Structure–Property Relationship in Injection‐Molded Recycled Polypropylene: Mechanical, Thermal, and Spectroscopic Evaluation

Recycled polypropylene (rPP) is increasingly considered as a sustainable alternative to virgin polymers; however, its application is often limited by variability in properties resulting from degradation during prior processing cycles. In this study, the mechanical, thermal, spectroscopic, and morphological behaviour of injection-molded rPP was systematically investigated to evaluate property retention and degradation mechanisms. Mechanical performance was assessed through tensile, flexural, impact, and punch–shear testing, while differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to examine thermal transitions and stability. The results indicate a reduction in tensile strength (45%) compared to virgin polypropylene, attributed to chain scission and molecular weight reduction during reprocessing, while impact performance shows relatively minor variation. Thermal analysis reveals that the melting temperature remains comparable (170 °C), and TGA indicates stable degradation behaviour with an onset temperature around 412 °C. Fourier transform infrared spectroscopy confirms that the primary chemical structure is largely preserved. Scanning electron microscopy reveals a predominantly brittle fracture morphology with limited plastic deformation, supporting the observed reduction in ductility. The findings provide insight into the structure–property relationship of rPP and indicate its suitability for non-critical applications under controlled processing conditions.