Nanosheet crisp packets you can recycle

14 June 2019

CRISPSBAG

An open packet of crisps on grass

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Fully recyclable packaging for crisps and foods has been developed by UK researchers...

When you open a crisp packet and see the shiny metallic colour inside, you would be forgiven for thinking that your crisp packet was made of aluminium foil. In fact, crisp packets are made of a “metallised plastic film”, which is plastic that has been coated with a thin layer of metal (usually aluminium) on the inside. Metallised plastic films are also used for chocolate bar wrappers and many other food types.

These materials offer weight and cost benefits over metal foil packets. But plastic alone is porous, so the metal film is essential to provide a barrier to oxygen and water, which prolongs the shelf life of foods. 

The downside is that this combination of plastic and metal makes crisp and other packets very difficult to recycle. The metal first needs to be separated from the plastic before either can be reused. But now a team at the University of Oxford, led by Dermot O’Hare, have developed an alternative barrier, which does not need to be removed to recycle the packet.

The new layer consists of nanometre (one millionth of a mm) thick sheets of aluminium and magnesium hydroxide compounds, dubbed nanosheets.  

Found naturally in rocks, these minerals need to be present only in very tiny amounts to form an effective nanosheet barrier. According to the team, 100-1000 times less material is required than when aluminum is used for the same purpose. This low-abundance means that a plastic packet can be recycled directly without having to remove the nanosheets first. The material will "just dissolve away when [the plastic] went through a reprocessing technology," says O'Hare.

The team have developed a technique to produce the nanosheets with very high aspect ratios. That means that the lengths and widths of the nanosheets are much much larger than the thickness, just like a piece of paper.

Imagine dropping some papers on a table so that they spread out in a messy pile. If you were a tiny particle and you wanted to get from the top of the pile to the bottom of the pile, but you could not go through the sheets, then on each layer you would have to travel along to the end of a piece of paper before you found a gap to go down to the next level. On the next level you would have to travel along again until you found another gap and so on. So It would be very difficult to find your way through to the bottom.

The nanosheets work the same way. Laid out on the surface of the plastic, "they all interlock beautifully, just like a deck of cards that's been spread on the table," says O'Hare.

This means that even tiny particles like molecules of oxygen and water cannot get through. To do so they would have to navigate a tortuous path winding around the interlocking nanosheets. In fact, tests show that the new coating is around 40 times better at blocking oxygen and water molecules than the existing metallised film technique. 

Over 300,000 people recently signed a petition demanding crisp-manufacturer Walkers - and other plastic packaging producers - to change the materials they use to ensure that they are fully recyclable. With O’Hare’s help, perhaps they now can...

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