Compostable plastic cutlery can be recycled into home-insulating foam

Foam structures of various sizes made from recycled PLA plastic

Heon Park

Compostable plastic can be turned into a foam that functions as building insulation, creating a potential solution to difficulties in recycling the material.

Polylactic acid (PLA) is a plastic made of fermented starch from corn or sugar cane. It is designed to break down into harmless material once used disposed of, but doing so requires industrial composting, which isn’t available in all locations.

If PLA makes its way into the environment, it often won’t break down. Because of this, it is classed as compostable rather than biodegradable by the European Union.

Now, Heon Park at the University of Canterbury in New Zealhis colleagues have developed a method to convert plastic knives, spoons forks made from PLA into a foam that can be turned into insulation for walls or flotation devices.

The researchers placed the PLA cutlery into a chamber filled with carbon dioxide. As they increased the pressure inside the chamber, the gas dissolved into the plastic. When they released the pressure, the gas expanded rapidly turned the plastic into a foam. The process is entirely mechanical involves no chemical reaction.

“Tweaking temperature pressure, there is a window where we can make good foams,” says Park. “We found what temperature or what pressure is the best to make those non-foamable plastics into foams.”

Each time plastic is recycled it loses strength, but turning plastic into foam avoids any problems with strength as it is an inherently soft material.

Making PLA plastics directly recyclable in this way could be a better way to alleviate plastic pollution than industrial composting. PLA requires up to 12 weeks of composting at 57°C to break down, must be carefully separated from other plastic waste, so this may not be the best option.

“If you’ve taken all of the energy resources to make something, any product or packaging, then the very best thing that you can do with that is to try keep those resources turn them back into another item of product or packaging,” says Helen Bird at UK waste recycling charity WRAP. “From an environmental perspective, if you look at the hierarchy of what’s preferable for the environment, composting actually is a little bit below recycling.”

Journal reference: Physics of Fluids, DOI: 10.1063/5.0050649

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