Sustainable Hydrophobic Coatings Using All-Lignocellulose Films Reassembled from Bark and Wood Biomass

We present a sustainable, binder‑free coating strategy using thermopressed films derived from Japanese cedar bark and Eucalyptus wood. This double‑layered film transforms wood surfaces from hydrophilic to highly hydrophobic, achieving water contact angles up to 127° without synthetic polymers. Surface characterization revealed that the biomass films possess low roughness (Sa 0.67 µm; Sz 4.24 µm) and markedly reduced surface free energy, with the polar component approaching zero due to the enrichment of bark‑derived lipophilic extractives. These physicochemical changes, together with matrix infiltration and cellulose fibril exposure during thermal compression, promote strong adhesion and stabilize the hydrophobic state. Mechanistic analyses (SEM, ATR‑FTIR, Tg‑DTA/MS, and GC/MS) further showed that enhanced hydrophobicity arises from thermal restructuring of lignocellulosic matrices and esterification of hydroxy and saturated fatty acids. Bark‑derived films from various species consistently exhibited contact angles exceeding 90°, demonstrating their versatility. This bio‑based coating, prepared via formic‑acid disintegration and casting, offers a scalable and eco‑friendly alternative to petroleum‑based coatings, contributing to sustainable innovation in wood protection and surface engineering. Formic acid-derived bark/Eucalyptus films undergo thermal restructuring to yield hydrophobic, binder-free coatings with strong adhesion and water resistance on wood surfaces.