Plastic Money

by Joe Clarke, PhD student. Originally published in issue eight of Resonance.

The money in our wallets is changing. The past two years have seen the conversion from old-fashioned "paper" banknotes to modern polymer banknotes, with the introduction of the £5 note in September 2016 and the new £10 note in September 2017 – but what makes these notes better and what heightened security measures do they offer?

A group of polymer notes, with chromatic effects in the crown image. © Bank of EnglandBanknotes have been a staple of our currency for over 300 years. Currency has always advanced with technology, and introduction of polymer notes is just the latest iteration. The old-fashioned banknotes are commonly referred to as "paper" - but this is itself a misnomer. Typical paper banknotes are constructed from 80% cotton paper, and cotton is comprised of the natural polymer cellulose.

Polymer banknotes are typically made from polypropylene, a versatile polymer that is also used in various commercial applications such as food and DVD packaging. The difference in banknotes is in the process. The polypropylene used in banknotes is known as Biaxially Oriented Polypropylene or BOPP, indicating the process of stretching along two axes leading to a stronger, more durable plastic. These are expected to last 2.5 times longer than paper notes, and are also more resistant to dirt and moisture, looking new for longer.

The Bank of England is only the latest to adopt polymer notes, with the initial research dating back to the 1960’s. Development of polymer notes is attributed to the Royal Bank of Australia and the Commonwealth Scientific and Industrial Research Organisation (CSIRO) combatting counterfeiters of the 10 AUD note in 1967. Their efforts birthed security devices known as OVDs or optically variable devices, which are now commonplace in any note and include effects such as holograms and colour changes. Australia were first to adopt the these in 1988, before they became known as polymer notes.

The exact details of all security measures in polymer banknotes are understandably closely guarded secrets, but several OVDs are apparent to the naked eye. For instance, the gold leaf on the £10 note varies in colour according to the angle viewed, appearing gold in reflected light and green in transmitted light. The smoother surface also makes conventional security measures and printing easier; with holograms, microscopic lettering and images appearing sharper.

The new notes are said to be 2.5x more durable than paper notes, but this is only applicable to everyday wear and tear, polymers are by no means indestructible. Professor Martin Poliakoff from Nottingham University, known for his 'Periodic' YouTube videos, demonstrated two ways to destroy the £5 banknotes. The first was to submerge the note in liquid nitrogen, cooling to -196 oC, before being struck with a hammer. This freezes the polymer strands which break on impact.

The structure of nitrocellulose.The second method involved submerging the note in fuming nitric acid, which acts as a powerful cleaning fluid. After submersion, the ink was removed leaving a pure sheet of polypropylene, the exact size of the £5 note. As a control experiment, a paper note was also submerged in this mixture. This is where the chemistry gets explosive. Addition of nitric acid to cellulose leads to nitration of hydroxyl groups to create nitrocellulose (above), also known as guncotton – a low-order explosive with uses as propellants and in film.

Polymer banknotes have been introduced to modernise British currency. A new £20 note is expected to be introduced in 2020, and combined with a shift towards contactless payments, the way we spend our money is becoming more and more plastic by the year.

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