Natural urushiol as a novel under-water adhesive

Abstract The curing and adhesion of natural urushiol, a catechol derivative, in water were studied; this work was inspired by the adhesive role of the catechol functional group, found in blue mussels, in water. We confirmed the curing reaction of urushiol in water in the presence of Fe3+ and showed the possibility of utilizing this material as an under-water adhesive. Urushiol becomes the cross-linked network urushi via the formation of chemical and Fe3+-coordination bonds, which were confirmed by FT-IR and Raman spectroscopy. The effect of the coordination bond between the catechol group in urushiol and Fe3+ ion was investigated by mechanical strength measurements. The adhesion strength of urushi formed in an FeCl3 aqueous solution depends on the pH and the thickness; an adhesion strength of 1.4 kgfcm-2 was found at a thickness of 100 μm at acidic condition, revealing a high potential for the applicability of natural urushiol adhesive that has environmental sustainability.