Upgrading (mixed) plastics through a composite approach

Abstract

Employing a circular economy model to plastic value chain remains a challenge because post-consumer plastics consist of various types and grades mixed together. This thesis describes the use of a composite approach to allow the reuse of mixed plastics, reduction of virgin plastics content, and recycling of post-consumer plastics. In Chapter 3, using polyethylene terephthalate (PET) and polypropylene (PP) as model immiscible polymer blend, the incorporation of short carbon fibres (CFs) increased the mechanical properties of the resulting composites. Cradle-to-grave life-cycle assessment showed that CF-reinforced PET/PP composites have a lower environmental impacts compared to the life-cycle scenarios of “doing nothing” and repurposing immiscible PET/PP blends as they are without CFs. This is due to the significant weight savings, which are a direct result of their high mechanical performance. In Chapter 4, the reinforcing potential of TEMPO-oxidised cellulose (TOC) with different fibre diameters for polypropylene (PP) are investigated. Micrometre-sized TOC fibres (TOCF) serve as better reinforcement for polymers composites compared to nano-sized TEMPO-oxidised nanocellulose (TOCN). PP composites reinforced with micrometre-sized TOCF possess the highest tensile modulus with 40% and 30% improvement over neat PP and PP/TOCN composites, respectively. This is attributed to the presence of surface microfibrillation on TOCF, which enhanced the quality of TOCF-PP interface through mechanical interlocking and the local stiffening of the PP matrix. Lastly, the structure-property relationships of HDPE blends containing virgin HDPE (vHDPE) with various reprocessed HDPE (rHDPE) are investigated. Blends of vHDPE and repeatedly-extruded HDPE exhibit tensile properties, complex viscosity and overall migration comparable to those of vHDPE. The chapter highlights the positive influence of blending the various grades of rHDPE with a virgin component to reduce the degree of discolouration, suppress the migration of low-molecular weight by-products of oxidation, as well as recover the tensile properties of the extensively reprocessed HDPE.