Underwater Bonding with a Biobased Adhesive from Tannic Acid and Zein Protein

Herein are presented several adhesive formulations madefrom zeinprotein and tannic acid that can bind to a wide range of surfacesunderwater. Higher performance comes from more tannic acid than zein,whereas dry bonding required the opposite case of more zein than tannicacid. Each adhesive works best in the environment that it was designedand optimized for. We show underwater adhesion experiments done ondifferent substrates and in different waters (sea water, saline solution,tap water, deionized water). Surprisingly, the water type does notinfluence the performance to a great deal but the substrate type does.An additional unexpected result was bond strength increasing overtime when exposed to water, contradicting general experiments of workingwith glues. Initial adhesion underwater was stronger compared to benchtopadhesion, suggesting that water helps to make the glue stick. Temperatureeffects were determined, indicating maximum bonding at about 30 & DEG;Cand then another increase at higher temperatures. Once the adhesivewas placed underwater, a protective skin formed on the surface, keepingwater from entering the rest of the material immediately. The shapeof the adhesive could be manipulated easily and, once in place, theskin could be broken to induce faster bond formation. Data indicatedthat underwater adhesion was predominantly induced by tannic acid,cross-linking within the bulk for adhesion and to the substrate surfaces.The zein protein provided a less polar matrix that helped to keepthe tannic acid molecules in place. These studies provide new plant-basedadhesives for working underwater and for creating a more sustainableenvironment.