Effect of repeated thermoplastic processing of polypropylene matrix on the generation of low-molecular-weight compounds

Abstract

This study investigates the effects of repeated reprocessing on polypropylene (PP) and the associated release of low-molecular-weight substances into liquid and gaseous phases. Specifically, virgin polypropylene (Mosten TB 003) was mechanically reprocessed 1, 5, 10, 15, and 18 times to simulate its participation in the mechanical recycling cycle. The mechanical properties of all PP samples were evaluated, along with their potential migration into water. The stability of PP was assessed using differential scanning calorimetry (DSC), while molecular weight distribution was determined via high-temperature gel permeation chromatography (GPC) with refractometric and viscosimetric detection. GPC and DSC analyses of PP's mechanical properties revealed that reprocessing led to chain shortening and reduced thermal stability, evidenced by material yellowing. For PP recycled 18 times—representing the maximum feasible number of reprocessing cycles—the molecular weight decreased by approximately 30 % compared to virgin PP. The potential release of volatile substances (odorants) into the environment during PP processing was monitored using headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC–MS). Two extraction temperatures were examined: 90 °C, simulating the maximum temperature PP reaches indoors, and 120 °C, representing outdoor conditions. The least volatile substances were detected in the most extensively reprocessed PP (18x). At 120 °C, the most significant compounds identified included phenolic substances such as 2,4-di‑tert-butylphenol, 2,6-di‑tert‑butyl‑p-benzoquinone, 4-hydroxybenzaldehyde, estragole, and 2-phenoxyethanol. The number of reprocessing cycles was indicated by the presence of 4-methyl-1,6-heptadien-4-ol.