Mechanical recycling of post-consumer polyethylene packaging waste

Doctoral thesis, 2024

The properties of mechanically recycled post-consumer flexible and rigid polyethylene packaging waste have been studied, using material collected and sorted on a large scale from two sources. The influence of processing conditions during washing and compounding were investigated using material unwashed and washed on a laboratory scale and also washed and further processed on a larger scale. The melt-compounding was done using a co-rotating intermeshing twin-screw extruder. The pellets produced on a laboratory scale were injection moulded, and the pellets produced with an upscaled washing and compounding procedure were blow moulded on an industrial scale.

Washing was necessary to remove the surface contaminants and,  as expected, it improved the properties in both waste streams, but washing on a large scale at a high temperature with harsh chemicals such as sodium hydroxide made the flexible polyethylene material more susceptible to further degradation during melt-compounding, especially when a high temperature was used, which was shown by a significant lowering of the oxidation induction temperature and a deterioration in mechanical properties. This was not, however, observed in the rigid polyethylene material. On both a laboratory scale and a larger scale, the washing medium had a greater influence on the properties of both streams than the washing temperature. The properties of the unwashed and washed flexible polyethylene materials were, in general, not influenced by the screw configuration during compounding, but the compounding temperature led to some changes especially in the case of the washed samples. Rheological characterizations suggested that different levels of chain branching occurred in the rigid polyethylene samples compounded with different compounding temperatures and with the application of vacuum, whereas the water stripping during compounding had no influence.

Overall, the results indicated that both post-consumer flexible and rigid polyethylene packaging waste have useful applications in new products, such as non-food packaging. 4L‑containers were successfully produced by blow moulding on an industrial scale using 100 % recycled rigid polyethylene material. However, the results of this work indicate that the washing and compounding conditions should be optimized for each stream since they are affected in different ways.