Effect of alkali treatment on the physicochemical and thermal properties of Calathea lutea fibers for composite reinforcement

Natural fiber reinforced composites are attractive sustainable materials, but their use is often limited by poor fiber-matrix adhesion and low thermal stability. This study investigates the effect of alkali treatment on the surface physicochemical characteristics and thermal behavior of Calathea lutea fiber (CLF) for composite reinforcement. CLF was treated with 5 wt% NaOH for 0, 2, 4, and 6 h and characterized by SEM-EDX, AFM, XRF, DTA, and TGA. The results showed that alkali treatment significantly altered the fiber surface and thermal response. Among the tested conditions, 2 h treatment (NT2H) produced the highest O/C ratio (1.53), indicating effective removal of lignin and hemicellulose and improved surface oxygenation. SEM and AFM revealed a cleaner and more homogeneous surface after alkali treatment, while AFM analysis showed that NT2H exhibited lower roughness values (Ra = 2.72 nm and Rq = 4.06 nm) and reduced topographical heterogeneity than the untreated fiber. XRF confirmed treatment-dependent elemental changes, and thermal analysis showed improved stability after alkali treatment, although NT4H exhibited the strongest thermal shift. Overall, NT2H provided the best balance between surface activation and structural preservation, supporting its potential for composite applications. • Alkali treatment improved surface chemistry and morphology of Calathea lutea fibers. • 2 h NaOH treatment yielded the highest O/C ratio and optimal surface reactivity. • SEM and AFM confirmed cleaner surfaces with enhanced roughness and pore uniformity. • Thermal stability increased due to removal of amorphous components and higher crystallinity. • Excessive alkali treatment caused fiber degradation and reduced thermal performance.