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

Feng, Can LU (2021) MTTM01 20211
Packaging Logistics
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... (More)
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. (Less)
Popular Abstract
Juice boxes. As easy as it is for us to accept liquid food and beverages coming in boxes, the engineering behind those carton packages is no simple feat – layers of plastic films sandwiching together paper and sometimes aluminum so the juice stays good throughout its shelf life. We don’t think twice about recycling the boxes as paper - the chief material recycled from of carton packages - but what happens to the rest?
The remainder mix of plastic (typically polyethylene, PE) and aluminum films is known as PolyAl, where the materials are still strongly attached to each other. In Europe, PolyAl is commonly incinerated for energy recovery, yet it is recyclable - in smaller volume, PolyAl is recycled as a composite material. It can even be... (More)
Juice boxes. As easy as it is for us to accept liquid food and beverages coming in boxes, the engineering behind those carton packages is no simple feat – layers of plastic films sandwiching together paper and sometimes aluminum so the juice stays good throughout its shelf life. We don’t think twice about recycling the boxes as paper - the chief material recycled from of carton packages - but what happens to the rest?
The remainder mix of plastic (typically polyethylene, PE) and aluminum films is known as PolyAl, where the materials are still strongly attached to each other. In Europe, PolyAl is commonly incinerated for energy recovery, yet it is recyclable - in smaller volume, PolyAl is recycled as a composite material. It can even be separated into their base components for higher value – this is where the green comes in.
Ever since the beginning of commercial PolyAl separation in 2010, the PE fraction had always had a dark green color. Meanwhile, recycled PE lighter in color enjoy higher prices on the market than its dark green counterparts, therefore the discoloration is a challenge against the commercial attractiveness of PolyAl recycling.
Understanding the problem is the first step to solving it. This study aims to provide a structured understanding of the green discoloration’s background, and to investigate the influences of additives on PE discoloration through experimental analysis.
A review of the PE materials throughout their lifetime revealed many potential sources of discoloration, including the materials themselves, additives, and impurities including inks and others at post-consumer levels; the manufacturing and recycling steps could also contribute to yellowing through oxidation of the materials and the additives. Through experimental analysis, ink was eliminated as a primary suspect, replaced with antioxidant additives. As the green color was found to be a mix of grey and yellow, the known phenomenon of antioxidant yellowing was more likely linked to the overall discoloration. Overall, the causes of discoloration in recycled PE from carton packages are more complicated than previously understood, further studies will help develop the solutions necessary to improve the business case of PolyAl recycling. (Less)
Please use this url to cite or link to this publication:
author
Feng, Can LU
supervisor
organization
course
MTTM01 20211
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Delamination, multilayered packaging, polymer discoloration, plastic film, recycling
language
English
id
9055885
date added to LUP
2021-06-17 15:17:35
date last changed
2021-06-28 11:09:58
@misc{9055885,
  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.}},
  author       = {{Feng, Can}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{The discoloration of polyethylene obtained from chemical separation of laminates: Investigating additives and other potential causes}},
  year         = {{2021}},
}