Hydrophobic and mechanically reinforcing coatings from palmitoylated lignin via waterborne spraying

Abstract

Softwood Kraft lignin was esterified with palmitic anhydride to produce palmitoylated lignin (p-lignin). Structural analyses by FTIR and quantitative ³¹P NMR confirmed high degrees of hydroxyl substitution (71% aliphatic and 82% phenolic). Thermal measurements revealed marked plasticization, with the glass transition temperature shifting from 170 °C to 61 °C. The p-lignin resulting from the modification was more hydrophobic, as confirmed by reduced dynamic vapor sorption and molecular dynamics simulations showing lower hydration free energy and significantly stronger binding free energy. When applied from acetone solution onto paper, p-lignin produced coatings with water contact angles (WCA, 1 min) exceeding 150°, although low-temperature curing (80 °C) was required to achieve adequate adhesion. In contrast, waterborne colloidal suspensions of p-lignin particles, prepared via solvent-shift and stabilized with hydroxyethyl cellulose (HEC), could be sprayed directly onto paper to yield uniform coatings with WCA values above 130° (1 min) and strong adhesion, without any need for post-treatment. The coatings greatly enhanced filter paper's mechanical performance, with dry and wet tensile strength increasing by 65% and 400%, respectively. Using palmitic groups (C16) as a model, we show that grafting long hydrocarbon chains onto lignin imparts durable hydrophobicity and mechanical reinforcement, highlighting palmitoylated lignin as a promising bio-based coating for sustainable applications.