Characterization of mechanical properties of five hot-pressed lignins extracted from different feedstocks by microscopy-aided nanoindentation

Lignin, a main component of plants, is produced in large quantities as a by-product of the pulp and papermaking industry. The abundance of this organic polymer makes it an excellent candidate for developing renewable materials such as lignin-based biocomposites. The mechanical properties of lignin, as well as the dependence on the extraction process and feedstock, are, however, still unknown and hamper its utilization. Therefore, five hot-pressed lignins, extracted by different processes from different feedstocks, are tested by means of grid nanoindentation at indentation depths ranging from 150 nm to 1200 nm. Statistical and microstructure-guided evaluation based on microscopy images of the indented areas allows for retrieving reliable indentation properties of the porous lignin, free of indentation size effects. The sought mechanical properties of the solid (pore-free) lignin are back-identified from the strong correlation between the property and the porosity using micromechanics homogenization theory. This correlation reveals that the solid lignin in all tested samples is mechanically similar, despite different chemical structures, with a virtually intrinsic Young’s modulus of 7.1 GPa.