Recyclability of thermoplastic polyurethane under repeated mechanical reprocessing: degradation mechanisms and volatile emissions

ABSTRACT Repeated mechanical recycling is a key strategy within circular economy concepts however the accumulation of degradation products and volatile emissions may limit the safe reuse of thermoplastic polyurethanes. This study presents a comprehensive, multi-analytical evaluation of the effects of repeated mechanical reprocessing on thermoplastic polyurethane (TPU), with particular emphasis on structural degradation, the formation of low-molecular-weight compounds and volatile organic compounds (VOCs). TPU samples were subjected to one and two consecutive mechanical reprocessing cycles and systematically characterised using gel permeation chromatography (GPC), differential scanning calorimetry (DSC), rheological analysis, X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), pyrolysis–gas chromatography–mass spectrometry (Py-GC-MS), migration testing, and headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry (HS-SPME-GC-MS). Mechanical reprocessing resulted in a pronounced decrease in molecular weight of the peak maxima (Mp; up to approximately 80%), accompanied by reduced viscosity and a narrower molar mass distribution, indicating extensive polymer chain scission and diminished processability. DSC analysis revealed moderate changes in melting temperature and crystallisation behaviour. Detailed VOC profiling identified several degradation-related compounds, including 1,4-butanediol, methylene diphenyl diisocyanate (MDI) and butylated hydroxytoluene (BHT). Notably, 1-hexadecanol may represent a potential molecular marker of repeated mechanical recycling, as its relative abundance increased consistently with each reprocessing cycle; however, this observation requires further validation, particularly given that it has not been reported in previous studies. Migration testing into water demonstrated limited release of low-molecular-weight compounds, while the persistent presence of 1,6-dioxacyclododecane-7,12-dione across all samples highlights its environmental relevance.