Hydrophobic, Mechanically Robust, and UV-Blocking Paper Enabled by Lignin Reinforcement

Traditional paper is sensitive to water due to the strong capillary action and abundance of hydrophilic hydroxyl groups in cellulose. Inspired by outstanding properties of lignin, i.e., hydrophobicity, UV-blocking, and thermal stability, lignin-filled hardwood paper (LHP) was produced for various advanced applications. To enhance the lignin retention on the cellulose fibers, we mechanically beat the fibers to different degrees. Results showed that intensive beating made the fibers more fibrous, which significantly increased the lignin retention on the fibers. Benefiting from these changes, more lignin interacted with cellulose fibers, resulting in an increase in the hydrophobicity of lignin-filled hardwood paper (water contact angle (WCA) increased from 42 to 109°). Meanwhile, the water vapor transmission rate (WVTR) of LHP-90 °SR was only half that of the control (786 g/m2·24 h), and its water absorption was as low as 49%. These data further confirmed the excellent hydrophobic properties of LHP-90 °SR, which was attributed to the phenylpropane units and aromatic groups in the lignin molecules. Due to the dense fiber network formed by intensive beating, the interaction between lignin and cellulose was strengthened by hydrogen bonding, thus leading to a remarkable improvement in the tensile strength of lignin-filled hardwood paper from 1 to 41.36 MPa. In addition, lignin filling not only made LHP-90 °SR effective in shielding UV rays but also endowed it with good thermal stability. This research revealed a viable approach to manufacture hydrophobic, thermostable, and UV-blocking paper with high strength for advanced food packaging applications.