AVS 70 Session BP-SuA: Biomaterials Plenary Session (INVITED SESSION)

Adhesives are wonderful materials, holding together our electronics, furniture, and packaging. However, most of these glues are petroleum-based, do not degrade, and prevent substrate separation, thereby meaning that most consumer products end up in landfills for centuries. These glues and the materials held together contribute to land-based and ocean microplastics. Given the amount of adhesive use, impact in the marketplace will arrive only when alternative adhesives are high performance, low cost, easy to make, and the components are already available at large scales. In order to begin down this path of adhesive sustainability, we have turned to sea creatures for design cues. Mussels, barnacles, and oysters all bond to rocks using chemistry not found in commercial glues. Characterization insights on biological materials have provided a starting point for adhesive design. Here we report new materials based upon epoxidized soy oil. Cross-linking with bio-based, multifunctional alcohols and acids was examined. Plant-based compounds such as malic acid reacted with the soy oil. However, catechol-like chemistry, found in marine mussel glue, was required to bring about the extreme degree of cross-linking needed for curing into viable adhesives. In the best of cases, bond strengths could be of quite high strength, similar to epoxies. Spectroscopic characterization showed that all components, including tannic acid, cross-linked with each other, although some reactions were more prominent than others. Water resistance studies showed a middle ground of being able to withstand being submerged for a short period of time while enabling debonding at later time points. Bulk scale costs worked out to be a modest ~1/3 premium over incumbent materials. Preliminary calculations indicate that, considering sourcing, manufacturing, and curing, this new adhesive system is, overall, carbon negative. By making new adhesives from bio-based components and demonstrating high bonding performance we hope that, soon, several environmental problems may be solvable.

Underwater environments are particularly challenging for adhesives, as water weakens the interfacial bonds that underlying adhesion. A promising strategy for improving underwater adhesion is the incorporation of multidentate Hydrogen bonding groups. However, it remains challenging to quantitatively relate the macroscale adhesive strength to cooperative hydrogen-bonding interactions at the interface, limiting our understanding of the bonding mechanisms. This presentation will discuss efforts to understand how the presence of multidentate hydrogen bonding groups leads to strong underwater adhesion. To do so we show a relationship between the bond activation energy to the macroscale adhesive strength and how this relationship can be employed in self-arresting crack measurements. We will also report on measurements using the Surface Forces Apparatus to show that epoxies modified with tridentate hydrogen bonding groups exhibit robust adhesion to both mica and ultra-smooth aluminum substrates in water. These results inform our understanding of how molecular debonding mechanisms impact macroscale adhesion, aiding our ability to connect adhesive chemistry to performance.