Underwater Bonding with a Bio-Based Adhesive from Tannic Acid and Zein Protein

Abstract

Here, we present several adhesive formulations made from zein protein and tannic acid that can bind to a wide range of surfaces underwater. Higher performance comes from using more tannic acid than zein, whereas dry bonding requires the opposite-more zein than tannic acid. Each adhesive works best in the environment for which it was designed and optimized. We show underwater adhesion experiments conducted on different substrates and in various water types, including seawater, saline solution, tap water, and deionized water. Surprisingly, the water type does not significantly influence performance, but the substrate type does. An additional unexpected result was the bond strength increasing over time when exposed to water, contradicting general observations with glues. Initial adhesion underwater was stronger compared to benchtop adhesion, suggesting that water helps improve adhesion. Temperature effects were examined, indicating maximum bonding at around 30 C, followed by another increase at higher temperatures. Once the adhesive was placed underwater, a protective skin formed on the surface, preventing water from immediately penetrating the rest of the material. The shape of the adhesive could be easily manipulated, and once in place, the skin could be broken to induce faster bond formation. Data indicated that underwater adhesion was predominantly induced by tannic acid, which facilitated cross-linking within the bulk material and adhesion to the substrate surfaces. The zein protein provided a less polar matrix, helping to retain the tannic acid molecules. These studies introduce new plant-based adhesives for underwater applications and contribute to creating a more sustainable environment