Natural fiber-reinforced polymer composites are increasingly investigated as suitable alternatives to conventional synthetic materials in the automotive sector due to their low density (1.1–1.3 g/cm³) and renewable origin. In this study, epoxy composites reinforced with hemp (HEMP) and coconut coir (COCO) fibers were fabricated with and without ∼0.78 vol% carbon black (CB) as a secondary reinforcement. Tensile properties were evaluated following ASTM D638, with statistical analysis performed using Kruskal-Wallis tests (α = 0.05). HEMP-based composites exhibited significantly higher ultimate tensile strength (UTS) than COCO-based composites: 82.9 ± 8.4 MPa for HEMP/epoxy vs. 50.8 ± 6.2 MPa for COCO/epoxy (p = 0.0007), representing a 63% increase. Conversely, COCO-based composites showed greater deformability, with strain at break of 46–51% compared to 13–17% for HEMP composites. CB incorporation increased Young's modulus by 5% (from 917 to 963 MPa) in HEMP composites and by 20% (from 628 to 754 MPa) in COCO composites, but reduced UTS by 16% in HEMP systems (p = 0.014). Thermogravimetric analysis revealed that CB delayed the onset of thermal degradation by 15–20 °C, with degradation temperatures exceeding 250 °C, meeting ISO 6722 requirements for automotive interior applications. Raman spectroscopy confirmed the presence of characteristic D and G bands of CB, while Voigt micromechanical modeling indicated that CB acts primarily through interfacial effects rather than direct elastic reinforcement. Overall, the combined selection of fiber type and CB content enables tunable mechanical and thermal performance, supporting the potential of these composites for lightweight automotive interior and semi-structural applications where weight reduction (20–30% lighter than glass fiber composites) and sustainability are prioritized.
Portillo et al. (Mon,) studied this question.