Saloplastics are plastics that break down over time when exposed to salt. A researcher at Wageningen University has devised a chemical trick that enhances the biodegradability of these plastics.

The greatest advantage of plastic is also its greatest disadvantage: the material is hardly biodegradable. This is because it consists of long carbon chains (polymers) that are firmly linked together. This makes the material particularly suitable for waterproof packaging, insulating electrical wires or providing sterile covers for medical instruments, but it is harmful when it ends up in the environment as waste.

After travelling around for a while, most plastic waste ends up in the ocean, where it becomes part of the infamous ‘plastic soup’. Every year, more than ten million tonnes of plastic waste are added to this, and it remains there for decades or even centuries.

A chemical trick

To address this issue, researchers have developed ‘saloplastics’ in recent years. These are plastics that have been modified so that they dissolve in salt. They consist of polyelectrolytes: molecules that are a combination of plastic-like chains (polymers) and electrically charged particles (electrolytes). Some of the bonds in saloplastics are based on the attractive force between oppositely charged particles. These electrostatic bonds are disrupted by the charges of the sodium and chloride ions in dissolved salt. The plastic then breaks down into shorter polymers.

During his PhD research at Wageningen University & Research, Julian Engelhardt devised a chemical trick to make saloplastics break down even further in salt water. This trick involved incorporating oxygen atoms into the carbon chains. This creates so-called ester bonds, which are known for their biodegradability. In May, Engelhardt defended his thesis, which focused primarily on how to produce polyester-based polyelectrolytes.

Additional processing steps

Engelhardt demonstrated that the new plastic he has developed remains strong and resistant to fresh water, but is soluble in salt water. Whether it breaks down completely or persists as microplastic remains to be seen. Scaling up also remains to be done: so far, the PhD student has only produced a few dozen grams of the material. The main challenge here is the cost, as producing the more biodegradable saloplastic requires quite a few extra processing steps that are not necessary when making ‘normal’ plastic.

As with other innovative forms of biodegradable plastic, such as plastic that degrades under LED light, or PISOX plastic that breaks down by absorbing water, competition from the dirt-cheap, non-recyclable variety is therefore ultimately the biggest problem once again...

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