maintained its adhesion on stainless steel even after immersion in boiling water. When this coating material was applied to a conventional two-component epoxy structural adhesive as a metallic primer, it improved the primer adhesion durability on the stainless steel sheet in water significantly, even if it was not chemically modified (without pretreatment). This indicated that a relative weak hydrogen bond between the hydroxyl group of the epoxy network and a surface hydroxyl group of the oxide layer of the stainless steel could be sufficient to prevent water penetration into the interface if the thermal mobility of the epoxy network is restrained. An investigation by X-ray photoelectron spectroscopy (XPS) indicated an interaction between a primer amino-group component and the metallic surface accompanied by proton transfer. However, the thermal mobility of the epoxy network could be the most important factor in its adhesion in wet conditions.