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Depolymerisation of real-world plastic waste

Project Description

Plastics are an integral and important part of the global economy and will continue to be critical in addressing the ongoing demands of our society. Most plastics recycling involves mechanical methods such as grinding and re-processing the material. These processes retain some value of the plastic but lead to degradation of the polymer chains and the incorporation of contaminants that lead to a decrease in molecular weight, loss of mechanical properties, chemical or food contamination, discoloration and ultimately lower-value plastic products. For example, poly(ethylene terephthalate), PET, is limited in how many times it can be mechanically reprocessed before it’s properties have degraded too much to be useful. While it is typically blended with virgin PET to extend that lifetime, it will eventually be too contaminated or have degraded too much to be useful and commands a lower economic value than virgin PET and ultimately finds its way to landfill or incineration. Another approach for treating post-consumed plastics relies on their thermal or photochemical conversion into small molecules. While photochemical conversion provides an exciting, low energy approach to plastic degradation, thermal degradation by pyrolysis, a treatment that requires elevated pressure and temperature, produces high calorific value fuels that when burnt to produce energy, release undesirable gases into the environment. While this process can depolymerise polyolefins (which make up a large proportion of plastics produced), degradation of other plastics to specific and well-defined products is challenging. While these methods all have a place in finding a solution to dealing with the problem of plastic waste, the use of solutions with a minimal energy input must also be considered to enable economic and environmental sustainability.

Chemical degradation presents an attractive potential method by which to create a sustainable polymer supply chain. The concept of the circular economy is one that is receiving a lot of attention. In an idealised circular economy, plastics would be degraded back to monomers that can be repolymerised to yield virgin materials that give the original performance back after processing. While this has been demonstrated for polymers such as poly(γ-butyrolactone) or polylactide, at present, of the commonly applied materials, only poly(ethylene terephthalate), PET, can be readily treated in this way. Recently, we have developed a highly efficient method to depolymerise PET using highly stable ionic organocatalysts that produce high purity monomer that can be directly repolymerised to virgin PET. There are however several complications that may prevent such a technology from being realised with real-world plastic wastes. For example, in furniture fabrics, PET is commonly woven as the fabric covering however, the legislation around flame retardancy requires that additional flame retardant additives are included into the fibres which significantly affect its recyclability.

As such, a process that uses waste plastic as a substrate must be tolerant to a wide range of impurities such as other waste, colourants and plasticisers that exist in many processed plastic wastes. If plastic wastes that contain such materials can be recycled by a chemical method such as those proposed herein, a wider range of (lower value) plastic waste feedstocks could potentially be accessed. While our initial focus will be to evaluate the effect of flame retardant additives on PET degradation, we will also identify the most commonly used additives in PET to build up a bigger picture of the translatability of this technology throughout the project. The potential of these small molecule compounds to deactivate the catalyst species will be evaluated by studying PET depolymerisation in the presence of added additives, both singly and in combinations at a range of loadings using the rate of depolymerisation (measured as outlined previously) to determine the effect. The interactions with the catalyst species will be determined and measured and new catalysts identified to overcome any identified limitations.

The candidate should possess a good (1st or 2.1 UK or equivalent) degree in chemistry from a well-regarded institution. Previous experience of polymer chemistry is not essential. The candidate should possess:
• interest in interdisciplinary research
• willingness to learn new techniques.
• ability to keep up to date with new developments in the fields relevant to the project.
• ability to work both independently and as part of a team on research programmes.
• excellent interpersonal and communication skills

Funding Notes

This project is part of the Global Challenges Scholarship.
The award comprises:

Full payment of tuition fees at UK Research Councils UK/EU fee level (£4,327 in 2019/20), to be paid by the University;
An annual tax-free doctoral stipend at UK Research Councils UK/EU rates (£15,009 for 2019/20), to be paid in monthly instalments to the Global Challenges scholar by the University;
The tenure of the award can be for up to 3.5 years (42 months).

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