Effect of fiber type and fiber loading on the mechanical behavior and microstructure of natural fiber reinforced polymer composites

Natural-fiber-reinforced polypropylene (PP) biocomposites are promising lightweight materials, but direct comparison among fiber types remains difficult because previous studies often used different materials and processing conditions. This study evaluated the effects of fiber type and loading on the tensile strength, Izod impact strength, and fracture morphology of injection-molded PP biocomposites reinforced with pineapple leaf fiber (PALF), banana fiber (BF), and coir fiber (CF). The fibers were treated with 5 wt% NaOH, ground, sieved through a 125 μm mesh, and compounded with PP and 5 wt% MAPP based on the PP/fiber blend mass. All formulations were processed under identical extrusion and injection-molding conditions at nominal fiber loadings of 10, 20, and 30 wt%. The ground particles had similar mean aspect ratios of 5.53–5.68. Processed neat PP showed tensile and impact strengths of 33.49 MPa and 1.24 kJ/m 2 , respectively. PP–PALF showed the highest tensile strength of 34.91 MPa at 10 wt% and impact strength of 2.75 kJ/m 2 at 30 wt%, corresponding to numerical increases of 4.2% and 121.8% relative to processed neat PP. Two-way ANOVA showed significant effects of fiber type and loading on both properties, with a significant interaction only for tensile strength. Tensile strength decreased at higher loadings in PP–BF and PP–CF but remained relatively stable in PP–PALF, whereas impact strength generally increased. SEM revealed loading-dependent differences in matrix deformation, cavities, clustered or fiber-rich regions, exposed fibrous particles, and localized openings. Overall, PP–PALF provided the highest mean mechanical properties among the investigated systems.