Coloured polymer coatings could help prevent fraud

The coating has a highly complex structure based on bespoke combinations of block co-polymers (essentially alloys of two different polymers), making it extremely difficult to forge.

Nevertheless, project lead Dr Andrew Parnell from Sheffield University said that now they understand the phenomenon in detail, it should be easy and cheap to scale up for things such as banknotes and passports.

Their transparent coating creates colour through the optical transmission of certain wavelengths of light — rather than simply reflecting light with metallic-coloured ink, which is currently used in some banknotes.

Comparing the existing technology with their method, Parnell said: ’For a specific colour you have to make a specific-sized particle, such as a sphere, and in order to make another colour you have to make an entirely new material — so from our point of view we can mix and match quite easily.’

They do this by combining co-polymers of differing molecular weights then ’shear aligning’ them using a curing process to form a unique optical structure.

To help the researchers understand how the colours were formed and to improve the method, they used the Diamond Light Source — the UK’s national synchrotron facility in Oxfordshire — to probe the ordered, layered structures using high-powered X-rays.

Based on that work, they can now finely tune the polymer mixture to achieve very specific effects, such a defined sequence of colours that appears as you change the viewing angle from left to right, for example.

Parnell says the resultant effect is akin to how the natural world creates intense colours in things such as opals, beetle shells and butterfly wings.

While fraud prevention and security are the most obvious uses for the coating, Parnell said it could have many other applications — keeping buildings cool by filtering out infrared light, for example.

‘The ultimate aim would be to have a polymer laser, because you’ve got these series of layers and cavities you can reflect back and forth with — this gives you a way of getting gain like you do in a laser,’ Parnell said.