title: Development of composites using waste mixed plastics and waste glass fibres
Creators: name: O'Rourke, Kit
Publisher: name: The University of Edinburgh
Year: 2025
Subjects: subject: mixed plastic packaging waste
subject: recycled plastics
subject: compression moulding
subject: mechanical testing
subject: fracture surface
subject: mixed polyethylenes
subject: Waste glass fibres
subject: Polyethylene blends
subject: Waste-based composites

description: Low-density polyethylene (LDPE) based packaging films, although recyclable, often end up in landfill sites after a single use due to their flexible nature, low strength and low cost, which make recycling economically unfeasible. Additionally, the necessity to separate and sort different plastic waste streams is the costliest step in plastic recycling, and is a major barrier to increasing recycling rates. Utilising waste mixed plastics (wMP) as a raw material can help mitigate this cost and improve overall recycling efficiency. To successfully integrate wMP as a commodity, a thorough understanding of the properties of wMP is required, particularly compared to virgin and individual recycled polyethylenes (PEs). The wMP studied in this work underwent a single sorting step, a float-sink separation method, resulting in a mix of plastics with densities lower than water. This PhD project aims to investigate LDPE-based wMPs from low-strength, low-density plastic film packaging waste. The properties of unreinforced wMP were investigated, along with its reinforcement with waste glass fibres (wGF), carbon nanotubes (CNT), and various fire retardants. This research is divided into four main investigations, which are summarised in the following paragraphs. The first study characterised wMP alongside individually recycled and virgin plastics. Differential scanning calorimetry (DSC) identified the different types of plastics that make up the wMP, thermogravimetric analysis (TGA) assessed its thermal stability to prevent degradation during reprocessing and rheometry measured the viscosities at different processing temperatures. wMP was processed by compression moulding in the as-received shred form, as well as after an intermediate extrusion step, to produce plaques. Processing parameters such as the pressure and cycle time were varied to understand their effects on the mechanical properties. The tensile, flexural, and compressive properties of wMP were measured, and the properties fell within the range of various virgin PE grades, indicating the possible variation between wMP batches. The intermediate extrusion step did not affect the tensile properties, but the failure behaviour noticeably differed; the wMP shred samples failed gradually, whereas extruded wMP failed suddenly, behaving like a single uniform material. No thermal degradation was observed during processing. Once characterised, wMP was used as a matrix with glass fibre (GF) reinforcement to produce composites. The GF was in the form of a non-woven mat and was out-of-spec production waste (wGF). A major concern with using waste materials is the variability in properties between different material batches. Reinforcement with wGF minimised this variability in wMP as the mechanical properties are primarily dictated by the fibres. The wGF used within this research consisted of short GFs that were not entirely randomly orientated, displaying a marginal difference in the machine direction (MD), and cross machine direction (CMD) due to the manufacturing process. The extent of this orientation and its effect on wMP/wGF composite properties was explored. wGF was coated with wMP via extrusion coating to produce consistent quality thermoplastic prepregs for laminate manufacture. The laminates were manufactured using compression moulding, with the processing pressure varied for optimisation. The wGF content was varied in the laminates from 6–26 vol%, and mechanical testing was performed to analyse the effects of the wGF content, fibre orientation and void contents on properties. A theoretical tensile modulus for wMP/wGF laminates (0–26 vol% wGF) was calculated using a modified Halpin–Tsai equation to account for the fibre orientation and void content of the laminates. Adding 6–26 vol% wGF to unreinforced wMP resulted in increases of up to 430% and 570% in the tensile strength and tensile modulus, respectively, up to 780% in the flexural modulus, and up to 350% and 700% in the compressive strength and compressive modulus, respectively. A 2-D dispersion of wGF was found to be superior to a 3-D dispersion for the tensile modulus, calculated via an empirical method incorporating the tensile results of the laminates. In addition to improved mechanical properties, any additional functionality can make wMP a useful material for second-generation products. Therefore, the next investigation explored wMP reinforced with CNT, or both CNT and wGF. wMP/CNT and wMP/CNT/wGF composites were manufactured via extrusion and compression moulding. The crystallinities were determined via DSC and the mechanical properties were measured. wMP/CNT composites were subjected to UV ageing for 500 hours to observe the effect of environmental conditions on flexural properties. The through-thickness electrical conductivity was measured and compared with equivalent virgin LDPE/CNT nanocomposites reported in the literature. Reinforcing wMP with 5 wt% CNT increased the tensile strength and tensile modulus by 46% and 23%, respectively. There were similar increases in flexural and compressive properties compared to unreinforced wMP. UV ageing had no measurable effect on unreinforced wMP but decreased the flexural modulus of wMP/CNT by 23%. Unreinforced wMP showed no measurable electrical conductivity as expected. wMP/CNT exhibited an average electrical conductivity of 1.65 × 10−2 S/m which decreased significantly with the addition of wGF. LDPE, with its low melting point and tendency to exhibit melt drip behaviour, can potentially contribute to fire spread. Therefore, the fire resistance of the wMP/ wGF laminates was assessed with and without various fire retardants (FRs) to understand their application potential in the construction sector. Suitable techniques were investigated to combine FRs with the laminates. Fire resistance was determined via cone calorimetry, which allowed the measurement of the heat release rate (HRR), total heat release (THR), burning time, effective heat of combustion (EHC), and carbon monoxide and carbon dioxide yields. The application of an inorganic hydroxide FR, aluminium trihydroxide (ATH), reduced the heat released during combustion by 18% and slightly shortened the burning duration compared to the untreated laminate. This is due to the release of water from ATH during burning. In contrast, an intumescent FR (IFR) veil was highly effective in reducing all hazardous fire characteristics while preserving the laminate’s structural integrity, with a reduction in the total heat released of 93% compared to an untreated laminate. This is due to the formation of a protective expanding char layer on the surface which shields the laminate from heat and blocks oxygen from reaching it. This work increases understanding of the fire behaviour of such waste materials. Such scientific understanding forms the basis needed to expand the potential applications of these waste materials, diverting them from landfill and creating a more circular economy.