News

Nov 06, 2023

There's Another Type of Plastic Money In Your Wallet

It starts with a bubble the height of a four story building. Made from a plastic

It starts with a bubble the height of a four story building. Made from a plastic called biaxially-oriented polypropylene (or BOPP for short), this mega-bubble is formed by melting hard pellets of BOPP, before letting it stretch and fall under the influence of hot air and gravity. What's produced is an ultra-thin, transparent film that can be pressed and rolled to produce a waterproof material that's very difficult to tear. When printed with high tech inks and cut into rectangles, BOPP becomes something very familiar to residents of (and visitors to) Australia, New Zealand, Canada, and Singapore, to name just four countries. And from September of this year, England will be counted among that number, because it will start using BOPP to produce plastic banknotes.

Sheets of the Bank of England's new polymer £5 note have been printed, ready to be issued in... [+] September. Scotland began issuing their polymer notes in March 2015 (Image credit: ‏Bank of England)

Plastics, or polymers, as they are more accurately known, are found everywhere in the city, from the threads of your blazer (polyester), and your travel mug (polyethylene terephthalate, PET) to the plumbing pipes under the pavement (polyvinyl chloride, PVC), and your car bumper (polypropylene). The ‘poly’ bit of each of their names means ‘many’, because all polymers are large molecules made from multiple repeating parts. Some occur naturally, such as amber and natural rubber, but most of those we use on a daily basis are synthetic. The properties of the final polymer depend on the type of molecules in the chain and the way they are processed – for example, cling film (food wrap) and Bakelite are both polymers, but they behave very, very differently. BOPP's bubble treatment turns a material used in car bumpers into something thin enough to bend, roll, and fit into wallets.

Polymer banknotes were an Australian invention – it was the country's national science agency, CSIRO, that first developed them, in response to a huge growth in counterfeiting in the late 1960s. By 1998, all Australian banknotes were polymer, but much of the rest of the world has struggled to catch up. If you’re in the US, the only polymers in your wallet are in store and credit cards. Your ‘paper’ banknotes are actually made from a mixture of cotton and linen fibers that are mashed to a pulp and bleached, before being pressed into sheets. This combination not only makes US banknotes durable, but it also gives them their distinctive, textile-like feel.

So, why would we want to make banknotes from polymers like BOPP anyway? Well, they are resistant to dirt and moisture, which makes them much cleaner than paper notes. They’re also more durable – in tests, polymer notes last between three and six times longer than those made from paper. And, perhaps most importantly, they’re more secure – the BOPP base material is pretty hard to make, and polymer notes themselves can host incredibly complex security features. Canadian banknotes even have a diffraction grating hidden in the transparent section of the maple leaf. This splits the light up, so that it can produce specific patterns – in this case, the denomination of the banknote. To see a video of this in action, head over to Steve Mould's YouTube page.

The Canadian $20 hides a bit of quantum mechanics in its design (Image credit: ‏@quark_and_laser,... [+] Twitter)

Earlier today, the Bank of England unveiled their design for their first polymer note – a £5, featuring former Prime Minister, Sir Winston Churchill. It includes a host of features that will make counterfeiting them much, much harder. Some include the color-changing border of the note's transparent window (it changes from purple to green), and the foil hologram at the base of Big Ben, which features the word ‘five’ when tilted in one direction, and ‘pounds’ when tilted the opposite way. Lettering small enough only to be seen with a magnifying glass appear under the Queen's portrait, and so-called intaglio printing has been used to produce raised text in various places.

Alongside these visible features, there are many others that will appear only when illuminated by a light of a very specific wavelength (e.g. infrared or ultraviolet). Judging by the polymer notes used in other countries, the new fiver is also likely to include so-called ‘covert’ features that are revealed only to banks and ATM manufacturers – the final layer of security checks to mark the note as genuine. Other polymer notes will follow after the fiver is launched in a few short months – the new £10, which features author Jane Austen, will be issued in September 2017. So the next time you pull out a banknote, maybe look a little closer – they contain a lot of interesting science!