Valorization of mixed polyolefin waste stream by addition of few-layer graphene

Abstract

This thesis aimed at upcycling a mixed polyolefin waste stream (MPWS) by addition of commercially available few-layer graphene (FLG), produced via mechanochemical exfoliation of graphite. The study commenced with designing polyethylene (PE)/ polypropylene (PP)/ FLG blend composites via melt-mixing, to have a basic overview of the influence of FLG on a controlled polyolefin blend system. Characterization of FLG-filled composites included the analysis of electrical conductivity, morphological evolution in molten condition and under shear-induced deformation, mechanical properties, and UV stability. Interestingly, a small amount of FLG, 4 wt.% and 5 wt.% of FLG induced electro dissipative behavior in PE/PP – 60/40 and PE/PP – 20/80 blends, respectively. Shear-induced deformation in molten condition did not affect the electrical conductivity, indicating the stability of the FLG network within the composite. Additionally, 1 wt.% of FLG maintained original mechanical properties in UV-exposed polyolefin composites. Comparatively pronounced UV stabilizing effect was observed in PErich blends than in PP-rich blends. Thereafter, the impact of adding FLG on the processability and mechanical properties of MPWS was examined. Results indicated that the addition of FLG facilitated the reinforcement of MPWS composites without compromising processability, a notable but unusual feature of rigid fillers. A maximum increase in tensile strength (by 9%) and flexural strength (by 23%) was observed in compounds containing MPWS, 4 wt.% of FLG, and 9 wt.% of prime PP. Higher FLG concentrations (>4 wt.%) in MPWS led to improved tensile and flexural properties while maintaining impact strength. Notably, 10 wt.% of FLG increased impact strength by 9%, attributed to FLG's resistance to crack propagation. Subsequently, the photo stabilizing effect of FLG was examined in both MPWS and a prime PE/PP blend. Despite the likely predegraded state of MPWS, UV-exposed composites containing 1, 4, 7, and 10 wt.% of FLG retained approximately 60%, 70%, 80%, and 90% of the original ductility, respectively. Unfilled MPWS retained only 20% of its original ductility after UV exposure. For prime polyolefin blends, presence of 0.5 wt.% was adequate to maintain surface finish and mechanical properties against photodegradation. Apparently, MPWS required higher FLG concentrations to achieve comparable property retention, possibly due to FLG's poor dispersion in MPWS and associated uncertainties such as impurities, predegradation, and polydispersity. Therefore, the concept of preconditioning prime polyolefins with a minimal concentration of FLG before their integration into practical applications arises, necessitating further feasibility assessment. Overall, this study highlights the potential of FLG as a versatile additive for improving the features of polyolefin blends, offering a promising and economically viable solution to mitigate plastic pollution.

Date	7 Aug 2024
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Nicole R. Demarquette (Supervisor), Éric David (Co-supervisor), Giovanna Gutiérrez (Co-supervisor) & Emna Helal (Co-supervisor)