The discoloration of polyethylene obtained from chemical separation of laminates: Investigating additives and other potential causes

Abstract: PolyAl, the material composed of laminated polyethylene (PE) films and aluminum (Al) in carton, is a challenge in recycling due to the strong adhesion of the layers and the low value of the end products. In Europe, recent efforts to improve the circularity of carton packages include the chemical separation of PolyAl, in order to obtain higher values of the PE and Al separately. However, an undiagnosed green discoloration had been observed in the separated PE fraction after reprocessing into granules. The study aims to explore the potential causes of the green discoloration and discuss the impact of the phenomenon. A review of literature combined with stakeholder interviews were conducted to identify potential causes of the discoloration throughout the base PE materials’ value chain; residues from the metallocene catalyst, phenolic antioxidants (AOs) and acid scavengers in additives, oxidation during extrusion melting, ozone treatment, peroxide bath, impurities, and oxidation during delamination were identified as the potential contributors to the discoloration. An analysis of the delaminated films through differential scanning calorimetry (DSC) and colorimetry (L'a'b', "∆E" , RGB) suggested that the green discoloration was a combination of yellow and grey, and the thermo-oxidation of AOs producing yellow species may be the root cause for a yellow discoloration. The second hypothesis was tested in subsequent experimental analysis; after oven-aging various metallocene PE grades with known additives, their color changes were measured and compared (L'a'b', "∆E" ). At 250ºC for 6 minutes, the samples without additives showed yellow discoloration which the AOs were able to protect against, although further treatment may result in aforementioned AO discoloration; calcium (Ca) stearate was linked to more advanced yellow discoloration when used with AOs. The thesis concluded that the cause of the final green discoloration was complex, with the choices of additives being a potential root cause. Future studies concerning other possible causes and the impact of different PolyAl recycling methods are recommended.