However, this can be stated up front: no two plastics are alike. And depending on a particular plastic’s physical properties, it will exhibit a different hardness, resilience, heat-resistance and thermal stability – which entails, of course, different fields of application. Transparent PET, for example, is predominantly used for producing beverage bottles. Hard-wearing polyolefins (POs), by contrast, serve primarily for making sturdier containers, toys, pipes and household goods. Precisely because of their wide dissemination it is essential to recycle the end-products after use and recover the plastics they are made of.
PET: washing and decontaminating
For PET, Krones has since 2009 already been closing a beverage bottle’s life-cycle by means of its MetaPure technology. The portfolio includes the company’s own modules for washing and decontaminating.
- The flakes are washed in the MetaPure W-PET. Here, they are passed through a number of process steps (pre-treatment, caustic washing and hot post-washing), so that ultimately pure flakes are obtained that are ideal for being turned into fibre or film.
- If the PET flakes are intended to be re-used in the beverage or food industries, they are passed to the MetaPure S. This is because the decontamination module treats the washed flakes in such a way that they can be turned into food-grade pellets, preforms and film. By means of solid state polymerisation (SSP), the intrinsic viscosity can be increased and matched to suit the end-product into which they will be made in each case. Since it is faster to decontaminate flakes than pellets, the MetaPure S excels in terms of low energy consumption and gentle material handling.
PO: upcycling instead of downcycling
Recycling PO (e.g. HDPE or polypropylene) entails numerous challenges for the treatment process, one of which is how the material is returned, with the concomitant side-effects. For the packages we’re talking about here are in Germany, for example, or the USA, collected in the household garbage, or in separate bins or sacks for plastics. Since the material thus obtained is usually not cleaned before recycling, the polyolefins have invariably been in contact with organic residues like ketchup, mayonnaise, oil, yoghurt, shampoo, soap, household cleaning agents, and many other substances. This creates an enormous biological and chemical loading, going hand in hand with an unpleasant smell. And that in turn renders it relatively difficult to make sure the end-product is odour-neutral.
Yet another challenge is posed by the physical properties of PO. As these plastics exhibit a density of lower than one gram per cubic centimetre, they float on the water’s surface. When washing PET, this is a huge advantage, since the closures and labels made of PO can thus be simply skimmed off the surface and removed, while the PET sinks to the bottom. When washing PO itself, by contrast, 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, meaning inferior washing quality cannot be compensated for during further processing of the washed flakes.
Density (in g/cm³)
|Polyolefins: < 1||PET: 1.38 to 1.40|
|o PP: 0.90 to 0.915
o LDPE: 0.915 to 0.935
o HDPE: 0.94 to 0.97
So as to avoid impurities caused by a wrong colour and residual soiling to a certain extent, the simplest solution often is: add colour, and then manufacture downcycled plastic products. But meanwhile there are growing aspirations to recycle polyolefins as well (e.g. polypropylene and polyethylene) and render them amenable to top-quality re-use. To meet exactly these requirements, Krones has for its MetaPure W-PO washing module modified the procedural approach accordingly and developed a recycling technology meticulously fine-tuned to the requirements posed by PO. For example, tried-and-tested individual units have here been matched to the PO-specific idiosyncrasies concerned, while the process-engineering features and the overall concept involved continue to be based on the extensive fund of experience gathered in the field of PET washing technology. In numerous trials on the company’s own washing system, optimum results in regard to purity and odour-neutrality could be achieved. Krones has thus provided proof positive that top-quality recovery of these polyolefins is indeed possible – and this is both ecologically and economically expedient.