rPET – A glimpse into a pilot project

PET is a plastic that can be 100 per cent recycled in a bottle-to-bottle process. This beneficial characteristic of PET has unfortunately been ignored in quite a few emotional debates on pollution of our natural environment and our general management of plastics. And it is, of course, correct that many millions of PET bottles are not properly re-used or disposed of but end up in our oceans. Whether this is due to the lack of political rules or to insufficient awareness-raising among the populace, to patchy infrastructures or gaps in the flow of re-usable materials – it is a condition that nobody can possibly wish for and that has to be urgently changed. Krones AG, too, is making its contribution here, by its successful production of PET recycling plants that goes back more than 10 years. These are up and running, for example, at our customers in Bangladesh, Japan, Africa or the USA.

The use of recycled plastic has only recently been rendered mandatory in various regulations worldwide – in quite different ways and at different times, but the trend is unambiguous: the use of recycled PET for containers has been rising, as has the demand for it. So we in the Plastics Technology Division at Krones AG have also been devoting our attention for years now to recycled PET (or rPET for short) – here, the focus is on the resultant container: our clients’ product.

Empirical data on how to handle rPET

What our customers want to know from the Krones experts first and foremost is whether – and if so – what influence rPET exerts on the process engineering and energy consumption levels involved, and on food safety. So as to research precisely this in even greater depth, the “PET Packaging Development and Consulting” group of Krones AG launched a pilot project in late 2019.

For the different series of trials, a variety of container geometries – ranging from simple to ultra-complex, and from typical to rather out-of-the-ordinary – could be produced by means of a modularised mould. So this makes for economical use of resources both for production and the test set-up itself – all in line with the higher-order imperative of sustainability. By running tests with disparate container geometries, we have been able to effectively check whether there are any restrictions in terms of container design and whether the bottle’s mechanical performance changes when rPET is added to the preform: “design from recycling” and “design for recycling” is the watchword.

Changes in this context could, for example, include a more viscous behaviour in the stretch blow-moulding process, different reshrink characteristics of ageing containers or visual impairments like what are called black specks. These latter are caused by plastic types other than PET encountered as contaminants in the PET recycling flow. These non-PET plastics carbonise due to further processing of the material and are then visible as black specks. It would be desirable, of course, if the material flows did not contain any contaminants of that kind in the first place – this could be assured either by suitable collection systems, of the type we have in Germany in the shape of the reverse vending machines, or by regulatory exclusion of certain materials for bottle dress and closure, etc. The questions of whether – and if so – what influence these plastic contaminants have on the food safety of the product filled were like examined in the pilot project, with assistance from the Fraunhofer Institute and found to be non-critical. For the time being, however, we will have to live with the possibility of visual impairments – but Krones AG can already offer a solution even for this so that our customers need not worry at all about the visual appearance of their products when filling them in PET bottles: the inspection technology in our machines and lines enables any contaminated preforms or bottles to be detected at an early stage, rejected and passed to another recycling process. Admittedly, experience has shown that this increases what is called the scrap rate – but the material concerned is not lost, and the additional energy needed remains very small as compared to using virgin PET.

The illustration below shows how much energy is consumed for obtaining virgin PET, and producing and converting containers made of it as compared to that consumed for bottles made of rPET:

Fig. 1: Energy consumed in the production of virgin PET right up to the finished bottle (left) and in the recycling process of the used bottles collected right up to producing a new PET bottle (right) (IM: injection moulding; SBM: stretch blow-moulding)

As a rule, visual impairments also include coloration changes in the PET material caused by the recycling process. Containers made of rPET appear somewhat darker, yellower – or also sometimes somewhat greyer. Some of our customers who are already producing their containers from rPET dye the material with blue or green pigments, so as to conceal the yellowing. But in our view, it can also have a beneficial effect when consumers can see that a particular container is a recycled one – just as with recycled paper, a slightly grey discoloration of a PET bottle may become a status symbol and address a quite specific user group.

Latest research at Krones

Aurélie Börmann is a chemical engineer and has been working at Krones AG in the “PET Packaging Development and Consulting” group since 2017. In the interdisciplinary team, she has been responsible for conducting and evaluating the series of trials to do with rPET performed so far.

Aurélie, what exactly was the goal of the current series of trials?

The goal of this series of trials was to examine the material, the process settings, the bottle performance, design restrictions if any, and product safety with a rising proportion of recycled PET.

Where there any results that came as a surprise to you?

Despite the rising content of rPET in the preforms, only minimal modifications had to be made to the blow-moulding process; what’s more, energy input remained the same. What was gratifying, of course, was the fact that all container geometries tested – ranging as you mentioned above from simple to ultra-complex – produced excellent results so that we could not find any restriction on container design.

A pleasant side effect likewise won me over: the greyish touch lends the material a certain degree of elegance, manifesting the bottle of tomorrow!

Are there any restriction on rPET in terms of process engineering?

In purely chemical terms, rPET is not any different from PET. The challenges involved are rather to be found in the quality of the input material. Correspondingly, there are no unambiguously anticipated restrictions in terms of processability, of the kind possibly encountered with other materials. In regard to our series of trials, we succeeded – as mentioned earlier – in proving that all bottle geometries complied with the container specifications to an equally good effect, irrespective of the rPET content.

What material exactly was examined?

The recycled material we examined comes from Europe and was produced mainly from bottles and possibly also from display trays that had previously been used for food and beverage packaging. A small proportion (< 5 %) could also be accounted for by non-food containers, like PET bottles for soap, detergents, etc. The Netherlands possess a PET deposit system with very high return ratios. So we presume that the majority of our material originates from this material flow. The advantage is that what’s involved here is possibly a very pure closed cycle with only a few contaminants. This had a beneficial effect on our results since the scrap rate was more or less zero per cent.

What are the next steps in the project?

During the first series of trials, an extremely fit-for-purpose preform-bottle combination was chosen, thus rendering possible a very large process window for the moulding function. Now it would be worth considering to what extent the material can be stressed, and what behaviour the process windows will show for lightweight containers, for example. Moreover, we will in cooperation with the Fraunhofer Institute be conducting further trials on product safety.

From the present point of view, what would you recommend to our customers?

Consumption of fossil resources should be avoided, and the use of sustainable materials encouraged. By using more recycled PET in beverage bottles, we can contribute our share to transforming the market and getting it closer to a closed-cycle economy. rPET has been processed on Krones blow-moulders for  years now and shows no significant restrictions in terms of processability, quality and food safety. The colour change caused by repeated heat-up and melting could in future even become a status symbol: we are committed to resource-economy use of resources!

Thank you, Aurélie, for the interview. I’m already looking forward to further results from our cooperation.