Polymer Processing in a Circular Plastic Economy

  • Selin Palali

Student thesis: Master of Philosophy


With the sharp increase of plastic waste, the current linear global plastic economy is putting strain on industries to adopt to a circular plastic economy. High density polyethylene (HDPE) has one of the largest shares (~ 20 %) in municipal waste streams owing to its favourable properties such as low cost and durability. One approach is to mechanically recycle these plastic resins via extrusion. However, plastics mechanical properties significantly deteriorate once the material is subject to harsh processing conditions such as high temperatures and shear force. In this project, the relationship between the mechanical, thermal and rheological properties of multiple virgin grades were analysed, and correlations between grade-based properties were confirmed. As such, this research can facilitate the development of a mapping system using a linear regression model to predict the properties of an HDPE grade and its recycling prospects. Additives such as antioxidants can reduce the rate of thermo-oxidative degradation but increase discolouration during recycling. Therefore, the yellowness index between recycling cycles were analysed, and preliminary Soxhlet extraction experiments were carried out to identify additive content and improve extraction parameters. Following this, various HDPE grades were exposed to several recycling cycles and the resulting degradation kinetics were compared and analysed. The highest melt-flow index (MFI) grade displayed increasing Youngs Modulus (5.6 %) and tensile strength (3.3 %), but a significant decrease in elongation at break (52 %) when recycled. The change in thermal properties of all grades were little to none. Likewise, nominal changes were seen in the rheological properties of the grades with increased recycling, however, the low MFI grade displayed unusual properties with variations in shear dependent viscosity. Nonetheless, low MFI grades showed better mechanical behaviour, and in general less degradation during recycling resulting from less variation in properties. Overall, a successful foundation of research was developed to understand the relationship between recyclate quality and MFI, and facilitate an efficient circular polymer processing system.
Date of Award31 Dec 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorMichael Shaver (Supervisor), Arthur Wilkinson (Supervisor) & Rob Lindsay (Supervisor)


  • High Density Polyethylene
  • Mechanical Recycling
  • Circular Plastic Economy

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