Biodegradable plastic alternative could be made from fishy waste

Polyurethanes are a highly versatile type of plastic derived from crude oil and found in everything from construction materials to clothing. The global demand for polyurethanes in 2019 was estimated at 18.4m tons, forecast to rise to 22.5m tons by 2024. These types of plastic have a serious downside in that they are very slow to break down and are not environmentally friendly.

In an effort to produce a greener alternative to polyurethanes, Canadian researchers have been investigating the possibility of deriving a biodegradable alternative derived from fish waste, such as the heads, bones, skin and guts - material which is often discarded.

The conventional method for producing polyurethanes presents several environmental and safety problems; it requires crude oil and phosgene (a colourless and highly toxic gas). The synthesis generates isocyanates (respiratory irritants) and, of course, the final product does not break down within a reasonable timeframe in the environment. When it does degrade, it also releases carcinogenic compounds. Given these problems, there is increasing demand for greener alternatives to polyurethanes – such as those derived from plant oils – among other plastic alternatives.

“It is important that we start designing plastics with an end-of-life plan, whether it’s chemical degradation that turns the material into carbon dioxide and water, or recycling and repurposing,” said Dr Francesca Kerton, who is leading the project at Memorial University of Newfoundland.

Kerton’s team started with oil extracted from the remains of Atlantic salmon – salmon farming being a key industry for coastal Newfoundland – after the fish had been prepared for sale. Leftover parts are often discarded, but sometimes oil is extracted from them.

In order to convert this fish oil into a polyurethane-like material, they added oxygen to the unsaturated oil through a controlled process to form epoxides (molecules similar to those in epoxy resin). The molecules are reacted with carbon dioxide and in a subsequent process the molecules are linked together with nitrogen-containing amines (ammonia derivatives) to form the new material. Through these chemical processes, the smell of fish vanishes, Kerton said.

Since the method was first described in a 2020 paper, Kerton’s team has been optimising it by swapping out the amine (which had to be derived from cashew nut shells) for amino acids (which already exist in nature). Preliminary results suggest that histidine and asparagine could fulfil the role previously played by amine.

The team has also started to study how readily the new material would break down once its useful lifetime is over. Soaking pieces of the material in water – and adding to some samples an enzyme which breaks down fats like those found in fish oil – they observed microbial growth even on the samples soaked in plain water. This is an encouraging sign that the polyurethane alternative may biodegrade quickly.

Kerton and her team will continue testing the use of an amino acids in the synthesis process, investigating the biodegradation of the material, and studying the physical properties of the material to assess how it could be used in real world applications, such as in packaging or clothing.

Last year, researchers from Leipzig’s Helmholtz Centre for Environmental Research-UFZ identified a strain of bacteria capable of breaking down some of the chemical building blocks of polyurethane.

The scale of plastic pollution in the world's oceans remains a serious global concern. E&T looked at the key issues and some of the potential solutions in our special plastics issue.