New plastic from Japan dissolves in salt water
Plastic waste is a heavy burden on the oceans. A team from Japan has developed a solution: a plastic that dissolves in salt water.
A piece of the new, transparent plastic
(Image: RIKEN)
Plastic waste can be found everywhere on the planet. The oceans are also heavily polluted with it. Over time, bags, bottles and fishing nets decompose in the water. What remains are microplastic particles that enter the bodies of sea creatures, for example, and damage them. They also end up in the human body via the food chain. Japanese researchers want to remedy the situation: they have developed a plastic that dissolves in salt water.
The material is similar to conventional plastics in terms of weight and strength: It is based on supramolecular polymers that form weaker and reversible bonds. This gives these polymers a number of practical properties, such as the ability to repair themselves by being compressed again. They are also easy to recycle. With certain solvents, the bonds can be broken at the molecular level. This means that supramolecular polymers can be easily reused.
Supramolecular polymer bonds are weak
However, these polymers also have a decisive disadvantage: "The reversible nature of supramolecular polymer bonds is also their weakness, as the materials break down too easily", says Takuzo Aida. "This has limited their potential applications." The materials scientist's team at the RIKEN research institute in Wako on the Japanese main island of Honshu developed the new plastic.
The aim was to create a supramolecular material that quickly breaks down into non-toxic parts under certain conditions, but still has the mechanical strength of conventional plastics. Aida thought of a reaction that would solidify the molecular bonds and a salt that would dissolve them again.
The team eventually came up with a mixture of sodium hexametaphosphate, a common food additive, and monomers based on guanidinium ions, which are used in fertilizers. When these two are mixed together in water, the solution separates into two layers, a viscous lower layer and an aqueous upper layer. The lower layer contained the two components, which were bound via salt bridges.
It was extracted and dried, resulting in a plastic-like film. The salt bridges make it as stable as conventional plastics. It is non-flammable, colorless and transparent.
Dissolves in less than nine hours
If the film is immersed in salt water, it breaks down into its components again: The electrolytes in the salt water dissolve the salt bridges again. The film dissolves after around 8.5 hours in salt water.
However, this poses a problem. The material can inadvertently come into contact with salt water. The scientists therefore propose a water-repellent coating. The end product remains recyclable, says Aida: it is enough to scratch the surface. Salt can then penetrate and decompose the material.
Nitrogen and phosphorus are released during the decomposition process. These could be metabolized by microbes and taken up by plants, say the researchers, who present their development in the journal Science.
(wpl)