Posted by: Papp Noémi Comments: 0 Post Date: 2023.05.21.
In addition to the basic functions of product protection, ease of transport and optimum storage, packaging must also be adapted to the latest sorting and recycling processes. This needs to be thought about in advance, and the raw material we want to recycle also matters.
Plastics can be recycled in two ways: ‘raw material’ or ‘mechanical’ recycling refers to physical treatment processes that preserve the basic chemical structure of polymers. In this case, the plastic waste is first sorted, cleaned, potential contaminants are removed, shredded and then remelted or blended into a new material. The other way is to opt for “chemical recycling”, where the polymer is chemically broken down into low molecular weight compounds, purified and then re-polymerised. The broad term ‘raw material recycling’ includes both mechanical and raw material recycling.
Much depends on the selection
The sorting or the sorting technology is the most important factor in the subsequent recycling process, so the primary goal of recycling-oriented design is to enable clear material sorting. The following standard technologies are used for sorting plastic types: magnetic sorting, induction separator, near infrared spectroscopy (NIR), flotation. In plastic recycling, near-infrared spectroscopy-based sorting is essential for correct [primary] sorting according to the material fraction of the packaging material. If material recognition is not possible, the packaging material cannot be sorted to the correct material stream and/or is not sorted to the correct destination or is rejected by the selector. This problem occurs, for example, when the entire surface of a bottle has been sleeved and the material does not match the bottle and/or the belt has been printed on the entire surface, so that the material colour of the bottle cannot be identified and therefore cannot be assigned to the correct material stream. Similar problems arise with the use of carbon pigments (black), which absorbs infrared rays and thus prevents material identification.
What else matters?
The second important distinguishing characteristic is the material-specific density. Different types of plastics have different densities, which are also used in sorting technology to differentiate them. If the density of the different types of plastic is artificially changed (e.g. by adding density increasing additives that increase the density of PP above 1 g/cm3), the sorting process can no longer be used in its normal form, as the distinguishing characteristic of the material has changed. The critical limit is density above or below 1 g/cm3 . The density of PET bottles is generally above 1 g/cm3, whereas the density of HDPE closures and PP labels is below 1 g/cm3.
Thanks to this difference, sorting can be carried out very efficiently and easily using the so-called settling-float-suspend tank method. Flotation is a density-based separation process in which shredded plastic flakes are separated, usually using water as a flocculant. In this way, polymers with densities of less than 1 g/cm3 (e.g. PP, PE) can be separated relatively easily from plastics with higher densities (e.g. PET, PS, PVC). Several research projects are currently working on chemical recycling. It is expected that chemical recycling processes will also be widely used in the next few years. Recycling is greatly simplified if the material is homogeneous, such as the triple-layer polypropylene already offered by Sipospack Ltd. and recommended to its partners. This is because it is fully recyclable, but also has the features required by the packaging industry, such as high gas barrier or strong vapour barrier, chlorine-free, easy to print, to laminate, to pasteurise.