Correlation Between Vinyl Content of Recycled HDPE and Polymer Degradation: An NMR Approach

Abstract

The present study investigates the relationship between vinyl content and polymer degradation in both virgin and recycled high-density polyethylene (HDPE) using 1H nuclear magnetic resonance spectroscopy. By analyzing multiple processing cycles of HDPE, thermo-mechanical chain scission and branching/cross-linking reactions are examined. The vinyl content of different HDPE grades is correlated with changes in rheological behavior, molecular weight distribution, and crystallinity. For virgin as well as recycled HDPE, it's confirmed that a higher initial vinyl content results in significant branching/cross-linking and molecular weight increase. On the other hand, low vinyl content helps retain the molecular structure in particular when antioxidants are present. The correlation is investigated to our knowledge for the first time for recycled plastics, whereas comparable studies have previously been conducted only on virgin HDPE. Additionally, changes in crystallinity and rheological behavior during multiple extrusion experiments further support the results as higher initial vinyl content is associated with a decrease in crystallinity due to branching/cross-linking reactions. Due to the inherent variability in recyclate composition and properties, model systems composed of single-stream recycled HDPE are utilized. The results offer improved insight into recycled HDPE degradation and support the development of tailored restabilization strategies to enhance its performance and sustainability.