Thermal Degradation and Fourier Transform Infrared Spectroscopy Characterization of Polypropylene under Multiple Mechanical Recycling Cycles

Abstract

This study investigates the effect of repeated mechanical recycling on the thermal and chemical properties of polypropylene (PP). Virgin PP (v-PP) was subjected to three recycling cycles (r1-PP, r2-PP, r3-PP) through repeated melting. Thermal degradation behavior was analyzed using thermogravimetric analysis (TGA), while chemical structural changes were examined using Fourier transform infrared spectroscopy (FTIR). TGA result showed a single step degradation for all samples. The onset degradation temperature increased from 418.73°C (v-PP) to 440.64°C (r2-PP), indicating improved thermal stability after initial recycling cycles, but decreased to 414.62°C in r3-PP due to stabilizer depletion and chain scission. Weight loss remained high (~97–98 wt%) with minimal residue (~1–2 wt%). FTIR analysis revealed increased intensity of C–H, C–C, and C=O functional groups in recycled samples, indicating oxidative degradation and structural modification. These findings suggest that limited recycling cycles do not significantly degrade PP performance, while excessive recycling reduces thermal stability.