Einfluss der Mehrfachverarbeitung auf die Eigenschaften von Polyolefinen und Polystyrol sowie Untersuchung der Eignung von inline Raman-Spektroskopie während des Verarbeitungsprozesses für die Feststellung von Materialkontaminationen

The ongoing transformation towards a circular economy within the European Union also demands significant adaptations from the packaging industry. However, numerous, primarily technical, challenges remain on the path towards achieving closed material loops for packaging. Within the FFG-funded project "Pack2theLoop," several of these challenges were addressed, including the production of optimized post-consumer recyclates, the development of suitable applications, and the assessment of toxicological safety. This study complements these aspects by investigating the material behaviour during multiple processing cycles and evaluating the use of Raman spectroscopy as a spectroscopic inline quality control method during the recycling process.
Multiple life cycles were simulated through tenfold processing of polypropylene- and polystyrene-post-consumer recyclates (rPP- and rPS-PCR). These were processed both without stabilization and in combination with antioxidant or impact-modifying masterbatch formulations. All five material variants (2× rPP, 3× rPS) were compounded ten times, the rPP variants (unstabilized/stabilized with 3 wt.% antioxidant masterbatch) additionally underwent ten complete processing cycles (compounding, injection moulding, shredding). The development of material properties was analysed using rheological tests, oxidation testing, tensile testing, and gel permeation chromatography to determine the molar mass distributions.
The analyses revealed significant changes in material properties over ten processing cycles, particularly in the unstabilized recyclates. For example, the weight-average molar mass of unstabilised rPP decreased by almost 60 % between the first and the tenth processing cycle (complete cyclic processing). Overall, the results demonstrated that targeted stabilization of recyclates can significantly slow down material degradation even over multiple processing cycles, thereby potentially enabling repeated cyclic use of the materials.
The establishment of Raman measurements as an inline quality control method faced numerous practical challenges and cannot yet be considered fully mature. In principle, the detection of polymer-specific spectra and the identification of mixtures (foreign polymer content 1–50 wt.%) of the investigated pure polymers PP, HDPE, and PS work reliably; however, difficulties remain at very low foreign polymer contents (1–2 wt.%). The chosen method, comparing the intensities of individual peaks with theoretical target spectra, does not provide sufficiently accurate results for determining material content.
Therefore, the implementation of a chemometric model is recommended. Additionally, regression models were developed for specific Raman peaks of the pure materials, which enable good predictions in two-component systems, for example, in determining polystyrene content.
In summary, Raman spectroscopy, after process optimization, can be considered a useful addition to inline quality assurance in the context of plastic recycling.