Extraction and characterization of natural fibers from Silybum marianum as a potential reinforcement for polymer composites

The aim of this article is to extract natural bast fibers from milk thistle (Silybum marianum) plant stems which are byproducts of bioactive silymarin production. Fibers have been obtained via different degumming methods including water- and alkali retting. The alkali-retted fibers were subjected to depectinization and dewaxing treatments. The chemical, physical, and mechanical properties of the produced fibers were examined. The cellulose, hemicelluloses, and lignin contents of the biological-degummed milk thistle fibers were found as 51.90%, 30.36%, and 7.69%, respectively. The ranges for the performance characteristics of the obtained milk thistle bast fibers can be listed as follows: the linear density values range between 27.49 and 34.32 tex, the initial moduli 414–902 cN/tex, breaking tenacity 13.0–19.4 cN/tex, breaking elongation 2.38–3.97%, moisture content 3.74–5.99%, and water absorption 7.06–7.46 g/g. The degumming method substantially affected the tensile properties of the milk thistle fibers, whereas the chemical treatments led to more efficient removal of lignin, hemicelluloses, pectin, and waxes as suggested by Fourier transform infrared (FTIR) spectra. Water retting resulted in finer, stronger, and stiffer fibers with lower elongation compared to alkali retting of the fibers. The chemical constituents and the tensile properties of the milk thistle bast fibers show similarity to those of kenaf fibers among conventional plant fibers. These findings suggest that the milk thistle stems can be leveraged as a renewable source of lignocellulosic fibers.