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Raw materials & technologies, Technologies

Electrochromic polymers create coatings that change colour

Wednesday, 11 February 2015

Artists, print designers and interieur decorators have long had access to a broad palette of paint and ink colours for their work.

Samples show some of the colours researchers have produced in electrochromic polymers. Source: Rob Felt

Samples show some of the colours researchers have produced in electrochromic polymers. Source: Rob Felt

Now, researchers have created a broad colour palette of electrochromic polymers, materials that can be used for sunglasses, window tinting and other applications that rely on electrical current to produce colour changes.

Other uses include window tinting

By developing electrochromic polymer materials in a range of primary and secondary colours and combining them in specific blends, the researchers have covered the colour spectrum - even creating four shades of brown, a particularly difficult colour combination. The materials could be used to make sunglasses that change from tinted to clear in a matter of seconds, at the press of a button. Other uses could include window tinting, signage and even greeting cards that change color through the application of low-voltage electrical current. Supported by BASF, the research was done in the laboratory of John Reynolds, a professor in the School of Chemistry and Biochemisty and the School of Materials Science and Engineering at the Georgia Institute of Technology.

Mixing different electrochromic polymers

"We've demonstrated the ability to create virtually any colour we want by mixing different electrochromic polymers, just like mixing paint," said research scientist Anna Österholm. "Using a simple coating method or even inkjet printing, we can create films that change colour with the application of a voltage." The many colours that have been developed by Reynolds' group over the years include magenta, cyan, yellow, orange, blue and green polymers that can be dissolved in common solvents. In addition, blends of these polymer solutions can be predictably mixed to target specific colours. To demonstrate the capabilities, the researchers created brown lenses for sunglasses using a five-layer sandwich of materials, including a film of the electrochromic material, a charge storage layer and a UV-curable electrolyte, with a cathode and anode layer on either side.

Applying a pulse of electrical current

The lenses can be switched between a coloured and colourless state by applying a brief pulse of electrical current and do not need a continuous power supply. To maintain the colourless state, a brief refresh pulse needs to be applied approximately every 30 min; however, the coloured state can be stable for up to several days. The materials can switch from about 10% transmittance to 70% transmittance - and back - in a few seconds. The brown shades are created by combining cyan and yellow primary colours with orange and periwinkle-blue secondary colors.

Colour switch at a press of a button

Photochromic sunglasses, which darken in response to light using a silver halide reaction, are already on the market. But many of these lenses respond to ultraviolet wavelengths that are filtered out by automobile windshields, require several minutes to transition - and can't be controlled by users. The passive switching time can be problematic for pilots, drivers, security officers or others who move quickly between light and dark environments. "In contrast, by using electrochromic polymers, we can create devices that by pushing a button, can be converted from dark to clear," said Österholm. "They are completely user-controlled, and it doesn't matter whether they are being used indoors or outdoors, in a vehicle or an aircraft."

Materials represent years of work

The electrochromic materials rely on a reduction-oxidation (redox) reaction triggered by the application of an electrical potential provided by a simple coin battery: a positive one volt causes the glasses to be clear, while a minus one volt switches to the color. "Essentially, we are just charging and discharging the device, which is what causes the color change," explained Eric Shen, a postdoctoral fellow. The electrochromic materials represent years of work by the Reynolds Laboratory to synthesise polymers whose repeat-unit structures provide the desired palette of colours. Because they can be dissolved in the same solvents, additional colors can be created by combining specific quantities of the primary and secondary colors.

Using blade-coating techniques

"Anything that you would want to have change colour at the push of a button would be an application for these," said Shen. "We have shown that we can switch them on and off thousands of times, and that we can shine strong light on them without causing degradation of the colour." The researchers have used simple spray and blade-coating techniques to create films of the colourful materials. They now are using ink-jet printing to create patterns and mix the polymers to create colors. "The ink-jetting is very versatile when you want to make patterns or very fine features with these materials," Shen said. "The fact that the polymers are so soluble makes it quite easy to process them using anything that would spread an ink."

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