New technique for recycling multi-layer packaging

CRISP packets, chocolate wrappers, and blister packets for pills are all extremely difficult to recycle as they are comprised of multiple layers, but a new technique can separate each layer and recycle 99% of the materials.

Packaging is important to preserve food and to protect pills from oxygen, light, and moisture, but the multilayer packaging used for these products is challenging to recycle. Blister packets consist of layers of plastic film and aluminium foils. They are classed as hazardous waste and are usually either landfilled, which has environmental consequence if the aluminium leaks into the environment, or incinerated, which can produce gas and ash exhausts that also have a negative environmental impact.

Researchers in Lithuania and Egypt have developed a process that uses a switchable hydrophilicity solvent to break the layers apart in food wrapping and blister packets. The solvent can switch from hydrophobic to hydrophilic, and different compounds are soluble in each form.

The team used six common types of multilayer flexible packaging (MFP) from crisps, chocolate bars, bakery products, ground coffee, ice cream, and biscuits. They also collected six different types of waste pharmaceutical blister (WPB) packets that had contained various different sizes, colours, and types of pill.

MFP is typically comprised of a coating layer to protect printed symbols, an outer layer with printed symbols, a structural layer to prevent tearing, a tie layer to combine two polymers that would otherwise separate, a barrier layer to prevent oxygen from entering the packet, and a seal layer for heat sealing the packet. Blister packaging usually has four layers; the forming film, the lidding material, the heat seal coating, and the printing ink.

The exact structure varies according to the product, and all of the samples were processed together under the same conditions to simulate an industrial environment where sorting and separating all the different types of waste would be a challenge.

The team used the solvent in N,N-dimethylcyclohexylamine (DMCHA) to break the layers apart and used ultrasonic treatment to accelerate the separation process. “The main idea of our technology is to break the mechanical and chemical bonds between all layers and then liberate all layers in the form of dissolution or films,” said Samy Yousef, a postdoctoral researcher in the faculty of mechanical engineering and design at Kaunas University of Technology in Lithuania.  “We can control by dissolution time. For example, after ten minutes we can separate the first layer then after ten more another layer and so on.”

The polarity of the solvent is then switched by adding water and CO₂, allowing the remaining dissolved adhesives and inks to be recovered.

“With common solvents, we need to heat the prepared solution above boiling point to extract the polymeric component form the solution which can cause some degradation in the recovered polymer,” said Yousef. “Extracting polymers using the switchable solvent can be performed by mixing with water at zero temperature and adding CO₂, and with this technique we can avoid any degradation or decrease degradation.”

The recycling rate using this method is >99%, and the recovered materials – aluminium flakes, EVA films, PET films, and PE powder – are of a higher quality than traditional recycling methods. This allows a much wider range of products to be produced and would increase the profits of recycling. The solvent can also be recycled by heating it overnight to remove the CO₂ and to evaporate water that has been added.

The researchers also outlined a design for a pilot plant based on current recycling technologies and currently looking for funders to develop the plant.

Green Chemistry http://doi.org/cst6

Journal of Cleaner Production http://doi.org/cst5