The present work discusses the mechanical and thermal properties of novel green hybrid composites manufactured from a polypropylene (PP) matrix, reinforced by bio-hybrid fibers-short woven flax fiber (SWF) and short basalt fiber (BF)-and agro-waste fillers, such as rice husk powder (RHP). Interfacial bonding was enhanced by a coupling agent, such as maleic anhydride grafted polypropylene (MAPP). These hybrid composites were prepared using the twin-screw extrusion and injection molding process. Extensive tests were conducted to study the effects of reinforcement and MAPP on tensile strength, flexural strength, tensile modulus, flexural modulus, and dynamic mechanical properties. The obtained results showed a large improvement in mechanical properties: tensile strength improved up to 57.68% (from 35.82 to 56.48 MPa), flexural strength improved up to 52.59% (from 58.08 to 88.65 MPa), the value of tensile modulus improved up to 147% (from 2.00 to 4.94 GPa), and flexural modulus improved up to 86.04% from 2.65 to 4.93 GPa—in comparison to a plain 25% SWF/PP composite. DMA represented that the storage modulus increased by 129% from 2159.69 to 4938.20 MPa, while tan delta values reduced by 15%, signifying enhanced stiffness in concert with a reduction in molecular mobility. Thermal analysis exhibited enhanced thermal stability as char residue was increased from 1.82 to 16.81% at 700 °C in the optimum composites containing RHP and MAPP. These morphological and structural characteristics were characterized by using techniques from FTIR and 3D-OM to SEM–EDS; indeed, the enhanced interfacial bonding and homogeneous distribution of reinforcement were verified with MAPP assistance. The findings of this study demonstrate the potential of these green composites for high-performance applications in the automotive and construction industries.

Syzgium cumini (L.) Skeels powder (S. cumini powder), also known as Jamun, is well-known for its various medical and health benefits. It is especially recognized for its antidiabetic and antioxidant properties. Thus, S. cumini powder is used in various industries, such as the food and cosmetic industries. In this work, the fruit of S. cumini was utilized; its seeds were extracted, dried, and ground into powder. The ground powders were subjected to various techniques such as physicochemical tests, Fourier transform infrared (FTIR) spectroscopy, X-ray diffractometry (XRD), particle size analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and antioxidant analysis. From the physicochemical tests, it was revealed that the jamun seed filler contained cellulose (43.28%), hemicellulose (19.88%), lignin (23.28%), pectin (12.58%), and wax (0.98%). The FTIR analysis supported these results. For instance, a peak at 2889 cm⁻¹ was observed and associated with CH stretching, typically found in methyl and methylene groups, characteristic of cellulose and hemicellulose structures. The XRD results demonstrated that the crystallinity index of the jamun seed filler was 42.63%. The particle analysis indicated that the mean (average) particle size was 25.34 μm. This observation was ensured with SEM results. The EDX spectrum results showed the elemental composition of the fillers. Regarding thermal degradation, the jamun seed filler had the ability to withstand temperatures of up to 316.5 °C. Furthermore, endothermic and exothermic peaks were observed at 305 °C and 400 °C, respectively. Furthermore, the antioxidant property of the powder displayed a peak scavenging activity of 91.4%. This comprehensive study not only underscores the viability of S. cumini powder as a sustainable and effective particulate filler in polymer composites but also demonstrates its potential to enhance the mechanical properties of composites, thereby offering significant implications for the development of eco-friendly materials in various industrial applications.