Auxetic behavior of eco-friendly natural fiber composites and their dependence on fiber surface modification

This study investigates the influence of sodium hydroxide (NaOH) surface treatment on the out-of-plane auxetic behavior and mechanical properties of hemp/polylactic acid (PLA) nonwoven composites. Untreated and alkali-treated hemp fibers (2.5 wt.%, 5 wt.%, and 7.5 wt.% NaOH) were used to fabricate needle-punched nonwovens with 50 wt.% PLA. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and Fourier-transform infrared spectroscopy (FTIR) were employed to analyze surface morphology, thermal stability, and chemical changes, respectively. Alkali treatment effectively removed hemicellulose and lignin, enhanced surface roughness, and improved fiber matrix adhesion. Mechanical testing revealed that 5 wt.% NaOH treatment yielded the highest tensile strength due to optimal interfacial bonding and mechanical interlocking. However, out-of-plane Poisson's ratio measurements indicated a decrease in auxetic magnitude with increasing NaOH concentration, with the maximum negative Poisson's ratio of approximately −6.5 observed for untreated composites and −2.3 for treated ones. The results demonstrate that improved interfacial bonding enhances strength but restricts fiber rotation and reorientation, leading to reduced auxetic behavior. This work establishes a direct correlation between fiber surface modification and auxetic tunability, providing valuable insight into designing sustainable, functionally adaptive composites for structural and protective applications.