The Colors of Money

Caren B. Les, caren.les@photonics.com

Who wouldn’t want a machine that prints $100 bills? But such a dream machine would have to outsmart advanced anticounterfeit technology – and this technology gets smarter all the time.

One recent advance involves pigment-free, intensely colored polymer materials and was developed by scientists from the Universities of Sheffield and Hull, both in the UK.

Colored polymer materials in robust layers on currency notes and passports could defeat the efforts of counterfeiters.

The researchers, headed by Andrew J. Parnell at the University of Sheffield, created colors using highly ordered polymer layers. By mixing alloys of different polymers called block copolymers, they could generate any color in the rainbow from two noncolored solutions. The polymers exhibit color as a result of their structure – the way natural forms such as butterfly wings and peacock feathers do – and thus would be highly difficult to copy.

“We now have a painter’s palette of colors that we can choose from using just two polymers to do this,” Parnell said. “We think that these materials have huge potential to be used commercially.”

Also, opallike optical effects result, as this type of polymer organizes itself into a layered structure. The color also changes depending upon the angle from which it is viewed, the investigators say.

The researchers used the high-power x-rays of the UK’s Diamond Light Source synchrotron to probe the ordered, layered structures and gain a better understanding of how the colors were formed and how to enhance them.

The two images demonstrate the angular dependence of the color shift and the effect the scientists hope to exploit as a security feature. The first image is of three example block copolymer blends at near-normal incidence. The second is of three example block copolymer blends tilted away from normal incidence. Images courtesy of the University of Sheffield.

“Small-angle x-ray scattering is a simple technique that, in this case, has provided valuable confirmatory information. By using Diamond’s x-rays to confirm the structure of the polymer, the group was able to identify the appropriate blends for the colors required, meaning that they can now tailor the polymer composition accordingly,” said Nicholas J. Terrill, principal beamline scientist for the I22 noncrystalline diffraction laboratory at the Diamond facility, which is based in Oxfordshire.

The chemistry involved in making the polymers is so complex, the scientists say, that it would definitely not be an easy code for counterfeiters to crack. The system could be advantageous in terms of cost, processing and color selection compared with existing technology, they add. In currency notes and passports, the technology could be a brilliant way to thwart unauthorized fabricators.