Effects of dopamine-containing curing agents on the water resistance of epoxy adhesives

Epoxy adhesive as an engineering material has more and more applications in industry. But to date, water resistance of the adhesive is still a very difficult problem to be solved. By mimicking mussel adhesive proteins, dopamine (DA) was applied as one of the two curing agents of epoxy adhesive to improve the water resistance of the adhesive, and the other was phenethylamine (PEA). The effects of the contents of DA on both adhesion property and water absorption of epoxy/PEA/DA adhesives were investigated in detail. The results of lap shear test (LST) showed that the adhesion strength of the epoxy adhesives after the introduction of proper amount of DA was still up to 7 MPa even after the sample was immersed in water for over 230 days. And then the effects of metal ions Zn(II) and Fe(III) on the adhesion properties of the epoxy adhesives were also further studied. It was found that the adhesion strength of the adhesives, when the content of DA was no more than 5 wt%, can be further increased through the complexation of a small quantity of ferric ions with DA. The coordination mechanism of catechol with Fe(III) ions in the system has been investigated by both UV/Vis spectra and XRD curves. The results testified that more bis-catecholate complexes form at high DA: Fe ratios and thus the adhesives have higher cohesion strength for having more regular structures after curing. In a word, the introductions of proper amount of DA and metal ion endow the epoxy adhesive better water resistance and excellent adhesion properties.