Characterization of natural cellulose fibers from the stem of Leonotis nepetifolia as reinforcement for polymer composites.

Leonotis nepetifolia stem (LNS) fiber was systematically characterized to assess its suitability as a natural strengthening agent in polymer composite applications. Scanning electron microscopy (SEM) analysis indicated that the surface of LNS fiber is moderately smooth, with only minor impurities resulting from the water-retting process. The fiber diameter varied between 30.87 and 32.56 μm with a variation of ±2.1 μm. The fiber density (0.945 g/cm3) is low compared to many other plant stem fibers. The chemical composition revealed a high cellulose content of 69.97%, followed by 12.46% hemicellulose, 11.24% lignin, and 2.25% moisture. Wax and ash content were very low, at 0.27% and 1.75%, respectively. This high cellulose content supports improved fiber-matrix interaction and thermal resistance in composites. Fourier Transform Infrared Spectroscopy (FTIR) spectra validated the existence of characteristic lignocellulosic functional groups. X-ray diffraction (XRD) analysis specified a crystallinity index of 58.75% and a crystallite size of 11.54 nm, confirming a semi-crystalline cellulose I arrangement. Thermal studies revealed a Tmax for LNS fiber of 347.28 °C, indicating good thermal stability. AFM analysis showed a textured surface, which supports good mechanical interlocking. Tensile tests revealed a tensile strength of 129.7 MPa and a Young's modulus of 45.0 GPa at a 40 mm gauge length. Overall, LNS fiber exhibits favorable mechanical, thermal, and structural characteristics, making it well-suited for polymer composite applications.