A Materials Science-Based Evaluation of Pet Bottle Recycling: Structural Degradation Analysis Via FTIR Spectroscopy and Shore D Hardness Testing

Research at a glance

Polyethylene terephthalate (PET) is a widely used thermoplasticpolymervaluedforitsstrength,lightweight nature, and recyclability. However, repeated thermo- mechanical recycling can lead to significant structural degradation. This study investigates the effects of five re-melting cycles on the structural and mechanical properties of post-consumer PET bottles. The objective is to evaluate the extent of degradation using Shore D hardness testing and Fourier Transform Infrared (FTIR) spectroscopy. PET samples were heated to 300°C at a rate of 10°C/min, held for 20 minutes, and cooled to room temperature. This process was repeated for five cycles. After each cycle, the hardness and FTIR spectra were recorded. The results showed a consistent increase in Shore D hardness, rising from 80 in the first cycle to 83.5 in the fifth, indicating increased brittleness due to polymer chain scission and crystallinity growth. FTIR analysis revealed reductions in peak areas and shifts in characteristic vibrational bands such as the carbonyl stretch (from 1400 to 1100 cm⁻¹) and weakening of C–H and O–C–C functional groups. These changes confirm progressive degradation of the PET structure, with an estimated 8.92% reduction in structural integrity per cycle. The findings support the concept of downcycling, where recycled PET loses quality and becomes suitable only for lower- grade applications. Although PET can be recycled mechanically, the absence of additives or stabilizers accelerates degradation, limiting reuse in high- performance products. This study did not include molecular weight analysis, crystallinity (DSC/XRD), or tensile strength testing. Future research should explore these parameters and evaluate the role of additives in enhancing recyclability.