Self-healing capability of novel eco-epoxy adhesives based on the modified tannic acid on Al adherends tested in a single lap joint

The aim of this paper is to study the self-healing capability of fractured Al joints bonded with novel eco-epoxide adhesives synthesized from a bio-renewable raw material (tannic acid – TA). Two synthesized eco-epoxy components based on TA, (A) glycidyl ether and (B) glycidyl phosphate ester of TA, were used as a replacement for the toxic epoxy component based on Bisphenol A. The effect of the eco-epoxy components on the self-healing capability was measured as a recovery of shear strength in a single lap joint (SLJ) test after complete failure, which was compared to the reference epoxy (R). The self-healing procedure was performed in an autoclave at 180 °C for 2 h and 2 bars. A combination of two monitoring techniques, Digital Image Correlation (DIC) and Acoustic Emission (AE), was used to monitor the strain distribution and damage propagation in the SLJ. The measured shear stress of A and B adhesives in the SLJ had values in the range of 2.3–5.1 MPa. A fracture analysis showed complete adhesive failure for all the tested adhesives, which was not affected by the self-healing process. Out of all adhesives, only the A adhesive demonstrated the capability to heal. The recovery of the shear strength for adhesive A was higher than 50% of the virgin case. In addition, the AE analysis managed to capture a clear distinction between the signals for the virgin and the self-healed tests for adhesive A. Results obtained in this study highlighted the promising potential of using bio-based epoxy adhesives in structural adhesive bonding with the possibility of using self-healing in the recovery of the strength of such bonded joints.