Nanochitin-Fortified Polyphenol Complexes for Dry and Wet Adhesion

Synthetic adhesives commonly used in shipbuilding, plumbing, and various industrial and household applications pose environmental and health concerns due to chemical leaching and other issues. In this work, we present a sustainable alternative using chitin nanofibers (ChNF) to enhance the networking and surface binding of biomolecules. We investigate aqueous-based formulations composed of tannic acid (TA), poly(vinyl alcohol) (PVA), and chitin nanofibers, which form robust adhesive complexes. These are driven by multiple interactions involving phenolic and hydroxyl groups, which are present at high densities and contribute to exceptional adhesion upon drying. Unlike most two-component structural adhesives, the ChNF-based adhesives introduced here do not rely on organic solvents and demonstrate versatility across surfaces with contrasting topologies and surface energies, including stainless steel, polypropylene, wood, and others. With an ultimate shear strength reaching up to 20 MPa, these adhesives rival commercially available structural adhesives commonly used for bonding metals, wood, and glass. The addition of chitin nanofibers enhances adhesion by up to 400%, depending on the PVA-to-TA ratio. Furthermore, these adhesives exhibit long-term structural integrity under wet conditions, showing no signs of swelling or degradation. To elucidate the mechanisms underlying adhesion in both wet and dry states, we conducted comprehensive analyses, including morphological, mechanical, rheological, spectroscopic, thermal, and surface characterizations. The findings highlight the potential of ChNF-based adhesives as a viable and sustainable alternative for diverse industrial applications.