The correlation between adhesion durability and viscoelastic creep-recovery behavior in epoxy coatings

Abstract

Abstract The durability of the adhesion between coating and substrate is of utmost importance in organic coatings. In the present study, we endeavor to find a correlation between adhesion durability (loss of adhesion and adhesion recovery) after wet conditions and viscoelastic properties of coatings. To that end, an epoxy resin was cured with four types of hardeners. Furthermore, Polyamine-based hardener, Polyamido amine-based hardener, Polyether polyamine-based hardener, and Cycloaliphatic Polyamine-based hardener, were employed in order to prepare the coatings with different viscoelastic properties. The storage modulus increased as H active of hardeners and crosslink density decreased. After applying these coatings to mild steel, the samples were placed in humidity chamber for 2000 h. Adhesion loss and recovery of adhesion were measured in wet and dry conditions during the time by pull-off test. We found that the higher the elastic module, the more the adhesion loss. Recovery of adhesion after wet and dry cycles correlated with glass transition temperature and elastic element of creep and recovery data. Polyether polyamine based sample and polyamine based samples indicated good adhesion durability through cycles. Moreover, creep recovery of these samples (response of polymer to constant load and its effect on removal of that load) demonstrated a more reversible behavior compared to polyamido amine and cycloaliphatic polyamine based sample. The microscopic delaminated areas of coatings were analyzed by breakpoint frequency method (another useful parameter to describe the extent of coating detachment from the substrate when exposed to humidity) using electrochemical impedance spectroscopy measurement. Finally, EIS measurements approved the adhesion data.