Chemists Develop Safer Electrochromic Inks

Estimated reading time: 10 minutes

Anyone who has a rear-view mirror that automatically dims blue in reaction to annoying high-beam headlights glaring from behind has seen an electrochromic film in action.

Image Caption: Samples show some of the colors researchers have produced in electrochromic polymers. The materials can be used for applications such as sunglasses and window tinting that can be turned on and off through the application of an electrical potential. (Credit: Rob Felt)

Now, chemists at the Georgia Institute of Technology have developed a new method to more safely and, by extension, easily produce these shear films, which change their color with the help of a tiny electric current. This could make them available to many industries that have not been able to feasibly use them before.

In manufacturing, electrochromic films are often coated onto other materials, such as the surface of a mirror, as inks. They are usually based in solvents that are flammable and have toxic fumes, making them unsuitable for many work settings that rely on printing and spraying machinery to apply colors.

Georgia Tech researchers have developed electrochromic film inks that are water-based, making them safer for diffuse application in settings where the kinds of safety precautions and protective equipment that are standard in handling volatile organic chemicals would be impractical.

Everyday Environments

“Where people print is not always in chemically safe environments,” said John Reynolds, a professor in Georgia Tech’s Schools of Chemistry and Biochemistry and Material Science and Engineering. So Reynolds and the study’s first author Brian Schmatz, who came up with the water-based method, set out to make electrochromic film inks safer for everyday environments.

There were some hurdles to pulling it off. The finished product had to electrically operate comparable to films that are applied in an organic solvent, and also be water-resistant in spite of the water-based production. Schmatz’s method also needed to be logistically and financially realistic for producers to implement.

The researchers published details on their solution and how it has met the criteria in the journal ACS Central Science on August 16, 2017.  Should the chemical process progress to production, the future may see more windows, prescription glasses, or even textiles that switch between colors and shades of darkness with the click of a button or with the help of a light-detecting switch.

That’s how many self-dimming rear-view mirrors work: The high-beams of the motorist behind you hit a light sensor that applies a mild electric field to the mirror, and that activates the color-changing, or electrochromic, film, which switches to a darker tint.

Electrochemical Rainbow

The Reynolds lab’s electrochromic films are made with conjugated polymers, colorful and electroactive organic molecules. They easily let go of a few of their more loosely attached electrons, and when they do, their colors shift.

If the colored films are on a clear surface, when the color vanishes, the surface becomes clear. The surface has to be conductive so that a small voltage (about 1 volt) can be applied to bump the electrons off the conjugated polymer or help them jump back on.

The tints don’t have to be gray, blue, brown, or otherwise straight-laced. “We can make any color,” Reynolds said.

‘Toxic,’ ‘Carcinogenic’

Because of previous inks’ organic solvents, applying electrochromic films in the past has come with significant safety requirements. Their costs could become prohibitive if the job is big, say, if a company wanted to cover the windows of an office building with an electrochromic film.

“Most research labs use chlorobenzene as a solvent. It’s pretty toxic. It’s carcinogenic, slightly volatile as well,” Schmatz said. “So, it’s not something people want to work with at scale.”

Also, people may simply find the smell of an organic chemical in their workplace unpleasant. Examples of organic solvent smells most everyone has experienced are kerosene, gasoline, or rubbing alcohol.

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