Structure–Property Relationships of Cashew Nutshell Biochar/Coconut Leaf Sheath Fiber-Reinforced Vinyl Ester Composites: Mechanical and Erosion Performance

The production of sustainable polymer-based composites from agricultural waste has gained great attention due to the growing demand for lighter and stronger materials. In this study, alkali-treated CLSF (30 wt.%) was used as reinforcement in vinyl ester composites containing 0, 5, 10 and 15 wt.% cashew nutshell biochar, fabricated by hand lay-up followed by compression molding. The effect of biochar content on the tensile strength, flexural strength, impact strength, Shore D hardness, erosion resistance and microstructure of the vinyl ester composites was systematically evaluated by mechanical testing, erosion testing, scanning electron microscopy (SEM) and X-Ray Diffraction (XRD). Biochar inclusion remarkably enhanced the properties of the composites when compared to the unfilled composite, depending on the optimum filler content. The composite containing 10 wt.% biochar showed the best flexural strength (31.88 MPa) and Shore D hardness (83.22), coupled with minimum erosion loss (0.021%), hence high stiffness and abrasion resistance. However, 5 wt.% biochar showed the best impact resistance, indicating effective energy absorption ability. SEM study showed enhanced filler distribution and fiber–matrix interface interaction for lower levels of biochar content, but higher amounts (15 wt.%) caused agglomeration of particles with the appearance of microvoids, resulting in reduced mechanical properties. X-ray diffraction showed that biochar addition leads to better structural ordering. In general, the obtained results clearly show that using 5–10 wt.% of cashew nutshell biochar increases multifunctional properties of coconut sheath fiber/vinyl ester composites.