Polylactic Acid Biocomposites Reinforced with Nanocellulose Fibrils with High Lignin Content for Improved Mechanical, Thermal, and Barrier Properties

Lignin, the second most abundant natural polymer on earth after cellulose, contains both hydrophilic and hydrophobic groups. In this study, the use of nanocellulose fibrils with high lignin content (NCFHL) has been explored to make polylactic acid (PLA) biocomposites with excellent mechanical, thermal, and barrier properties. Different amounts of NCFHL aqueous suspensions (5–20 wt %) were wet mixed with PLA latex to form composite films by casting and hot pressing. The presence of lignin imparted a strong compatibility between NCFHL and the PLA matrix, which overcame the major issue of poor interfacial bonding associated with nanocellulose fibrils without lignin previously reported by literature studies. Atomic force microscope infrared spectroscopy (AFM-IR) characterization results showed an effective coupling between NCFHL and PLA at the nanoscale. With 5–10 wt % of NCFHL additions to the PLA matrix, a significant improvement in mechanical, thermal, and water vapor barrier properties was observed for the resulting biocomposites. The addition of 10 wt % of the NCFHL increased the modulus and strength by 88% and 111%, respectively, and the water vapor transmission rate was reduced by 52%, compared to neat PLA.