Valorization of waste cotton fiber in flax fiber-reinforced polylactic acid (PLA) hybrid composites: Mechanical and thermal properties

This research presents a sustainable approach for valorizing waste cotton fibers discarded during the blow-room stage of textile spinning through incorporation into flax-reinforced polylactic acid (PLA) hybrid composites. Composites were fabricated via nonwoven carding, needle punching, and compression molding with varying fiber compositions. The results demonstrated substantial enhancements in mechanical performance compared to neat polylactic acid (PLA), with the best-performing formulation among the investigated compositions, 30/20/50 (waste cotton/flax/PLA), achieving a tensile strength of 77.56 MPa, a tensile modulus of 6.11 GPa, a flexural strength of 169.25 MPa, a flexural modulus of 7.33 GPa, and an impact strength of 11.25 J/mm. These values represent improvements of 171.9% in tensile strength, 188.2% in tensile modulus, 193.3% in flexural strength, 105.3% in flexural modulus, and 296.1% in impact strength, respectively, compared to neat PLA. Scanning electron microscope (SEM) analysis revealed strong interfacial bonding and uniform fiber dispersion at 30/20/50 (waste cotton/flax/PLA) composition, whereas higher fiber loading resulted in defects such as agglomeration and voids. Thermal analysis indicated only marginal changes in degradation behavior, with peak decomposition temperatures remaining within 396–399 °C. Differential scanning calorimetry (DSC) results showed increased glass transition temperature and crystallinity (up to 27.06%), indicating restricted chain mobility and fiber-induced nucleation. Fourier transform infrared (FTIR) confirmed the presence of polylactic acid and cellulose without new bond formation, suggesting predominantly physical interactions. These results demonstrate that waste cotton can be effectively valorized with flax to develop high-performance, sustainable hybrid composites.