Mechanical, thermal and interfacial performance of sustainable polypropylene hybrid composites reinforced with short woven hemp fibers, basalt fibers and wheat straw ash

Abstract

The growing demand for sustainable lightweight materials has encouraged the development of hybrid polymer composites reinforced with natural fibers and agricultural waste. In this study, polypropylene (PP) hybrid composites reinforced with short woven hemp fibers (SWHFs), chopped basalt fibers (BFs), wheat straw ash (WSA), and maleic anhydride-grafted PP (MAPP) were developed and systematically investigated. The experimental program was conducted in two stages. First, the BF content was optimized by incorporating 5–25 wt.% BF into PP containing 25 wt.% SWHF. The optimized formulation was then modified by adding 6, 12, and 18 wt.% WSA with a constant MAPP content. The composites were fabricated by twin-screw extrusion followed by injection molding and characterized using tensile and flexural tests, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). Incorporation of 20 wt.% BF increased the tensile strength and modulus of the hemp fiber-reinforced PP composite by approximately 21% and 94% (from 35.62 to 43.00 MPa and 1.80–3.50 GPa, respectively), together with 30% and 50% increases in flexural strength and modulus (56.48–73.28 MPa and 2.42–3.62 GPa). Incorporation of WSA further increased tensile and flexural modulus by up to 41% and 55% relative to the BF-optimized composite, while MAPP addition raised tensile strength by up to 27% (43.00–54.50 MPa), consistent with improved matrix-reinforcement compatibility. SEM revealed fewer visible interfacial voids in the examined regions of the MAPP-containing composites, and FTIR revealed spectral changes consistent with modified interfacial interactions between the constituents without direct evidence of new covalent-bond formation. TGA showed that the residual char yield increased from 0.20% for neat PP to 13.83% after BF incorporation and up to 16.0% for the WSA/MAPP-modified composites, indicating an increased thermally stable residual fraction. Among the WSA-modified formulations, the 6 wt.% WSA composite exhibited the highest tensile and flexural strength (54.50 and 86.40 MPa, respectively), whereas the 12 wt.% WSA composite provided marginally higher tensile and flexural moduli (4.94 and 5.62 GPa) together with a more balanced thermal response; the latter (25SWHF/20BF/PP-6MAPP-12WSA) is therefore identified as the formulation offering the best overall property balance rather than the single highest-strength composition. These findings demonstrate the potential of WSA as a sustainable inorganic functional filler for high-performance PP hybrid composites.