A promising bioplastic derived from indole, a molecule found in feces, is currently under development that performs better and is more durable than both regular plastic and other bioplastics – and is potentially better suited for recycling.

Almost all plastic is made from crude oil, and plastic production currently accounts for 4-6% of global oil consumption. The development of renewable bioplastics is progressing, but relatively few are actually being used.
A strong candidate among bioplastics is polyethylene furanoate (PEF). Instead of oil, PEF contains the hydrocarbon, furan, which can be extracted from maize, wood and certain types of grain. The main market for PEF is packaging. Experiments have shown that PEF is superior to standard polyethylene terephthalate (PET) in protecting against oxygen, carbon dioxide and water, which gives products enclosed in plastic greater durability.

The success of PEF made researchers at Sweden’s Lund University interested in other renewable materials that could potentially be used for plastic production. Chemical engineering doctoral student Ping Wang has produced a plastic based on indole, a heavier hydrocarbon molecule than furan, that is present in human faeces and smells accordingly. The compound is also found in lower concentrations in certain flowering plants and has a more agreeable aroma. This effect is due to our sense of smell decoding the aroma differently depending on the amount and combination.

The research team is thought to be the only one researching indole polyesters, and their results are promising. A regular PET bottle’s glass-liquid transition temperature – when the material softens and deforms – is 70 degrees. The most successful PEF experiments withstand about 86 degrees. However, one of Ping Wang’s indole plastics is stable up to 99 degrees.

“These are preliminary results, but we have seen that polyester plastic has better mechanical properties, which makes it more sustainable. This can lead to better recycling in the future. At present, PET bottles can only be recycled once, then they must be used for something else such as textiles”, said associate professor Baozhong Zhang, who is supervising the research team.

Currently, indole is only produced on a small scale and used mainly in perfumes and drugs. It may be possible to use bioengineering methods to produce indole from sugar through fermentation. However, such a process would first need to be analysed more thoroughly before the production cost can be calculated.

Ping Wang is continuing her research by examining the indole plastic’s potential in other application areas.

“We obtained good results, but are not satisfied. Now we are trying to find methods for making higher quality indole polymers that can be used in more ways, not just for plastic bottles”, she concluded.

Publication: Indole as a new sustainable aromatic unit for high quality biopolyesters

https://rsc.li/2D9xpTQ