Study advocates for multilayer film sortation to improve plastic recycling

A new study has determined a multilayer plastic film recycling method that acknowledges the different materials and layers in complex film packaging. The research stipulates that conventional mechanical recycling of multilayer films fails to acknowledge the complex mix of materials.

The researchers propose the mechanical sortation of  plastic films based on material composition using hyperspectral near-infrared spectroscopy combined with principal component analysis. 

The study, published in Sustainable Materials and Technologies, finds that recyclers can attain higher-value recovery options by sorting multilayers into plastic compositions. This is said to have significant consequences for solvent or melt-phase separation of high polymer content layers, chemical recycling with reduced contamination, and energy recovery with optimized emissions. 

“Flexible multilayer packaging materials are estimated to be around 17–20% of plastic packaging materials. Some estimate implies that roughly 100 million tons (that is 25 % of global production) of plastic is used annually in single-use multilayer plastic products,” say the researchers.

“Even if these approximates are highly overestimated, the considerable share of multilayers in plastic products is evident.”

Unpacking plastic 

Plastic film is useful for protecting food during transport and storage. Polymers are beneficial because they are lighter and more flexible than paper or metals. 

Plastic film packaging is typically made from multilayers consisting of polymers, challenging recyclability. Two-dimensional plastic packaging materials can be divided into mono- and multi-material films according to the number of materials used or the amount of layers. 

The researchers collected packaging waste materials for analysis before they entered recycling streams.Monolayers are made from a single material type and grade, whereas multilayers generally comprise various materials and layers. In specific cases, player plastic films can have up to 17 polymer layers. Moreover, the presence of fillers and additives further increases the complexity of these materials, impacting recycling efficiency. 

In the study, the researchers separated multilayers from monolayers and also found several other multilayer classes in the process — indicating the complicated chemical structure of plastic films. 

Under a microscope

The researchers collected packaging waste materials before they entered municipal waste systems. One hundred fifty-four post-consumer plastic packaging films were analyzed, consisting of 83 monolayer and 71 multilayer samples. 

Each sample was studied using transflectance near-infrared hyperspectral imaging data collected on a laboratory scale sorting line and principal component analysis. 

Moreover, the researchers analyzed layer thickness and material type to specify further the components that made up the plastic films. 

Specific sorting 

The study concludes that recycling processes are more effective through specific multilayer plastic film sorting. 

Sortation can reduce chemical and mechanical recycling contamination and improve energy recovery through optimized combustion and emissions.

The study distinguishes between monolayers and multilayers and between main material classes and their subclasses, enabling more precise sorting.

The researchers point to a three-stage process for more efficient plastic film recycling: “The first step involves collecting reference data from virgin or post-consumer multilayers with known compositions to develop classification models. Secondly, transferring these models to inline mechanical sorting could efficiently sort the target multilayer material stream.”

“Finally, the precise compositional information of the resulting fractions could enable the implementation of optimized recycling processes to recover specific polymers or monomers.”