Sustainable polypropylene composites reinforced with Citrus limetta peel: Effects on mechanical performance and crystallinity

This study investigates the effect of Citrus limetta peel (CLP) bio-filler on the moisture diffusion behavior, structural characteristics, and mechanical performance of polypropylene (PP) composites. The composites were prepared using CLP particles (100–250 μm) at filler loadings of 10–30 wt% through an extrusion–compression molding process. Water absorption behavior was evaluated according to ASTM D570 at ∼23 °C, and diffusion was analyzed using Fickian theory. The results show that equilibrium moisture uptake increased from 2.01% for virgin PP to 7.03% for PP–30CLP due to the hydrophilic nature of the lignocellulosic filler. Correspondingly, the diffusion coefficient increased from 1.8 × 10 − 8 cm 2 /s to 4.8 × 10 − 8 cm 2 /s with increasing filler content. Mechanical testing revealed that tensile strength decreased from 26.99 MPa to 17.80 MPa, while tensile modulus increased from 425 MPa to 446 MPa at 10 wt% CLP, indicating improved stiffness at moderate loading. Similarly, flexural strength decreased from 43.31 MPa to 31.94 MPa, whereas flexural modulus increased to 1569 MPa at 10 wt% before slightly decreasing at higher loadings. Statistical analysis using one-way ANOVA confirmed that the changes in mechanical properties are significant (p < 0.05). X-ray diffraction analysis indicated a reduction in crystallinity due to restricted polymer chain mobility, while scanning electron microscopy revealed filler agglomeration and increased porosity at higher filler contents. Overall, CLP waste demonstrates potential as a sustainable bio-filler for PP composites, although optimization of filler loading is required to balance mechanical performance and moisture resistance.