Composites based on virgin and recycled polypropylene (PP and rPP) reinforced with 15 wt% sisal fibers, with and without alkali treatment, were prepared by compression molding in a mat composed of a three-layer sandwich structure. The sisal was characterized by Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The composites were characterized according to physical and mechanical properties. Additionally, a factorial experimental design was used to statistically evaluate the mechanical properties of the composite. The FTIR and XRD indicated the partial removal of amorphous materials from the surface of the sisal after alkali treatment. The composites’ density results varied from 0.892 to 0.927 g·cm⁻³, which was in the desirable range for producing lightweight automotive components. A slight decrease in the hardness of the pure rPP and rPP composites in relation to the PP was observed. The water absorption was higher in rPP composites, regardless of the chemical treatment. Moreover, the impact resistance of PP and its composites was higher than the values for rPP. Statistical analysis showed that the alkali treatment was a significant factor for the hardness of the rPP and PP composites, and that the addition of the sisal layer was relevant to improve the impact resistance of the composites.

This article presents the development and characterization of environmentally friendly composites comprising polypropylene (PP) reinforced with Bahia Beige (BB) marble waste. The composites were prepared using different PP/BB weight ratios and analyzed for their chemical, physical, mechanical, microstructural, and thermal properties. X-ray fluorescence (XRF) analysis revealed the composition of BB, which exhibited a significant concentration of CaO, indicating the presence of calcite and other oxides. X-ray diffraction (XRD) analysis confirmed the presence of PP and identified calcite, dolomite, and quartz phases in the composites. Due to enhanced ceramic reinforcement, the composites displayed increased crystallinity with higher BB content. Fourier-transform infrared (FTIR) analysis demonstrated the interaction between PP and BB, with the bands corresponding to PP being replaced by bands related to BB as filler content increased. The density tests indicated a slight increase in composite density without deviating significantly from pure PP, which is advantageous for low-density applications. The hardness of the composites increased with filler content, while the impact resistance decreased notably. Scanning electron microscopy (SEM) images showed the good distribution of BB within the composites and the presence of ductile characteristics on the composite surface. The heat deflection temperature (HDT) results revealed that adding BB up to 40% by weight increased HDT, whereas a significant reduction occurred at a 50% BB content. These composites demonstrated favorable properties for engineering applications, offering a sustainable solution through utilizing natural waste resources and contributing to Brazilian sustainability efforts.