Understanding the impact of recycled content in plastic packaging

Abstract

The linearity of the global plastics economy is both an environmental crisis and socio-economic challenge. Global recycling rates lie as low as 16%—most plastics are produced, used, and sent to landfill. We must better retain the value of plastics and close the loop by re-directing plastics for reuse and recycling. However, current mechanical recycling approaches negatively impact recyclate properties and price, rendering it undesirable for remanufacture. This thesis explores two complementary approaches to circumvent key barriers—quantification and degradation—that inhibit incorporation of recycled plastic in packaging. Chapter 2 presents a novel fluorescence-based analytical approach to verify recycled content in plastic packaging. Through exploitation of solid-state aggregation-induced enhanced emission (AIEE) of a common optical brightener, a linear relationship between recycled content and three different fluorescence parameters is established. The effects of this dye doping on the material (e.g. mechanical and thermal properties) are also reviewed. It is demonstrated that supramolecular interactions between the dye (guest) and polymer (host) govern the level of AIEE within the system, and thus the efficacy of recycled content determination. Additionally, using further characterisation, the competitive influences of polymer crystallinity, aromaticity, and dye solubility on this fluorescence based methodology are explored, and AIEE response is shown to be governed by aromaticity. Industrial viability is also investigated, and it is revealed that the analytical method is unaffected by key industrial parameters such as additive package, colour, and waste stream source but displays a mild vulnerability to UV-based degradation. This fluorescence methodology offers a robust quantitative approach, while showing applicability to a range sample types with minimal impacts on properties and waste-sorting infrastructure. Chapter 3 is centred on understanding the degradation pathways in poly(ethylene terephthalate) (PET) during typical recycling processes, and how these pathways affect plastic properties. Initially, a baseline understanding of mechanical recycling under standard conditions (280 °C in air) is pursued. The effects of degradation in PET begin to dominate after three consecutive recycles. Following this mechanical recycling work, the effects of changing the gaseous environment during high temperature processing are probed through extensive rheological testing, simulations, and molecular weight characterisation. The presence of N2 or CO2 imparts a protective effect by mitigating chain scission, and CO2 is revealed to have an additional plasticising effect. This work is expanded to test the effect of competing reactions in mixed gas systems, and the presence of O2 (from air) is identified as the largest contributor to degradation in mechanical recycling. These concepts were then translated to gas-mediated extrusion trials, and a similar trend is found: the molecular weight of the PET recyclate depends on the O2 content within the extrusion environment. Overall, the work presented within this chapter aims to support circularity by providing further insight into PET degradation mechanisms to facilitate creation of improved PET recycling processes. The overarching conclusions to this work are summarised in Chapter 4 and followed by suggestions for potential future work. Experimental details, including materials, methods, and characterisations, are presented in Chapter 5.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Shaver (Supervisor) & Lee Fielding (Supervisor)