Recycling PHA for Second life (RePHASe)

Transitioning towards a more circular economy is an important goal if we are to achieve a more sustainable future, especially for plastics. With the introduction of bio-based plastics as a more eco-friendly alternative to fossil-based, we must ensure that the circular economy model is able to support the re-use, recovery, and recycling of bio-based plastics as well.

One key family of bio-based plastics which may be able to replace conventional plastics are polyhydroxyalkanoates, or PHAs, as they have very similar properties to conventional plastics, and so can be used in similar applications. PHAs are manufactured by bacteria, and as well as this, they are biodegradable and can completely break down within 6-12 months in a compost bin, on an industrial scale, or even in the sea. The polymer chains break down into CO2 and H2O, and the amount of CO2 released is equivalent to that absorbed by the bacteria during production, thus contributing to net-zero targets.

Despite this biodegradation, this disposal route is still highly linear as it requires manufacturing new PHAs from scratch (see Figure 1). Academics within the Birmingham Plastics Network are working towards creating a circular economy for bioplastics by exploring the potential to depolymerise it at the end of its life.

The chemical process for RePHASe uses hydrolysis. As a solvent, water is very cheap, safe, and has a low environmental impact. It has been proven in small batch quantities (see Figure 2) and there are plans to investigate opportunities to scale-up production – using a continuous setup where water and catalysts are preserved and recycled into the process. The end product of the reaction is a mixture of depolymerised material, water, and catalyst. In theory, the monomers can be recovered, and re-polymerised into PHA. These polymers would have the same properties as the fully virgin monomers and go through the same quality checks before creating new material. Design software will be used to map out the full chemical process, the heating and total energy demand, as well as a full design of the process to demonstrate the industrial scale. A Life Cycle Analysis (LCA) and techno-economic assessment (TEA) will be done with the data gathered to determine if it is technically feasible and economically viable. Ideally, the selling cost would be less than the virgin PHA, and any petroleum-based alternatives.

Undergraduate Summer Research

Over the summer of 2023, a project was conducted with undergraduate researchers, Alessia and Aadam, to recycle the bioplastic P3HB (one of the most common types of PHA). To drive a circular economy, the aim is to get the smallest “loops” for maximum resource efficiency. Although mechanical recycling has a place in future infrastructure, over time this degrades the properties of the plastic, whereas chemical and biological recycling can recover virgin-quality monomers. 

A diagram depicting chemical and biological recycling in the natural environment and in landfill.Figure 1: Usually the smallest recycling loop would be mechanical processing, however chemical and biological recycling can produce virgin-quality monomers.

This project found that soaking the polymer in methanol for 12-16 hours, and subsequently letting it dry, halved depolymerisation time, which likely lowers resource expenditure. The depolymerisation products entered solution, which allowed Alessia and Aadam to graph the reduction in mass of the solid material.

A series of 4 images depicting crystals in various states of absorption over time, from dry matter to fully dissolved.Figure 2: Pellets breaking down via depolymerisation.

Modelling was conducted to understand the reaction and identify the limiting factors. The rate constant (speed of reaction) was then calculated, which is an important parameter when designing an industrial scale system.

A graph plotting matter pre-soaked in methanol compared to not soaked.Figure 3: Graph for depolymerisation at 70℃.

Alessia and Aadam have also given their take on the project:

"The results and trends we have identified during this internship are an important stepping stone for informing the scale-up of chemical recycling processes. Ultimately, these insights will play a key role in addressing significant challenges within the industry related to the sustainable recycling of plastic waste."

"The project was an engaging and practical experience which taught us a variety of skills such as design of experiment, curve fitting, data analysis and many more. The project was very important because it attempts to tackle a global issue: addressing polymer waste. Essentially, we can overcome such a pressing issue if chemical recycling can be carried out in a cheap and effective way compared to landfill or incineration."


Two students in a research laboratory, weighing substances and measuring outputs.Alessia and Aadam in the lab

Future work

Dr Matt Keith, leader of this summer project, has applied for funding to run future research. Entitled “Recycling PHA for Second life” (RePHASe), he hopes to expand the work to consider the engineering requirements behind a commercial process. He believes that the technology is available, but the logistical, social, and policy issues of plastics waste also need strong consideration in order to transition towards a circular economy. For more information, he can be contacted through his University of Birmingham webpage