Fluorine free surface modification of tannin-furanic foams by silylation

Tannin-furanic foams are a promising and biogenic alternative to oil-based porous materials. Their hydrophilic character, typically indicated by a contact angle to water of 70°, limits some potential applications (for instance outdoor thermal insulation). To overcome this, a post-synthetic surface modification step with different fluorine-free organosilanes at 323 K was investigated with a focus on the final, hydrophobic performance. On the one side, methyltrimethoxysilane, and 3-(chloropropyl)trimethoxysilane, which undergo self-condensation as well as bonding to the hydroxy groups of the tannin polymer, were applied. A modified surface structure and a 25 to 50 % weight increase, depending on the molecular weight of the silylation agent, were observed. Contrary, a mono-functional silane, precicely trimethylchlorosilane, shows only a slight increase in weight, yet also condenses onto the polymer surface without forming a protective surface coating layer. Contact angle measurements using water show an increase from 70° (unmodified) up to 145° for a silane-modified foam. Nuclear magnetic resonance and infrared spectroscopy show the formation of covalent bonds between the silane and the biogenic polymer matrix. The obtained material is less prone to absorb water from a humid atmosphere (reduction of 75 %) and is highly efficient for the removal of non-polar contaminants from water, featuring new possible applications in humid surrounds.