Highly selective microwave decomposition of plastic waste

Nelson, Matthew 2026. Highly selective microwave decomposition of plastic waste. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Growing amounts of plastic waste material pollute the planet contributing to a global ecological and environmental crisis. Common hydrocarbon based plastic materials do not quickly or safely biodegrade, but their high hydrogen and carbon contents present an opportunity for extraction of valuable materials. Of the potential materials that could be produced, two of the most interesting are hydrogen gas and carbon nanotubes, which themselves could be used for a range of useful applications such as clean energy generation, improved battery systems and biomedical applications. This thesis presents an investigation into the use of microwave heating to rapidly produce valuable products from waste plastic material. Microwave heating offers benefits compared to conventional methods including rapid heating and material selectivity. In particular, the use of the magnetic field of a microwave resonant cavity is explored for its potential to be even more rapid and selective even when compared to standard microwave dielectric heating mechanisms. With relatively few materials that can be effectively heated by a high-frequency magnetic field, the useful energy can be more effectively directed into a chosen catalyst, increasing efficiency and maximum temperatures. Cost effective and abundant iron oxide powders are chosen for the core exploration. These are magnetite (Fe3O4) and haematite (Fe2O3) and are characterised using microwave cavity perturbation techniques to determine their complex permeability and permittivity. This is performed across a range of frequencies and at elevated temperatures up to 198 °C to provide a detailed electromagnetic profile and investigate potential frequency and temperature dependencies. Practical demonstrations of microwave magnetic heating are performed, illustrating the nature of selective heating. Magnetite is assessed for its suitability as a catalyst for rapid microwave assisted plastic decomposition, showing rapid heating potential to temperatures above the thermal decomposition requirements of common household plastics (> 620 °C). This is achieved with limited power inputs typically under 100 Watts. This heating technique is applied to mixtures of magnetite and common plastics, focusing on polyethylene. Rapid decomposition of plastics was achieved with produced gaseous mixtures and solid residues analysed. This revealed limited hydrogen content and the most significant quantities consisting of alkane hydrocarbons including methane, ethane and propane. Analysis of solid residues revealed frequent reduction of magnetite to wüstite (FeO) and iron (Fe) which was theorised to be enabled by available hydrogen or carbon monoxide. No crystalline carbonaceous materials were detected. Microwave magnetic heating has the potential to be a useful tool for a range of uses across fields of catalysis, chemical synthesis, and material treatment, including for the application of rapid plastic decomposition. This work demonstrates that magnetically-driven microwave heating enables selective, low-power catalytic decomposition pathways distinct from conventional dielectric microwave approaches.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Engineering
Uncontrolled Keywords:	1. microwave heating 2. microwave characterisation 3. magnetite 4. magnetic field 5. plastic waste 6. resonant cavity
Date of First Compliant Deposit:	30 September 2026
Last Modified:	30 Sep 2026 15:30
URI:	https://orca.cardiff.ac.uk/id/eprint/189872

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