Valorisation of thermoformed PET packaging waste through mechanical recycling technologies

This thesis aimed to investigate the recyclability of PET (polyethylene terephthalate) thermoformed plastic packaging waste, shortly named “PET trays”. Nowadays, the management of PET tray waste is still challenging since it consists of multi-material objects. Indeed, this packaging waste affects the consolidated recycling of mono material PET bottles because it represents an impurity in the stream. This happens especially in countries like Italy, where the DRS (Deposit Refund System) for beverage bottles is not implemented, and the sorted product streams of bottles and trays can overlap. Also, the increased demand for high-quality rPET underscores the urgency of developing recycling processes to transform sorted PET trays into transparent, food-grade rPET, in a closed-loop pathway. Likewise, introducing a specialised process for PET trays promises not only the optimisation of bottle flows but also the facilitation of the valorisation and the recovery of trays. In order to answer the scientific question, two main approaches were adopted in the thesis: the laboratory activities on PET tray waste and the environmental assessments performed through LCA (Life Cycle Assessment) analyses. In the experimental phase, 4 representative samples have been studied: 2 lightweight packaging waste (LWP, composed of beverage cartons, plastic packaging and metals) taken from a cross-docking station and 2 samples of PET tray sorted products taken from a sorting plant. The samples were sorted into 20 categories representing their packaging use and material build-up. The material composition of at least 10 representative PET trays from each category, considering only the transparent categories, was characterised. Afterwards, samples of each category were separately mechanically recycled on bench-scale and the recycling products were analysed with a list of thermal, optical and spectrophotometric analysis techniques. The data generated were assessed with a multi-criteria decision analysis (MCDA) approach to identify the categories most suited for the mechanical recycling process. At the end of these activities, the PET tray categories were classified into three levels of mechanical recyclability (high, medium and low). A select few less common PET trays were found to have the potential to yield transparent recycled PET through mechanical recycling methods. Though, the most ubiquitous PET trays in LWP were only found to be suitable for producing opaque recycled PET with mechanical recycling processes. PET trays composed of multiple objects and inseparable materials render the lowest quality recycled PET and correspond to the least desired results. Based on the categories’ recyclability level, four different feedstocks were defined to scale up the mechanical recycling process on a pilot-scale, applying extrusion treatments into pellets and injection moulding into plaques. The scale-up process aimed to understand the impurity removal efficiency of the applied process and the impact of the level of impurities present in the feedstocks on the properties of the obtained rPET. Following the experimental study, two distinct life cycle assessments were conducted. The first assessment purpose was to underscore the advantages of introducing PET tray sorting in the facilities where it is not practiced. The second assessment aimed to examine the impacts and benefits of employing the mechanical recycling process on PET trays. This research has enhanced understanding of thermoformed PET packaging flow waste, offering remarks for recycling and paving the way for future replications. This path holds promise for PET trays to emerge as an excellent substitute material for virgin PET in the food packaging sector, contributing to a more sustainable and recyclable system.