Supramolecular materials that fully degrade when soaked in saltwater could tackle the microplastics pollution crisis.
Plastics circulate throughout all the Earth’s oceans. Rather than biodegrade, they simply break down into tinier and tinier pieces, producing microplastics that measure less than 5mm long.
According to US environmental advocacy group Ocean Conservancy, 11 million tonnes of plastics enter our oceans every year. This is in addition to the estimated 200 million tonnes already there.
As well as polluting our environment, microplastics have also been found in our bodies, including in our blood and brains.
Researchers at RIKEN, a national research and development agency in Japan, intend to tackle this problem with a new plastic-like material that biodegrades in saltwater.
Similar in weight and strength to conventional plastics, the new material could help reduce plastics pollution, as well as reduce greenhouse gas emissions associated with burning plastics, according to Takuzo Aida, a materials scientist who heads the emergent soft matter function research group at RIKEN.
Aida has spent three decades researching supramolecular polymers. Polymers in plastics comprise small molecules bound into long chains by strong covalent bonds that require extensive energy to break. In contrast, supramolecular polymers have weaker, reversible bonds. As Aida puts it, these materials are “like sticky notes that you can attach and peel off”.
This gives them the ability to ‘self-heal’ when broken, and they can also be easily recycled and reused as their bonds can be broken down at the molecular level.
However, the challenge is that disintegrating too quickly means they couldn’t be used as a replacement for plastics.
Aida and his team set out to discover a combination of compounds that would create a supramolecular material with good mechanical strength, but break down quickly under the right conditions into non-toxic compounds and elements.
Aida had a specific reaction in mind, one that would lock the material’s molecular bonds and only be reversed with the use of salt.
Following various experimentations, the team found that a combination of sodium hexametaphosphate (a common food additive) and guanidinium ion-based monomers (used for fertilisers and soil conditioners) formed ‘salt bridges’ that bind the compounds together with strong cross-linked bonds.
According to Aida, these types of bonds serve as the ‘lock’, providing the material with strength and flexibility.
The team produced a small sheet of this supramolecular material by mixing the compounds in water. The solution separated into two layers, the bottom viscous and the top watery, a spontaneous reaction that surprised the team.
The viscous bottom layer contained the compounds bound with salt bridges. This layer was extracted and dried to create a plastic-like sheet.
Not only was the sheet as strong as conventional plastics, but it was non-flammable, colourless and transparent, giving it great versatility.
The sheets degraded back into raw materials when soaked in saltwater, as the electrolytes in the saltwater opened the salt bridge ‘locks’.
The team’s experiments showed that the sheets disintegrated in salt water after eight and a half hours.
According to Aida, even when waterproofed, the material dissolved just as quickly as non-coated sheets if its surface was scratched to allow the salt to penetrate.
When broken down, the new material left behind nitrogen and phosphorus, which microbes can metabolise and plants can absorb.
Aida said: “With established infrastructures and factory lines, it’s extremely challenging for the plastics industry to change.
“But I believe there will come a tipping point where we have to power through change. And a technology like this will be needed when that time comes.”