More energy-efficient material recycling of thermoformed products through annealing

Annealing as a possibility for optimized material recycling of thermoformed plastic products using the example of PLA (disposable beverage cups) and evaluation using life cycle (gap) analysis.

To enable effective material recycling of the biopolymer poly(lactic acid) (PLA) from disposable beverage cups, the ordinary process chain for mechanical recycling of plastics (washing, shredding and recompounding) was extended by integrating an additional annealing step. The thermal pretreatment, which leads to a relaxation in the molecular structure of the biopolymer, enables the PLA material to be subsequently comminuted with less energy input (total energy savings > 10%) and to generate flakes that exhibit improved flowability for further processing (production of rPLA pellets using an extruder). This makes it possible to recover PLA in new-product quality and significantly reduce the corresponding life cycle gaps (LCGs) (> 60%, see Fig. 1). A pictorial comparison of the materials with and without annealing step is shown in Fig. 1. The considered disposable cups are examples of thermoformed products. From an engineering point of view, it is in principle conceivable to test the additional process step of annealing prior to shredding on other products and thermoplastic polymers as well. Thus, any thin-walled component, such as PET bottles or thermoformed trays, could be subjected to additional thermal treatment before shredding in order to achieve higher process efficiency by means of an optimized molecular structure of the polymer and to further reduce the gaps in the cycle in an energy-efficient manner.

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The results of the work were described in the publication by Dieterle & Ginter (2022) and the LCA and LCC results for poly(lactic acid) (PLA) disposable beverage cups were evaluated from a circular economy (CE) point of view. For this purpose, the findings of the master's thesis by Ginter (2019) on "Mechanical reprocessing of post consumer poly(lactic acid) using the example of disposable beverage cups" were combined with the Life Cycle Gap Analysis (LCGA) (Dieterle, et al. 2018 Thanks to a targeted interpretation of results, LCGs > 82% could be identified according to the assumed disposal route for post consumer PLA beverage cups (thermal recycling), suggesting significant potential for further improvement from a CE perspective. Figure 2 below illustrates the results from the LCG perspective.



Dieterle, M., & Ginter, J. (2022). Life cycle (gap) analysis for advanced material recycling of PLA cups. Procedia CIRP 105, 13-18.

Ginter, J. (2019). Masterarbeit - Mechanische Aufbereitung von Post Consumer Poly(lactic acid) am Beispiel von Einweggetränkebechern. Karlsruhe: Karlsruher Institut für Technologie Institut für Angewandte Materialien - Werkstoffkunde (IAM-WK).

Dieterle, M., Schäfer, P., & Viere, T. (2018). Life Cycle Gaps: Interpreting LCA Results with a Circular Economy Mindset. Procedia CIRP Volume 69, 764-768.