Last week the UK put its first polymer note into circulation, and it plans to replace all of its paper banknotes with polymer notes by 2020 (with the current exception of the £50 note). It’s far from the first country to introduce polymer notes, however; in fact, Australia has been using them since 1988! What then is the advantage of polymer notes over paper ones? Here we take a look at the chemistry and the benefits.
Paper banknotes aren’t made of paper in the usual sense – wood pulp paper wouldn’t be durable enough, and would absorb water too easily. Instead cotton paper is used, sometimes mixed with other textiles in small amounts. In a way, paper banknotes could be considered to be polymer banknotes too; cotton consists of around 90% cellulose, a natural polymer formed from a large number of glucose units, and commonly found in the cell walls of plants.
Actual polymer bank notes are usually made from biaxially oriented polypropylene. ‘Biaxally oriented’ doesn’t, as you might expect, refer to the structure of the polymer, but due to the process it undergoes during its manufacture – particularly the manner in which it is stretched in two different directions during manufacture. This stretching enhances the strength and transparency of the film produced. The film’s transparency is often preserved in a window on the note, but the majority is coated in a white pigment to make it opaque before printing on the notes.
So why are countries looking into replacing paper banknotes with polymer notes? Part of the reason is cost. Polymer notes remain usable on average 2.5 times longer than paper ones, due to their higher durability. They’re more difficult to tear, waterproof, and do not absorb dirt or other substances. Their longer life offsets the fact that they are slightly more expensive to produce. They also have the advantage that when they reach the end of their usable life, being plastic they can be recycled – unlike paper notes, which must be shredded or burned.
Another benefit of polymer notes is the fact they they are more difficult to counterfeit than paper notes. Many of the security features they use are similar to those used on paper notes, but the increased difficulty of producing some of these on plastic notes makes them harder to replicate. But what are these security features?
It’s a little hard to go into specific detail, due to the fact that specific compounds or compositions used are understandably kept a close secret. However, we can look in general terms at some of the security features. A group of features are dubbed ‘Optically Variable Devices’ (OVDs), and all work on the principle that their appearance changes when something external to the note changes (usually light or viewing angle). Very thing gold foil is a simple example of this – in reflected light the foil appears gold, whereas in transmitted light it appears green. Diffraction gratings are also OVDs, and involve thin patterned lines (as many as 12,000 per centimetre) coated with a reflective metal such as aluminium, which gives a variety of colours depending on the angle from which the note is viewed.
Another commonly used security feature is impregnating portions of the note with compounds that appear a different colour under ultraviolet light. Probably the most well-known example of this is the use of Europium complexes in Euro notes. Europium is a member of the lanthanide group in the periodic table, and many lanthanide compounds fluoresce under UV light. The UV light excites electrons in the compounds to higher energy levels (known as an excited state) before they lose this excess energy and fall back to their original position. The excess energy is lost as visible light, giving the appearance of fluorescence.
It’s sadly unclear whether the choice of Europium for Euro notes was coincidence, or a deliberate choice by whoever was entrusted with coming up with the notes’ security features. Either way, the exact chemical nature of the compounds used remains unconfirmed, though there has been speculation that it may be europium complexed with two beta-diketone molecules.
Of course, this is just the tip of the iceberg as far as security measures on banknotes are concerned – there are many more details provided in the links below, as well as speculation as to what future security measures might be employed. What’s clear is that it’s likely more countries will adopt polymer notes in the future, but for now, if you’re in the UK, it’s a fun new chemistry novelty!
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References & Further Reading
- Advanced materials for banknote applications – B Hardwick and others (£)
- Australia’s plastic banknotes: fighting counterfeit currency – E Prime & D Solomon
- Counterfeiting countermeasure – S Rovner, C&EN
- Plastic banknotes – new fingerprint technique – P Kelly, The Conversation
- Polymer banknotes Q&A library – The Bank of England