Plastic cash

If you’ve been keeping up with the news, you’ll know that Britain, too, recently released a new fiver. The new polymer banknote was issued in September 2016, with a 10-pound note due to be released in 2017 and a 20-pound note in 2020. They’re just the latest to switch to plastic cash: polymer banknotes are now used in many countries across the world.

While the Brits have stuck with their pound, it’s been five decades since Australia gave it the flick. When Australia switched to dollars in 1966, its new banknotes had a number of security features designed to deter counterfeiters. Printed on quality paper, they included a watermark and intaglio (raised) printing, as well as state-of-the-art metal threads. Yet, within a year, over $100,000 worth of forged $10 notes had been collected by authorities, with over 1,500 forgeries passed in the state of Victoria alone. 

In 1967, the Governor of the Reserve Bank of Australia approached senior Australian scientists, including some at CSIRO, to help develop a more secure banknote. It was to be the beginning of a decades-long journey, resulting in the release, in 1988, of Australia’s first polymer banknote. 

Not everyone was enamoured with the idea of plastic cash. Indeed, while the science was up to scratch relatively quickly, with technical problems resolved within 10 years of beginning research, it took a further decade until the banking industry accepted and released the revolutionary new notes. One perception was that plastic looked ‘cheap’ and therefore not suited for use in the nation’s official currency. In the end, a series of ‘blind feel’ tests convinced the doubters.

But the invention of a unique polymer substrate (as well as a number of other security devices) put science squarely at the centre of currency production—making forgery a more difficult prospect. To make a passable fake note, forgers would need to know about polymer properties, replication techniques and the chemistry and physics behind optically variable devices.

A unique polymer substrate

If you’re thinking a potential counterfeiter need only get their hands on a good roll of plastic book-covering, think again. The polymer substrate used for Australian banknotes is a unique polypropylene material—it’s produced using specialised technology and isn’t commercially available. 

The substrate contains several film layers which are created using a ‘bubble’ process. Clear plastic beads are melted down and blown into a large bubble. During this process the film is stretched in both length and width to orientate the polymer molecules. The walls of the bubble are then pressed together and cooled, resulting in a clear polymer film. This base film is known as biaxally oriented polypropylene (BOPP), and has uniform tensile strength both horizontally and laterally.

The use of a clear polymer base also enables the inclusion in the banknote of a see-through window or windows. These are very difficult to counterfeit. Australia’s banknotes begin as a large roll of clear material onto which inks are applied, leaving the window clear. Each denomination of the existing series of Australian banknotes has a small, clear window. The new $5 note released in 2016 also has a clear window which runs the full height of the note and a third window in the shape of a bird.

Polymer science also comes to the rescue in tackling the thorny issue of the banknote’s feel. Initial concerns that banknotes would feel ‘plasticy’ were addressed by coating notes in a special polymer varnish. The chemistry of the varnish was controlled by varying its hydrophobic/hydrophilic ratios (that is, the tendency of the material to repel or bond with water molecules), while its physics were controlled by adding fine silica, giving the note a less slippery feel.

If you run your finger over an Australian banknote, you’ll also notice that it’s not completely smooth: the printing is raised up slightly from the surface. This is the result of what’s known as intaglio printing. Metal plates are deeply engraved with a design (say, the portrait of ex-convict Mary Reibey on the Australian twenty) and ink is applied to the plates. The ink is transferred to the sheets of polymer film under pressure, leaving a raised print. 

• How money is made

  
  The creation of an Australian banknote begins with a large roll of clear polymer film. 

  Special inks are applied which make the film opaque, and which will help the next layer of ink stick. Some parts of the plastic are left clear; these will become the see-through windows. 

  The reels are cut up into sheets and flat background designs are then printed onto both sides of the sheet at the same time. On the existing series of banknotes, this simultaneous printing allows the creation of a security device: a circle containing diamond-shaped patterns on each side of the note which align exactly to make an entire Federation star when you hold it up to the light.

  For the new banknotes, at this stage a continuous strip of holographic foil is applied in the full-length window. A rolling colour effect is also screen printed onto a smaller window on the banknote using optically variable inks on one side, creating a half window. 

  Intaglio print is applied separately to each side of the sheet, giving designs a raised texture. Some of the microprinting (the tiny words that appear on the note and can only be seen with a magnifying glass) and embossing is also carried out at this stage.

  Serial numbers are added using letterpress printing.

  A coat of protective varnish is applied which controls the feel of the banknote and protects it from wear and tear. 

  For the most recent banknotes, a tactile feature for use by vision-impaired people (raised bumps on the long edges next to the full-length window) is embossed onto the note.

  Finally, the large sheets are cut up into the individual notes.

Now you see it: optically variable devices

Australia’s commemorative ten-dollar note released in 1988 was the first polymer banknote in the world to include another difficult-to-forge security measure: an optically variable device, or OVD. An OVD is an image that changes appearance when the angle at which it’s viewed is changed. In the case of the 1988 note, the OVD was an image of Captain Cook that moved when tilted. Because OVDs change depending on the angle at which they’re viewed, and a copier or scanner can only copy a document at one angle, they can’t be effectively counterfeited by being scanned or copied.  

Optically variable devices depend on the diffraction of light. Diffraction is where a light wave encounters an obstacle, creating a particular effect. To make the optically variable Captain Cook, a device known as a diffraction grating was used. This is a pattern of extremely small lines which are coated with a thin film of a reflecting material such as aluminium. When light hits the pattern, it is split into many beams travelling in different directions, creating a rainbow of different colours. It’s a similar effect to what you may have seen when light hits a CD, throwing a rainbow of coloured lights onto the wall. 

After this initial venture into the use of OVDs, it was decided not to include them in Australia’s banknotes—not only were they very expensive to make, the unique polymer material and clear window were to prove sufficiently effective against counterfeiters. But in the new Australian polymer banknotes OVDs have made a reappearance. Several of these devices appear on the new, full-length clear window. They rely on materials such as foils which reflect light at different wavelengths depending on the angle at which they’re viewed. Optically variable inks, for instance, which contain pigments that reflect various wavelengths in white light differently depending on the angle at which the light strikes the surface, are used to create a ‘rolling colour’ effect. 

Another feature on the new banknotes which relies on the properties of light is an embossed transitory image in the smaller clear window. Like the raised print on the note, this is created by intaglio printing—but without the ink. The engraved printing plates are left uninked, giving an embossed effect when the feature is ‘printed’ at high pressure onto the banknote. The image is made up of small lines, which creates a transitory effect—that is, the image can be viewed in transmission (when looking through the window) as well as in reflection. 

If you happen to have a UV light handy, you may be able to see some further features not visible in normal daylight: a bird, the serial number and the year of print all fluoresce under a UV backlight with a wavelength around 365 nm. Fluorescent materials produce light when the atoms in them become excited, releasing energy in the form of light as they return to normal. These usually-invisible features on Australia’s new banknotes are created using fluorescent inks—they contain chemicals which absorb energy from UV light, become excited, and release energy as light, producing the ‘glow’ you’ll see if you put them under a UV lamp.  

Over the rainbow

While finding the pot of gold at the end of the rainbow may be the stuff of fairy tales, with their colourful and distinctive plastic cash Australians carry their own pocket-sized rainbow wherever they go. Though we may one day see the demise of physical money, the science behind the invention of the polymer banknote will no doubt persist, already finding new life in the burgeoning field of flexible electronics.