Salmon oil makes sustainable plastic

Derived from crude oil, toxic to synthesise and slow to break down, conventional polyurethanes are not environmentally friendly, but they are nearly everywhere.

Researchers from Memorial University of Newfoundland are now investigating ways to develop safe, biodegradable plastic alternatives to polyurethanes, derived from fish waste including heads, bones, skin and guts.

If developed successfully, a fish-oil based polyurethane could help meet the need for more sustainable plastics. That’s according to PhD student Francesca Kerton, the project’s principal investigator, who said, “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.”

Kerton’s team started out with oil extracted from the remains of Atlantic salmon, after the fish were prepared for sale to consumers.

“I find it interesting how we can make something useful, something that could even change the way plastics are made, from the garbage that people just throw out,” said graduate student Mikhailey Wheeler, who presented the work at the spring meeting of the American Chemical Society.

The conventional method for producing polyurethanes presents a number of environmental and safety problems. It requires crude oil, a non-renewable resource, and phosgene, a colourless and highly toxic gas. The synthesis generates isocyanates, potent respiratory irritants, and the final product does not readily break down in the environment. The limited biodegradation that does occur can release carcinogenic compounds. Meanwhile, demand for greener alternatives is growing. Previously, others have developed new polyurethanes using plant-derived oils to replace petroleum. However, these too come with a drawback: the crops (often soybeans) that produce the oil require land that could otherwise be used to grow food.

Leftover fish struck Kerton as a promising alternative. Salmon farming is a major industry for coastal Newfoundland, where her university is located. After the fish are processed, leftover parts are often discarded, but sometimes oil is extracted from them. Kerton and her colleagues developed a process for converting this fish oil into a polyurethane-like polymer. First, they add oxygen to the unsaturated oil in a controlled way to form epoxides, molecules similar to those in epoxy resin. After reacting these epoxides with carbon dioxide, they link the resulting molecules together with nitrogen-containing amines to form the new material.

But does the plastic smell fishy?

“When we start the process with the fish oil, there is a faint kind of fish smell, but as we go through the steps, that smell disappears,” Kerton said.

Kerton and her team described this method in a paper last August; since then, Wheeler has been tweaking it. She has recently had some success swapping out the amine for amino acids, which simplifies the chemistry involved. And while the amine used previously had to be derived from cashew nut shells, the amino acids already exist in nature. Wheeler’s preliminary results suggest that histidine and asparagine could fill in for the amine by linking together the polymer’s components.

In other experiments, the researchers are examining how readily the new material would break down once its useful life is over. Wheeler soaked pieces of the material in water, adding lipase — an enzyme capable of breaking down fats like those in the fish oil — to speed up the degradation of some pieces. Under a microscope, she later saw microbial growth on all of the samples, even those that had been in plain water — an encouraging sign that the new material might biodegrade readily, Wheeler said.

Kerton and Wheeler plan to continue testing the effects of using an amino acid in the synthesis, and studying how amenable the material is to the microbial growth that could hasten its breakdown. They also intend to study its physical properties to see how it might potentially be used in real-world applications, such as in packaging or fibres for clothing.

Image caption: Using fish oil, researchers have made a polyurethane-like material. Image credit: Mikhailey Wheeler.