Improving epoxy adhesion with steel adherends using a tannic acid-based additive: Impact on resin properties and interfacial bonding

Traditional adherend surface treatment techniques such as grit blasting, acid etching, phosphating and primer treatments alone are often inadequate for maintaining interfacial bond durability of adhesively bonded joints. Therefore, the existing interfacial bonds between epoxy adhesive and the adherend should be strengthened. Tannic acid (TA) is a plant-based polyphenol that is inexpensive and easily available in nature. Its potential as an additive to improve the interfacial bond durability of epoxy adhesives has not been studied adequately. In the present study, an additive (TA-AGE) was synthesized by grafting allyl glycidyl ether (AGE) onto phenolic hydroxyl groups of TA. With 1 wt% TA-AGE additive in the epoxy, the single lap shear strength improved by 37%. Further, accelerated aging studies were performed to understand the impact of water uptake on the mechanical and viscoelastic properties and interfacial bond durability of the epoxy with 1 wt% TA-AGE additive. TA-AGE modification can increase hydrophilicity and enhance the water uptake characteristics of epoxy. This can negatively impact its mechanical properties and glass transition temperature (T_g) due to plasticization effects. Nevertheless, Boeing wedge test and single lap shear tests reveal that the strengthened interfacial bonds to steel adherends with the additive were robust and durable.