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.

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A mild alkaline treatment followed by microgrinding is employed to isolate lignin (23 wt %) containing nanocellulose fibrils (LNFs) from the bark of western red cedar. Microscopic images of the cross section of the bark revealed the high abundance of thin-walled parenchyma cells followed by fiber cells. The mild alkaline treatment resulted in separated cell corners, reduced cell wall thickness, and loose and layered cell wall structure for all of the cells. Confocal and atomic force microscopy-infrared images confirmed that the lignin mainly moved out from the cell corners, parenchyma cells, and S1 layers of fiber cells. Within a few passes through a grinder, most of the bark was disintegrated to smaller fibrils. The extracted bark was homogenized to LNFs with a diameter less than 15 nm. The dynamic mechanical analysis and thermogravimetric analysis results showed the high thermal stability of LNF films, compared to those made from bleached nanocellulose fibrils. In addition, the LNF films retained the bulk of mechanical and water vapor barrier properties at high humidities. The films made from LNFs exceeded the tensile performance of most of the other biopolymers reported in the literature.

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