Different plastics exhibit different physical properties, such as differing hardness, resilience, heat resistance and thermal stability, thereby entailing varied fields of application. Transparent PET is primarily used for producing beverage bottles, whereas hard-wearing polyolefins (POs) are used to make sturdier containers, toys, pipes and household goods. Because of their wide dissemination, it is essential to recycle the end products after use and recover the plastics they are made of.
For PET, Krones closes a beverage bottle’s lifecycle using its MetaPure technology, with the portfolio including the company’s own modules for washing and decontaminating. The flakes are washed in the MetaPure W-PET, where they pass through a number of process steps including pre-treatment, caustic washing and hot post-washing. Through this process, pure flakes are obtained that can be turned into fibre or film.
PET flakes that are to be re-used in the beverage or food industries are passed to the MetaPure S, as the decontamination module treats the washed flakes so they can be turned into food-grade pellets, preforms and film. Solid state polymerisation (SSP) can be used to increase the intrinsic viscosity and match it to suit the end product into which they will be made. The process of decontaminating flakes is faster than decontaminating pellets, and it is facilitated by the MetaPure S, which provides low energy consumption and gentle material handling.
Recycling PO (eg, HDPE or polypropylene) presents many challenges for the treatment process, one of which is how the material is returned, with the associated side effects. For packages collected from household garbage, the material obtained is usually not cleaned before recycling, meaning the polyolefins have been in contact with organic residues like tomato sauce, mayonnaise, oil, yoghurt, shampoo, household cleaning agents and other substances. This creates a sizeable biological and chemical loading and produces an unpleasant smell, making it difficult to produce an odour-neutral end product.
The physical properties of PO present another challenge. Exhibiting a density lower than one gram per cubic centimetre, plastics often float on the water’s surface. This is advantageous when washing PET plastics, as the closures and labels made of PO can be skimmed off the surface and removed, leaving the PET to sink to the bottom. Conversely, when washing PO by itself, it is essential to adjust the process appropriately. It is this step in the process, especially, that is of crucial importance as far as the recyclate’s quality is concerned. Inferior washing quality cannot be compensated for during further processing of the washed flakes.
To avoid impurities caused by a wrong colour and residual soiling, the simplest solution is to add colour, then manufacture downcycled plastic products. There are plans to recycle polyolefins (such as polypropylene and polyethylene) and render them amenable to top-quality re-use. To achieve this, Krones has modified the procedural approach for its MetaPure W-Po washing module and developed a recycling technology fine-tuned to the requirements posed by PO. For example, individual units have been matched to PO-specific idiosyncrasies, while the process-engineering features and the overall concept involved continue to be based on the experience gathered in the field of PET washing technology.
Trials on the company’s washing system achieved high results in regard to purity and odour neutrality, thus demonstrating Krones’ capacity to recover polyolefins in an ecologically and economically expedient manner.