Improving interfacial adhesive bond durability of a structural epoxy with steel using a tannic acid‐based additive for underwater applications

Abstract This study explores the applicability of a tannic acid‐based (TA‐AGE) additive to improve interfacial bond durability of a structural epoxy (Ep.) adhesive for underwater applications. A modified epoxy M(Ep.) was prepared from Ep. by incorporating 1 wt% (optimized) TA‐AGE additive. Although the cohesive properties of Ep. were slightly affected by the modification, underwater adhesion was improved. Incorporation of TA‐AGE additive in the adhesive can strengthen the interfacial bonds by forming more noncovalent interactions with the adherend surface. In addition, a 6‐month durability study in distilled water at 20°C, 40°C, and 60°C was performed to study the effect of water uptake on the evolution of viscoelastic, tensile properties and interfacial bond durability of Ep. and M(Ep.). With aging, the tensile properties of M(Ep.) degraded more rapidly than Ep. due to higher water uptake. Steel joints bonded with M(Ep.) in air, but cured underwater, had strength retention of 97%, 99% and 107% in 20°C, 40°C, and 60°C water, respectively, after 6‐month aging. Meanwhile, joints prepared underwater with M(Ep.) had a strength retention of 77% in 20°C water. Joint strength retention (%) in both cases was significantly higher than Ep. bonded joints aged under similar conditions.