Lignin-polyphenol epoxy layer: a multi-functional protective coating cascade-constructed by ionic liquids

Industrial lignin-derived polyphenols are promising platform chemicals, especially as bisphenol A substitutes for bio-resin production. Nevertheless, there is an urgent need to address the issues caused by traditional volatile and toxic epoxy curing agents. Herein, a novel lignin-polyphenol epoxy layer with multi-functional protective performances was successfully constructed through cascade reactions by ionic liquids (ILs). Specifically, lignin polyphenol obtained from demethylation of alkali lignin in ethanolamine acetate was cross-combined with salicylol (SA) and epichlorohydrin to synthesize lignin-based epoxy prepolymers by tetrabutylammonium bromide. And the epoxy layer was eventually obtained by solidification with dicarboxylic acid based PILs, [EOA]₂[Asp]. The resin layer has demonstrated excellent anti-corrosion, anti-ultraviolet and anti-bacterial properties. The iron sheet coated with resin layer (contact angle = 103.23°) exhibited corrosion resistance more than 2  months without any damage. A thin resin layer (0.5  mm) can achieve both excellent UV absorption properties (≥0.66 a.u.) and high UV blocking rate (96%). The positive Zeta surface potential of the resin layer is conducive to the adsorption of bacteria contributing additional antibacterial properties. Compared to lignin, lignin polyphenols can create more epoxy-binding sites and thus be cross-linked (ρ = 60.33 × 10^-3 mol/cm³) and cured with high density by more dicarboxylic acid based PILs. Meantime, the anions of the dicarboxylic acid based PILs contain multiple carboxyl groups, which can form additional dynamic physical crosslinking with the pre-polymer. This study explores new ways to utilize industrial lignin and develop ILs as green solvents for lignin-based processes.