Microwave Pyrolysis of Polymers in a Supercritical Medium

Plastics are ubiquitous in modern society. From food packaging to clothing, car interiors to children's toys, this versatile material is one of the bedrocks upon which society rests. Plastic products are composed from a range of polymers, long chains consisting of repeat units of identical chemical units, which exhibit properties such as flexibility, transparency and chemical inertness.

Such diverse properties are a large factor in the popularity of plastic materials. However, they also present a problem. Durability and chemical inertness of plastics, combined with very high consumption levels, have resulted in large amounts of waste material requiring treatment. This is a global issue; 335 million tonnes of plastic was produced worldwide in 2016 and this is expected to rise to 1.124 billion tonnes by 2050. This colossal scale of plastic waste is a symptom of the linear take-make-waste model where valuable resources are left to persist as waste. The circular economy model aims to disrupt the linear take-make-waste model, through restructuring current markets to prioritise the prolonging of a product's lifetime and reducing in consumption of raw materials. One of the changes that will need to occur in the transition to a circular economy is improvement in plastic recycling.

Mechanical recycling enables waste plastic to be processed into useful materials, but the intense processing conditions degrade the material resulting in it only being available for less valuable applications post-recycling. Pyrolysis can produce useful chemicals from plastic waste feedstocks, selectivity being given by variation in temperature and processing time, but the heterogeneity of plastic waste feedstocks necessitate complex separation steps. Furthermore, the broad range of polymers in plastic materials demands bespoke processing conditions for each material.

Microwaves have two principal advantages over conventional thermal heating Firstly, they heat a substance volumetrically, allowing high temperatures to be reached on a short timescale. Secondly, they are able to heat selectively, making it possible to target one component of a mixture.

The high temperatures microwave heating of a substance can achieve are essential for overcoming the chemical inertness of polymers, whilst the selectivity could present a pathway towards tackling the heterogeneity challenge associated with plastic recycling. A material must possess at least one of free charge carriers or a permanent dipole to be susceptible to microwave heating.

Both of the polymers that will be studied in this project, Poly(ethylene terephthalate) and Poly(bisphenol A carbonate), possess permanent dipoles in their structure. One such example where this selectivity could be useful is if either were processed as part of a heterogenous mix which contained apolar polymers, like polyolefins, as they would not respond to microwave heating and be left untouched.

However, there is one key barrier to successfully heating polymers which microwaves: their size. Polymer chains are very long structures, and they can be closely packed or interwoven in higher density plastic products. This is a problem when we consider using microwave heating to degrade polymers as the alternating EM field of the microwaves induces spinning of the polymers, with heat being generated by the friction of the chains spinning against each other. Close packing of polymer chains presents a mechanical barrier to this spinning.

One strategy to overcome this is the utilisation of supercritical carbon dioxide as a swelling agent, and a solvent for the recycling process. Supercritical CO2 is a unique state of matter, achieved above 31.1 degrees C and 72 bar, where the substance takes on a mixture of gas and liquid properties. Due to the high pressures needed to reach the supercritical state, the CO2 is able to permeate into polymeric structures and move individual polymer chains apart.

This CDT will deliver impact aligned to the following agendas:

People
A2P will provide over 60 PhD graduates with the skill sets required to deliver innovative sustainable products and processes into the UK chemicals manufacturing industry. A2P will inspire and develop leaders who will:
- understand the needs of industrial end-users;
- embed sustainability across a range of sectors; and
- catalyse the transition to a more productive and resilient UK economy.

Economy
A2P will promote a step change towards a circular economy that embraces resilience and efficiency in terms of atoms and energy. The benefits of adopting more sustainable design principles and smarter production are clear. For example, the global production of active pharmaceutical ingredients (APIs) has been estimated at 65,000-100,000 tonnes per annum. The scale of associated waste is > 10 million tonnes per annum with a disposal cost of more than £15 billion. Consequently, even a modest efficiency increase by applying new, more sustainable chemical processes would deliver substantial economic savings and environmental wins. A2P will seek and deliver systematic gains across all sectors of the chemicals manufacturing industry. Our goals of providing cross-scale training in chemical sciences with economic and life- cycle awareness will drive uptake of sustainable best practice in UK industry, leading to improved economic competitiveness.

Knowledge
This CDT will deliver significant new knowledge in the development of more sustainable processes and products. It will integrate the philosophy of sustainability with catalysis, synthetic methodology, process engineering, and scale-up. Critical concepts such as energy/resource efficiency, life cycle analysis, recycling, and sustainability metrics will become seamlessly joined to what is considered a 'normal' approach to new molecular products. This knowledge and experience will be shared through publications, conferences and other engagement activities. A2P partners will provide efficient routes to market ensuring the efficient translation and transferal of new technologies is realised, ensuring impact is achieved.

Society
The chemistry-using industries manufacture a rich portfolio of products that are critical in maintaining a high quality of life in the UK. A2P will provide highly trained people and new knowledge to develop smarter, better products, whilst increasing the efficiency and sustainability of chemicals manufacture.
To amplify the impacts of our CDT, effective public engagement and technology transfer will become crucially important. As a general comment, 'sustainability' styled research is often regarded in a positive light by society, however, the science that underpins its effective implementation is often poorly appreciated. The University of Nottingham has developed an effective communication portfolio (with dedicated outreach staff) to tackle this issue. In addition to more traditional routes of scientific communication and dissemination, A2P will develop a portfolio of engagement and outreach activities including blogs, webpages, public outreach events, and contribution of material to our award-winning YouTube channel, www.periodicvideos.com.

A2P will build on our successful Sustainable Chemicals and Processes Industry Forum (SCIF), which will provide entry to networks with a wide range of chemical science end-users (spanning multinationals through to speciality SMEs), policy makers and regulators. We will share new scientific developments and best practice with leaders in these areas, to help realise the full impact of our CDT. Annual showcase events will provide a forum where knowledge may be disseminated to partners, we will broaden these events to include participants from thematically linked CDTs from across the UK, we will build on our track record of delivering hi-impact inter-CDT events with complementary centres hosted by the Universities of Bath and Bristol.